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Indoor humidity and human health - Part I: Literature review of health effects of humidity-influenced indoor pollutants

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Abstract

Standards for indoor thermal conditions and ventilation include upper limits for relative humidity (RH) that typically are in the range of 60% to 80% RH. Although the reasons for the limits are often not explicitly stated, it is generally known that they were set out of concern for the health effects that might occur should the humidity become too high. The primary health effects of high humidity are caused by the growth and spread of biotic agents, although humidity interactions with nonbiotic pollutants, such as formaldehyde, may also cause adverse effects. This literature review identifies the most important health issues associated with high humidities and presents humidity requirements, typical contamination sites within buildings, and remediation measures for each pollutant. Part two of the paper addresses the physical causes of moisture-related problems in buildings.
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Title:
Indoor Humidity and Human Health--Part I: Literature Review of Health Effects of Humidity-
Influenced Indoor Pollutants
Author:
Baughman, A., University of California, Berkeley
Arens, Edward A, Center for the Built Environment, University of California, Berkeley
Publication Date:
1996
Series:
Indoor Environmental Quality (IEQ)
Permalink:
https://escholarship.org/uc/item/5kz1z9cg
Additional Info:
ORIGINAL CITATION: Baughman, A., and E. Arens. 1996. Indoor Humidity and Human Health
— Part I: Literature Review of 0Health Effects of Humidity-Influenced Indoor Pollutants. ASHRAE
Transactions, Vol. 102, Pt. 1, pp. 193-211.
Abstract:
Standards for indoor thermal conclitions and ventilation include upper limits for relative humidio,
(RH) that O~pically are hz the range of 60% to 80% RH. Although the reasons for the limits are
often not explicitly stated, it is generally known that they were set out of concern for the health
effects that might occur shouM the humidio, become too high. The primal3, health effects o~ high
humidio, are caused by the g~vwth and spread of biotic agents, although humidiO, interactions
with nonbiotic pollutants, such as formaldehyde, may also cause adverse ~ffects. This literature
review ident~’es the most important health issues associated with high humidities and presents
humidio, requirements, ~,pical contamination sites within buiMings, and remediation measures for
each pollutant. Part ~vo of the paper addresses the physical causes of moisture-related ptvblems
in buildings.
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3951
lndeer Humidity and Human Health Part
Literature Review of Health Effects
Anne V. Baughman
Student Member ASHRAE Edward A. Arens, Ph.D.
Member ASHRAE
ABSTRACT
Standards for indoor thermal conclitions and ventilation
include upper limits for relative humidio, (RH) that O~pically
are hz the range of 60% to 80% RH. Although the reasons for
the limits are often not explicitly stated, it is generally known
that they were set out of concern for the health effects that
might occur shouM the humidio, become too high. The pri-
mal3, health effects o~ high humidio, are caused by the g~vwth
and spread of biotic agents, although humidi
O, interactions
with nonbiotic pollutants, such as formaldehyde, may also
cause adverse ~ffects. This literature review ident~’es the most
important health issues associated with high humidities and
presents humidio, requirements, ~,pical contamination sites
within buiMings, and remediation measures for each pollut-
ant. Part ~vo of the paper addresses the physical causes of
moisture-related ptvblems in buildings.
INTRODUCTION
Standards for indoor thermal conditions and for ventila-
tion have traditionally put upper limits on the amount of
humidity permissible in interior spaces because of concern for
the health effects that might occur should the humidity become
too high. Such limits are found in past versions of ASHRAE
Standards 55-1992 (ASHRAE 1992) and 62-1989 (ASHRAE
1989) and in most international standards. The values set for
the upper limits have typically ranged from 60% to 80% RH,
although boundaries of absolute humidity have also been used.
To date, the relationship of high humidity to the full spectrum
of air quality issues and to the relevant characteristics of
building envelopes and conditioning systems has not yet been
addressed in a comprehensive manner’. This situation affects
our ability to set rational standards and building specifications.
Human health is not affected by high levels of humidity
per sen Known health effects related to high humidity are
primarily caused by the growth and spread of biotic agents
under elevated humidities, although humidity interactions with
nonbiotic pollutants, such as formaldehyde, may also cause
adverse effects. Existing limits appear to be based on engi-
neering experience with such humidity problems in buildings.
The position of any upper humidity limit has great
economic significance, particularly in hot and arid parts of the
country, where evaporative cooling is an energy-conserving
option. In the West, it affects the need for billions of dollars of
new peak electrical generating capacity that could be offset by
noncompressor-based cooling. It also directly affects a
substantial fraction of the cooling load in hot, humid climates.
Under such economic imperatives, it is desirable to carefully
examine the position of any upper humidity limit. Ideally, one
would be able to assess the health risks against the economic
benefits for any given humidity limit. At present, there is not
enough information on this subject to even begin such an anal-
ysis.
This review of the literature identifies a number of health-
related agents that are affected by indoor humidity. All of them
affect human health primarily through their inhalation fiom the
air, although some of them have lesser effects through the skin.
Biological agents require appropriate conditions in the building
lbr their germination, growth, release to the air’, and transport to
the human host. Airborne levels of nonbiological pollutants,
such as formaldehyde and ozone, may also be affected by
humidity through influences on offgassing and surface reaction
rates. Finally, the occupants’ susceptibility to these agents may
also be a function of humidity, although this appears to be a
problem primarily at low humidities, when respiratory
ailments result from dry mucous membranes (Green 1985).
The health implications of low humidities are not addressed in
this paper. Part two of this paper addresses the relationships of
the environments within buildings and conditioning systems to
the growth of biological pollutants.
Anne V. Baughman is a graduate student researcher and Edward A. Arens is a professor in the Department of Architecture at the University
of California, Berkeley~
ASHRAE Transactions: Research 193
© 1996. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.
org). Reprinted by permission from ASHRAE Transactions 1996, Vol. 102, Part 1. This material may not
be copied nor distributed in either paper or digital form without ASHRAE’s permission.
OVERVIEW OF HUMIDITY-RELATED
HEALTH CONCERNS
The primary influences of humidity on health are through
biological pollutants. The following outline describes the
health issues most commonly associated with biological
pollutants.
Infectious disease (pathogens)
bacteria (e.g., Streptococcus, Legionella)
viruses (e.g., common cold, flu)
fungi (e.g., Aspergillusfumigatus)
Allergic reactions (e.g., asthma, rhinitis)
dust mites (dried body parts and fecal excreta)
fungi
Nonallergic immunologic reactions (e.g., hypersensitivity
pneumonitis)
fungi
bacteria
Myctoxicosis
fungi
Infectious disease can occur when viable pathogenic
organisms enter (usually through inhalation) and colonize
the body of a susceptible host. The most commonly found
pathogens are bacteria or viruses, although fungal pathogens,
such as Aspergillus fimffgatus, also exist (Flannigan 1992).
Most pathogens are transmitted through human-to-human
contact when droplet nuclei form as a result of sneezing or
coughing and are subsequently inhaled by a human receptor: A
few pathogens, most notably the bacterium Legionella, can
colonize abundantly within moist environments outside the
human body and become airborne given proper conditions.
Noninfectious health conditions related to biological pollut-
ants include allergic, immunologic, and toxic responses, The
primary sources of these adverse health effects are the byproducts
of organisms rather than the viable organism itself. The term
allergy is used specifically to refer to illnesses that take place as
a result of the formation of IgE antibodies in affected persons. All
human beings have some IgE antibodies, but only a fraction of the
population responds readily to allergen exposure and produces
enough IgE antibodies to cause an allergic reaction. Once the
antibodies form, the person becomes sensitized and re-exposure
to the allergen can then trigger larger’ immune reactions resulting
in allergic symptoms. This IgE-mediated reaction develops in
20% to 30% of the people in the United States (Seltzer 1995). The
allergic diseases with well-documented links to indoor air quality
(IAQ) include allergic rhinitis (rhino conjunctivitis), primarily
affecting the nasal area, and allergic asthma and bronchopulmo-
nary aspergillosis (ABPA), both of which affect the lower airways
and alveolL The majority of patients suffering from asthma are
allergic to dust mites, mold, and/or animal dander. The estimated
overall prevalence of asthma and rhinitis may be as high as 20%
of the population (Berglund et al. 1992) and the American Lung
Association estimates that the number of people with reported
asthma in the U.S. has greatly increased in recent years, with a
49% increase since 1982.
Nonallergic immunologic responses, characterized by
recurrent flu-like symptoms (e.g., hypersensitivity pneu-
monitis, farmer’s lung, and humidifier fever), seem to be unre-
lated to the IgE antibody. They occur as a result of repeated
pollutant exposures that trigger other antibody-dependent
mechanisms as well as cellular immune responses. Although
there seems to be no genetic predisposition, only a fraction of
those exposed develop overt symptoms (Burge 1988).
Mycotoxins are produced by fungi and can lead to respira-
tory irritation, interference with pulmonary macrophage cells,
and/or higher risks of cancer (Flannigan and Miller n.d.). Many
fungi also produce volatile organic compounds (VOCs) that
may be respiratory irritants and have been suggested as a
contributing factor to sick-building-type symptoms in microbi-
ally contaminated buildings (Bjurman 1993; Sorenson 1989).
Nonbiological pollutants, such as formaldehyde, ozone,
oxides of nitrogen, and sulfur, affect humans primarily through
chemical irritation of the mucous membranes. Formaldehyde is
released into the indoor air from building materials in ways that
are dependent on atmospheric humidity. Surface reactions, and
consequently the amount and toxicity of ozone and nitrogen
oxides (NOx) and sulfur oxides (SO
x) in the air, may be influ-
enced by humidity levels. The extent to which humidity
increases or’ decreases the health impacts of these pollutants,
however’, is relatively small compared to other environmental
factors, such as air change rates and outdoor pollutant levels.
For exarnple, the use of direct evaporative cooling leads to a
rise in indoor humidity levels, which may reduce ozone by
increasing surface reactions. However, this effect is relatively
insignificant compared to the increased influx of outdoor air;
which tends to increase indoor ozone concentrations to levels
near those of the outdoor’ air’ (Stock et aL 1993).
DUST MITES
Introduc~ion
Mites are considered one of the most important allergens
in house dust, particularly in regions with high humidities and
temperate climates. The most common genus of mites found in
house dust in North America and Europe is Dermatopha-
goides, of which there are two species, D. pteronyssinus and D.
farinae. It is estimated that 10% of the population in the U.S. is
allergic to house dust and 70% of these people are specifically
allergic to mite allergens (Bates et al. 1993). The actual
allergen is not the mites themselves, which are approximately
1/3mm in length at maturity, but the dried fragments of their
body parts and fecal excreta. These by-products are initially 10
to .50/.tm in diameter but break down into smaller fragments
that become airborne when dust is disturbed. According to one
study, more than half of the weight of mite allergens within a
home were found to be less than 5 p.m in length (Reed et al.
1986). These particles are the primary health concern since
they can be inhaled into the lower airways of the lungs and, if
quantities are significant, IgE antibodies can form, leading to
allergic reactions in the susceptible portion of the population.
194 ASHRAE Transactions: Research
A number of studies have demonstrated a high prevalence
of sensitization to mite allergens among patients with asthma
and nonspecific respiratory symptoms (Voorhorst et al. 1964;
Korsgaard 1983b; Platts-Mills et alo 1989; Smith et al. 1985;
Arlian et al. 1992). In most of these studies the patients were
referred to the researchers by clinics and compared with
control subjects randomly selected from the same or a similar
population base. Sensitization to mite allergens was demon-
strated by a positive skin prick test. The study of Danish homes
by Voorhorst et al. (1964) was the first to establish a definitive
link between the presence of mite allergens and respiratory
symptoms. Korsgaard (1983a, 1983b) confirmed this finding
when he found significantly higher concentrations of dust
mites in the homes of 25 asthmatic patients compared to 75
randomly selected homes. Arlian et al. (1992) conducted a five-
year study of 252 homes inhabited by dust-mite-sensitive
people in eight different regions of the UoS. They found that
83% of the homes had average mite densities greater than the
estimated sensitivity threshold of 100 mites/gm of dust.
Studies in the literature also provide evidence to support a
connection between damp housing and sensitivity to dust mites
and childhood respiratory symptoms. For example, Murray et
al. (1985) studied 774 homes inhabited by children with respi-
ratory symptoms in British Columbia and found that more than
90% of these children lived in areas defined as "humid" (i.e.,
indoor humidity estimated to be 50% or greater for four or
more months out of the year). In addition, there was a signifi-
cant difference in the number of mite-sensitive children in the
"humid" areas (skin prick test positive for D. farinae in 31%
and D. pteronyssinus in 40%) as compared to those living in
areas defined as "dry," with an indoor RH of 50% or higher for
no more than two months per year (skin prick test positive for
D. farinae and D. pteronyssinus in 3% and 2%, respectively).
Verhoeffet al. (1995) conducted a study that included 259 chil-
dren with chronic respiratory symptoms and 257 control chil-
dren. There were more cases of mite and mold sensitization in
the children with respiratory symptoms. These children were
also slightly more likely than the controls to have been living in
homes where mold or damp was reported or observed.
Along with respiratory symptoms, high levels of dust mite
allergens have also been correlated with atopic dermatitis
(AD), characterized by itchy, irritated skin (Harving et al. 1990;
August 1984)m In general, these studies suggest that those
susceptible to mites (i.e., those likely to form IgE antibodies)
are also likely to develop skin sensitization if exposed to high
concentrations of mite allergens. For example, Colloff (1992)
examined the density of dust mite populations in mattresses in
the homes of 23 people with AD who were mite-sensitive and
found that counts were significantly higher than in the
mattresses of the nonatopic control group° Colloff also cites a
number of references that link atopic dermatitis to high dust
mite exposure, including IgE antibody responses to mite aller-
gens, among patients with AD and marked clinical improve-
ment following intensive eradication of mite allergens.
The regional diversity of mite studies in the literature
suggests that mites occur indoors all over the world, from arctic
Greenland to tropic Africa (Anderson and Korsgaard 1986).
The study by Arlian et al. (1992) included eight different
geographic regions within the U.S. They found that D. pteron-
yssinus and D. farinae were by far the most common species,
with D. pteronyssinus predominating in humid regions with
moderate climates and D. farinae predominating in areas with
prolonged periods of dry weather. This finding was supported
by Lang et al. (1977), who examined mite populations in four
different climatic zones of southern California and found
significant numbers of D. pteronyssinus and D. farinae in 14 of
15 of the coastal homes and in 9 of 15 of the inland valley
homes. D. pteronyssinus was the predominant species in the
coastal region, while D. farinae predominated in the inland
regions.
Mites are relatively sparse in regions with low outdoor
humidity, such as at high elevations and in desert areas (Brun-
drett 1990; Murray et aim 1985; Lang et al. 1977). However, if
the indoor humidity is allowed to rise due to internal sources
such as direct evaporative cooling, mite populations, particu-
larly D. farinae, can become significant even when outdoor
humidities are low. For example, O’Rourke et aim (1993) evalu-
ated 190 evaporatively cooled homes in Tucson, Arizona, and
detected mites in more than half of the homes, with D. farinae
being the overwhelmingly predominant species (greater than
98% of all mites recovered).
Environmental Requirements
Mites contain about 70% to 75% water by weight and
must maintain this in order to reproduce (Arlian 1992). Their
primary source of water is ambient water vapor, which they are
able to extract directly from unsaturated air by means of a
hygroscopic salt solution in the supracoxal gland (Fernandez-
Caldas et al. 1994). The amount of water gained through inges-
tion of moist food is relatively small. Laboratory studies of D.
pteronyssinus suggest that optimal conditions for growth and
development occur between 70% and 80% RH at 25°C, with
acceptable ranges of 55% to 80% RH and 17°C to 32°C
(Anderson and Korsgaard 1986). The upper humidity limit
constrained by the possibility of mold growth, particularly
above 88%, which can inhibit mite development (Brundrett
1990).
Arlian (1992) performed laboratory studies of both
farinae and D. pteronyssinus and found that the critical equilib-
rium humidity (CEH) for fasting mites, defined as the lowest
RH at which mites are able to maintain their water balance, was
73% and 70% RH at 25°C for D. pteronyssinus and Dfarinae,
respectively. The CEH was found to be influenced by tempera-
ture and ranged from 55% at 15°C to 75% at 35°C for D.
farinae. According to Arlian, this temperature relationship,
together with the fact that feeding mites do gain small amounts
of water from food, may explain why significant populations of
mites are found in environments with relative humidities below
70%. Survival of mites for prolonged periods at lower humidi-
ASHRAE Transactions: Research 195
ties may also be explained in part by the crystallization of salts
within the supracoxal gland, which may slow down the rate of
dehydration (Fernandez-Caldes et al. 1994).
Under optimal conditions, mites live for three months with
three different larval stages. The survival of active adult mites
(both male and female) is limited to 4 to 11 days at humidities
below 50% RH at 25°C (Arlian et al. 1982). The protonymph,
however, which is one of the dormant larval forms, can survive
for months at low humidities and then evolve to the more active
forms when optimal conditions return (Arlian 1992). This
observation is supported by the field study by Lang et al. (1978)
in which the different stages of mite development were quanti~
fled. In this study a higher number of protonymphs were found
when the RH fell below critical levels (50% to 65% RH). These
protonymphs are particularly difficult to remove with normal
vacuuming since they can bury themselves within surfaces
(Arlian 1992).
As one might expect, most mite allergens are fot~ned by
adults during their active phase° Thus, for a given number of
mites, the highest levels of allergens found in the environment
usually correspond to optimal humidity conditions. Arlian
(1992) examined the effect of RH on mite metabolism for
range of relative humidities between 22% and 95% and
observed that feeding rates, and consequently the amount of
fecal matter produced, increased with increasing RH~ The
effect was pm~icularly significant between 75% and 85% RH,
for which there was a fivefold increase in the weight of food
consumed for’ both D. pteronyssbms and Dfarinae. Below the
CEH, Arlian found that mites fed sparingly and produced little
fecal matter. These results suggest that significant reductions in
the level of mite allergens, which consist primarily of meta-
bolic by-products, may occur ifRH is reduced below the CEH.
(For more detailed information on the mite life-cycle and
metabolism, see Arlian [1992].)
Laboratory studies suggest that temperatures within the
range typically found in occupied spaces have little direct effect
on the length of the mite’s life-cycle and that mites are able to
survive extreme temperature conditions for limited periods. For
example, under laboratory conditions, more than half of D.
pteronyssinus survive after 12 days when continuously
exposed to 34°C and 75% RH, while at 2°C and 75% RH
approximately 64% of D. farinae adults survive after 72 hours
(Lang et al. 1977). Mites are also able to reproduce at temper-
atures as low as 17°C, albeit mote slowly than at 25°C (Murray
et al. 1979).
Mites subsist primarily on shed human and animal skin
scales. It is believed that mites cannot digest lipids withi~ the
skin scales themselves and require the aid of xerophilic fungi,
of the genus Aspergillus, to dissolve the lipids for them (Flan-
nigan 1992; Hart and Whitehead 1990; Platts-Mills et al.
1989). This suggests that conditions for’ mite survival must also
be suitable for these fungi.
Absolute vs. Relative Humidity
Some researchers have suggested that absolute humidity
rather than relative humidity is the limiting factor controlling
mite metabolism (Korsgaard 1983a, 1983b; Platts-Mills et al.
1987b)~ However, the predominant evidence from laboratory
studies and field work suggests that relative humidity is the
controlling factor. Mites have a high surface-to-volume ratio
and are poikilothermal (i.e., theh" body temperature is identical
to that of the surrounding environment) (Anderson and Kors-
gaard 1986). Since there is no temperature gradient between
the mite and the surrounding environment, the relative differ-
ence between the air vapor pressure and the mite’s internal
saturation vapor pressure is proportional to the relative
humidity rather than the absolute humidity. Arlian (1992) has
demonstrated that the driving force in the uptake of water frotn
unsaturated air is the number of water molecules impinging on
the mite’s uptake surface. Arlian also performed laboratory
studies that suggest that mites are able to maintain a water
balance at 20°C and 79% RH but die at 27°C and 56% RH (i.e.,
the same absolute humidity).
Field Studies: Residential
Field studies within homes generally support the labora-
tory findings that indoor relative humidity is the most signifi-
cant environmental condition associated with high mite
populations and allergen concentrations. However, it is not
clear from these studies what specific level of humidity is crit-
ical. Significant concentrations of mites and allergens have
been found at indoor humidities as low as 40% RH (O’Rourke
et al. 1993) but more often at indoor humidities above 50% RH.
For example, in a study of homes in Vancouver; Murray et al.
(1979) detected significant numbers of mites only when the
was greater than 50% for at least part of every day during the
month of collection. Smith et al. (1985) also found a direct
correlation between mite population and indoor RH in a study
of 20 homes of mite-sensitive children, with mite populations
peaking at RH of 50% or greater; Hart and Whitehead (1990)
evaluated 30 homes in the United Kingdom and found that mite
populations were most strongly correlated with indoor RH and
that bedrooms with humidities above 64% RH contained
significantly more mites in mattresses than those with humidi-
ties below this level.
Regional studies suggest that dust samples from different
homes within the same region can exhibit wide differences in
mite concentration due to differences in indoor humidity alone.
For example, Lintner et al. (1993) evaluated 424 homes across
the U.S. and found that the greatest variations in mite popula-
tion occurred as a result of differences in the indoor relative
humidity among homes rather than regional climatic differ-
ences. Korsgaard (1983a, 1983b) conducted a four-season
study of 50 Danish apartments, all within the same region, and
found that seasonal variation in dust mite populations in
mattresses con’elated with the indoor humidity, while homes
with the lowest indoor humidities did not contain detectable
levels of mite populations. Ellingson et al. (1995) studied the
196 ASHRAE Transactions: Research
effect of direct evaporative cooling on the prevalence of mite
allergen in Colorado homes. Their results show that during the
peak cooling season 48 of 95 samples from homes with evapo-
rative coolers (average interior RH of 51% or greater) had
levels of Der p 1 and Der f 1 of 2 gm/gm dust or greater, but
only 5 of 95 control samples (average interior RH of less than
45%) had levels of 2 gm/gm dust or greater.
Long-term studies suggest that seasonal trends in mite
populations correlate with seasonal variations in indoor
humidity. For example, in a 19-month study of six homes in
southern California, Lang et al. (1978) found seasonal differ-
ences in species composition, with D. pteronyssinus more
abundant from July through November and D. farinae more
abundant from August through December. Both species were
found to be less prevalent from late spring to July. Although
monthly population fluctuations correlated with indoor relative
humidity, the population increases lagged one to two months
behind the time that conditions first became favorable, while
population declines correlated directly to the time that relative
humidity levels fell below critical levels (47% to 50% RH for
D.farinae and 60% to 65% RH for D. ptero~o,ssinus). Arlian et
al. (1982) also found significant seasonal fluctuations in the
two-year study of 19 homes in Ohio, with highest densities of
mites occurring during the humid summer months and lowest
densities occurring during the drier late heating season. In the
study by Korsgaard (1983a, 1983b), those apartments that had
low absolute indoor humidities in the winter did not contain
noticeable concentrations of mites in the summer and autumn
despite the fact that the humidity conditions increased to levels
that were high enough to support peak populations. This
suggests that, in this case, winter conditions may have been
severe and long enough to kill off even the dormant proto-
nymph, assuming other eradication steps were not taken.
Based on a number of field studies it is also apparent that
allergen levels correlate with seasonal variations and that
changes in allergen levels lag behind both increases and
decreases in indoor RH. Lintner and Brame (1993) studied 424
homes across the U.S. and found a distinct seasonal fluctuation
of mite allergens for both D. pteronyssinus and D. farinae
species, with the D. pteronyssinus allergens peaking in July and
the D. farinae allergens peaking in September. In a study by
Friedman et al. (1992) of homes in the upper Connecticut River
Valley, there was a marked seasonal increase in total D. ptero-
nyssinus allergens from June to September. In a one-year study
of 12 homes in central Virginia, Platts-Mills et aL (1987a)
found that increases in both mite populations and allergen
levels lagged approximately one month behind increases in
indoor humidity and that several months passed before a fall in
allergen levels was detected after a drop in indoor humidity.
In all of the field studies cited, the highest concentrations
of mites were found in mattresses, thick carpeting, and/or
heavily used fabric-upholstered furniture. This suggests that
mites thrive best within micr0environments that contain a
source of food (shed skin scales) and have a relatively high and
consistent moisture level. For example, bedding is a common
site for mites because there is an ample supply of food and the
humidity within an occupied bed is higher than that of the air of
the surrounding space. This is also true of furniture upholstered
with permeable fabric that can absorb and retain the moisture
given off by an occupant. Field studies have found less varia-
tion over time in the number of mites within bedding as
compared to other sites (Smith et al. 1985). Murray et
(1979) also found significant numbers of mites in mattress dust
during the winter even when the indoor RH fell below 50%.
Carpeting can also be a localized site of increased
humidity and consequently may be an important reservoir for
allergens in both homes and schools. Studies conducted in
schools have demonstrated that carpets contain high levels of a
variety of allergens including pollen, cat and dog dander, and
mite and mold allergens (Fernandez-Caldas et aL 1994). This
may be the primary source of exposure for young children, who
generally live closer to the floor and do not have high exposures
in bedding since they usually sleep on plastic-covered
mattresses. Arlian (1992) studied the microenvironment of
carpeted floor and found that the relative humidity within the
carpeting was 9.6% higher than that of the ambient air 1 to 2
meters above the floor. This was attributed to the decreased
temperature of the floor (3.7°C lower on average than the
ambient air), which drove the relative humidity up. In this
study, Arlian also found that long-pile carpeting contained
significantly more mites than short-pile carpets and tile or
wooden floors (i.e., short-pile carpets did not contain signifi-
cantly more mites than floors without carpets). This finding is
also supported by a study by O’Rourke et al. (1993) in which
house mites were found four times more frequently in homes
with wall-to-wall carpets than in homes with other floor types.
Field Studies: Office Buildings
The few studies of mite populations that have been
conducted within commercial buildings have shown that mite
levels are generally low (Menzies et alo 1992). In a study
office buildings in the mid-Atlantic states, Hung et al. (1992)
found moderate to high levels of mite allergens within
carpeting and chairs of one of the five buildings studied. In a
study of buildings in the New England area by Friendman et al.
(1992), very low population levels of dust mites were found
within the carpets of workplaces. The observation of low mite
levels within commercial buildings may not be surprising since
these buildings tend to be less humid than residences due to the
frequent use of air conditioning and fewer internal sources of
moisture (e.g, cooking, showering, etc.). In addition, commer-
cial buildings do not usually contain bedding and thick
carpeting, the most common sites for mites in residences. It has
also been suggested that mites tend to be more common on
ground floors than on upper stories and are rare in hotels (Reed
et al. 1986).
ASHRAE Transactions: Research 197
Remediation
The strong correlation between indoor relative humidity
and dust mite population has led to recommendations to reduce
indoor humidity. However’, exactly where the upper limit
should lie is not obvious. Most of the field studies suggest that
when indoor humidity is kept below 50% RH, mite populations
do not grow to significant levels. Laboratory studies, on the
other hand, in which the microenvironment of the mite is in
equilibrium with the surrounding air, suggest that mite popula-
tion growth and metabolism (related to the amount of allergen
produced) can be significantly reduced if relative humidity is
kept below 70% RH at 25°C (Arlian 1992).
One reason for the discrepancy between field and labora-
tory studies may be the difference in relative humidity between
the mite’s microhabitat, which can be considered to be within a
few millimeters of the horizontal surfaces on which they lie,
and that of the surrounding air’ due to differences in tempera-
ture as well as the ability of certain types of surfaces to retain
moisture. The humidity measurements for the field studies
cited were taken from the air within the core of the room, which
may not correlate with the RH within the rnicroenvironments
from which the mite sa~nples were taken. The time fi’ame of the
RH measurements was also not indicated for most of these field
studies---instantaneous measurements taken at the time of
sampling may not be representative of long-term conditions. In
addition, seasonal changes in indoor humidity have a signifi-
cant effect on mite populations and allergen levels, as
suggested by the long-term field studies; however, the specific
time constraints have not yet been resolved.
’Iqaere are a number of effective remedial methods directed
at reducing allergens and mites within their microhabitat in
addition to control of relative humidity. These include special-
ized vacuuming procedures, removal of long-pile carpeting and
heavily used upholstered fur:niture, regular hot-water cleaning
of bedding, encasement of mattresses and pillows, and applica-
tion of acaricides (Htut 1994). For example, in a study
laundry procedures, McDonald and Tovey (1992) found that all
mites were killed by water at 55°C or higher. In a study by
Platts-Mills et al. (1989), a tenfold or greater reduction in mite
allergen levels was achieved in many houses by hot-washing all
bedding at least every 10 days and removing carpets and uphol-
stered furniture. Wickman et al. (1994a) found a significant
decrease in mite allergen levels on mattress surfaces six months
after they had been encased with a semipermeable polyure-
thane cover’. Based on the observed seasonal effects for
temperate climates, it seems that late winter and early spring
are the best ti~nes to clean mattresses and carpets aggressively
to kill the few mites that survived the winter. Theoretically, this
should reduce the chances of having a large infestation in the
summer months.
Vacuuming is effective only if central vacuuming systems,
HEPA filters, or systems that entrain dust in a liquid medium
are used. Conventional vacuuming does not help to reduce mite
populations and allergens within carpets and can actually
aggravate the problem. Allergen particles in the size range of
the greatest health significance (< 2 gm) easily pass through
the filter bags of conventional vacuums, causing a significant
increase in the concentration of airborne allergens during and
shortly after vacuuming (Kalra et al. 1990). In a two-year study
of ~nites in 19 homes in Ohio, Arlian et al. (1982) found
significant correlation between the number of mites and the
frequency or thoroughness of cleaning, amount of dust, or age
of furnishings or dwelling. Arlian (1992) suggested that the
ineffectiveness of cleaning may also be related to the difficulty
in removing larval forms of mites adhering to surfaces.
Acaricides are now available that have been specially
designed to eliminate mites from carpeting. One such product
uses benzyl benzoate as the active ingredient. It is formulated
as a moist powder with a wax to bind mite fragments and excre-
ment so that they can be vacuumed. It is designed to be reap-
plied every six to eight months. Results from a number of
studies suggest that this product has been successful in
reducing mite populations (Htut 1994). Benzyl benzoate was
initially marketed in Europe and has been approved for use in
all states in the U.S. except California (eL 1994). Fungicides
such as natamycin kill the fungi required by mites to digest
lipids in the skin scales and have also been used with some
success (Flannigan 1992; Platts-Mills et al. 1987b; Htut 1994);
however, in one study in which a double-blind, placebo-
controlled method was used, no significant improvement was
observed (Reiser et al. 1990). Other surface treatments that
have been used include liquid nitrogen, benzyl benzoate in
combination with tannic acid, and benzoic acid (Htut 1994).
Other possible methods of reducing mite levels include the
use of electric blankets, which can reduce the local humidity
within bedding (Hart and Whitehead 1990), and dehumidifica-
tion/air conditioning (Lintner et al. 1993). According to a study
by de Boer and van der Geest (1990), a reduction in dust mite
populations of 19% to 84% can be achieved by heating the
mattresses with electric blankets when the beds are not in use.
In a field study by Cabrera et al. (1995), dust mite allergens
were reduced by more than 50% with the use of a dehumidifier.
Improved ventilation systems within homes can also help
reduce mite levels by counteracting internal sources of
humidity, such as cooking and showering, in climates where
outdoor humidity is not the major source of moisture.
Wickman et al. (1991) suggest that house dust mite infestation
used to be rare in Stockholm; however, mite-sensitive children
are now frequently observed, which may indicate an increased
infestation rate. The authors attribute this to a reduction in the
ventilation rate resulting from the energy conservation
programs. In a follow-up study, Wickman et al. (1994b) looked
at the concentration of dust mites in 70 homes in Stockholm
belonging to two major house types--4.hose with crawlspace
basements and those with concrete floor slabs--and deter-
mined that the highest risk factors for allergen concentration
exceeding the median were unimproved natural ventilation
(i.e., no mechanical exhaust), concrete floor slabs, and conden-
sation on windows. In a study of Danish homes, Harving et al.
(1994) found that decreases in indoor humidity levels through
198 ASHRAE Transactions: Research
the use of supply-and-exhaust ventilation systems significantly
reduced dust mite levels.
FUNGI
Introduction
Fungi (via airborne fungal spores, fragments of hyphal
mat, and metabolic by-products) have been linked to a number
of adverse health effects, including allergic reactions, hyper-
sensitivity pneumonitis, mycotoxicosis, and pathogenic
disease. In general, however, fewer people are allergic to fungi
than to dust mites and animal dander (Flannigan et aL 1991).
Beaumont et al. (1985) demonstrated that many more respira-
tory patients with suspected allergies react to animal dander
(34%) and house dust (44%) than to molds (3%). The
common genera known to cause asthma and rhinitis include
Alternaria, Aspergillus, Cladosporium, and Penicillium (Flan-
nigan and Miller n.do). A few genera, such as Aspergillus
niger and A. fi#nigatus), Histoplasma, and Cryptococcus, are
pathogenic and can infect the lungs, ears, or eyes in susceptible
hosts; however, reported cases are relatively rare (Gravesen
1979; Flannigan 1992; Miller 1992). Metabolic gases produced
by fungi contain volatile organic compounds (VOCs) that are
responsible for the mildew odor. These VOCs may be a
contributing factor to sick-building-type symptoms, including
eye, nose, and throat irritation; headache; and fatigue (Bjurman
1993). In a study of microbially contaminated buildings, VOCs
of the type commonly associated with indoor man-made mate-
rials were actually found to be the metabolic by-products of
fungi growing in the buildings (Bayer et al. 1993). High indoor
spore levels of fungi such as Cladosporium and the dry rot
fungus Sepula have been associated with cases of hypersensi-
tivity pneumonitis. Fungal spores and vegetative mycelium are
also known to contain toxic substances (mycotoxins) that can
lead to respiratory symptoms unrelated to allergic mechanisms
(Flannigan and Miller n.d.). Flannigan et al. (1991) list
number of toxigenic species isolated from the indoor air of
houses.
Most fungi originate outdoors and are saprophytic (i.e.,
grow on substrates of dead or dying plant and animal matter).
Outdoor concentrations vary with the season, the time of day,
local weather conditions, and whether the site is rural or urban.
For example, phylloplane (leaf-loving) fungi, which include
Cladosporium and Alternaria, are more common in rural areas
and show a strong seasonal activity with peak concentrations in
the summer. Penicilliu~n and Aspergillus are the most common
soil fungi found in urban environments, and airborne spore
concentrations of these species remain relatively constant
throughout the year (Brundrett 1990). Fungal spores are typi-
cally in the range of 3 to 30 p.m in diameter and, once they are
released into the air, can travel intercontinental distances.
Airborne spores enter buildings through ventilation equipment
and can set up colonies on surfaces where moisture and nutrient
conditions are favorable.
Buildings with no internal sources of fungi have nearly the
same proportion of fungal species as outdoor air, with total
counts reduced due to settling and filtration within air-condi-
tioning equipment. In a study of Canadian office buildings,
Miller (1992) found that those buildings not associated with
microbial problems had micofloral counts that were qualita-
tively similar and quantitatively lower than those of outdoor air,
while contaminated buildings tended to have a higher propor-
tion of nonphylloplane fungi, particularly Penicillium and
Aspergilluso In a study of fungal concentrations within daycare
centers and dwellings, Hyvarinen et al. (1993) found that the
total concentration of airborne fungal spores was higher in
moldy buildings. In addition, the concentrations of Aspergillus
and Oidoidendron in the fall and Aspergillus and Penicillium in
the winter were higher in the buildings with mold problems
than in the reference buildings. The presence of wet-habitat
fungi, such as Phoma, Stachybotrys, Trichoderma, and Ulocla-
dium, in significant quantities suggests the existence of either
rotting vegetation near the air intake or an extremely damp
amplification site within the building (Flannigan 1992). Xero-
philic species, including the toxigenic species Penicillium
auranteogriseum and Aspergillus versicolor, can form an
appreciable percentage of the population within indoor dust
samples. These had not been widely detected until the recent
use of new sampling methods designed for detection of xero-
philic species (Miller 1992).
Environmental Requirements
Fungi need water, carbon, and nitrogen for growth, as well
as minute amounts of other nutrients normally present in
natural environments~ Typical construction materials
containing nutrients used by fungi include wood, cellulose,
wallpaper, organic insulation materials, textiles (especially
natural fibers), and glues and paints containing carbohydrates
or proteins. Although materials such as metal, concrete, plas-
tics, fiberglass, and other synthetic products cannot be used
directly by fungi, they can collect organic debris that serve as a
nutrient source for fungi. For example, despite air filtration,
some dust containing living microorganisms passes through
air-handling units and settles on porous insulation within ducts.
If this insulation material then becomes wet (e.g., due to
condensation), fungi will grow and release spores into the
ventilation air (Morey et al. 1991; Pasanen et al. 1993).
Fungi acquire most of their nutrients through a solvent
process (Griffin, 1981). Thus the moisture on and within
substrate is the important factor determining fungal growth
rather than the moisture of the ambient air (Block 1953)o Labo-
ratory studies support this observation. For example, Pasanen
et al. (1991) measured colony diameters for both Penicillium
sp. and Aspergillus.fitmigatus as a function of RH in the range
of 11% to 92% and found that fungal colonies grew on wet
substrates even at low levels of atmospheric humidity. The
authors conclude that growth is dependent on substrate mois-
ture and is not directly affected by atmospheric moisture.
Systems containing water, such as the water reservoirs of
humidifiers, favor the growth of bacteria, algae, protozoa, and
certain types of fungi, especially yeasts. Most fungi, however,
ASHRAE Transactions: Research 199
prefer surfaces of moist materials to liquid water (Pasanen et al.
1992). Thus, since nutrients and airborne fungal spores are
abundant within buildings, the availability of moisture on and
within surfaces appears to be the limiting factor for growth.
Fungi are able to withstand dry periods to some extent by
becoming dormant or by utilizing metabolically generated
water that they add to the substrate. For’ example, wood will not
decay if the moisture content is less than 20% to 25% of its dry
weight, except when it has been invaded by a dry-rot fungus
such as Merulius lacrymans, which is able to translocate water
(Moore-Landecker 1982). Fungi that inhabit soil and wood
grow better in moderate rather than high moisture contents
since soil aeration (and therefore oxygen supply) is limited
when the moisture content is high (Moore-Landecker 1982).
In general, fungi can grow at temperatures between 0°C
and 40°C. Below temperatures of 0°C the fungi may survive
but will not continue to grow, and above temperatures of 40°C
fungi cannot survive for long periods (TenWolde and Rose
1993)~ Temperature variations within the range found in most
conditioned buildings do not appear to be a limiting factor but
may affect growth rates. For example, the study by Pasanen et
al. (1992) demonstrated that both Penicillium sp. and
Aspergillus fi~migatus grew at all temperatures from 10°C to
30°C, with Aspergillus growing fastest at 30°C and Penicil-
lium growing fastest around 20°C.
The potential for fungal growth on a substrate has often
been attributed to moisture content (MC), defined as the ratio
of "fi’ee water" in the material to the material’s dry weight after
being dried in an oven (free water refers to water held in
porous material by van der Waals forces [i.e., hydrogen
bonding] or’ capillary attraction, as opposed to water of hydra-
tion, which is chemically bound to the materials). For wood
products, percent moisture content is defined as the weight of
removed water divided by the weight of oven-dried wood.
Wood consists of hygroscopic cell walls surrounding cellular
spaces filled with water and/or air~ Below fiber saturation,
when the cell walls are fully hydrated yet have no water
contained in the cellular’ spaces, fungal growth is inhibited
since the fungi are not able to readily extract the water held by
van der Waals forces (Wilcox 1995). The fiber saturation point
for wood occurs at MCs of 25% to 30%.
Although moisture content is commonly mentioned, it is
not the most appropriate measure of a substrate’s potential to
support fungus. This is because materials differ in how tightly
they hold fi’ee water; and the measurement of moisture content
may var3~ depending on the procedure used. For example,
Block (1953) evaluated fungal growth on a number of different
materials, including leather’, wood, cheese, wool, and cotton,
and found a common mold growth threshold value of about
MC = 0.1, which has often been quoted in the literature. This is
significantly below the fiber’ saturation point for’ wood. More
recently, Foar’de et al. (1993) demonstrated a critical MC
0.055 to 0.065 for Penicillium growing in porous ceiling tiles.
It has been suggested that for biological purposes the more
physically meaningful parameter is water activity, a~ defined
as the ratio of the water vapor pressure at the surface of the
moist material to that of a pure liquid water surface at the same
temperature and pressure (Ayerst 1969; Griffin 1981; Flare
nigan 1992). This is also r’efe~x’ed to as the equivalent relative
humidity (ERH) when written in the form of a percentage (i.e,
an aw of 0.80 is the same as an ERH of 80%). ERH is equal to
RH at the surface of the material only when the system is
confined to the extent that the atmosphere above a moist
surface is at the same vapor pressure and temperature as that
directly at the moist surface. In actual environments, however,
there is usually a gradient of vapor pressure from the surface
into the air above or vice versa. In this case, the relative
humidity of the air has only an indirect influence through
drying and moistening of the materials it contacts.
In general, favorable conditions for fungal growth depend
on the species and the type of substrate on which it is growing.
In addition, fungal growth does not occur" in isolation but rather
within a complex microbiological system that includes yeasts
and bacteria as well as molds. The following excerpt from
Flannigan (1992) presents his ecological classification
molds in terms of their moisture requirements, and describes
the process by which different types of molds take hold and
grow.
Although all molds growing on surfaces in buildings grow
most rapidly in pure culture at a high ~w .... individual species
can be classified as:
o Primary colonizers, which are able to grow at an ~ of
less than 0.80 and also are [eferred to as xerophilic
fungi because they are able to grow at lower ~ than
other molds, e.g., species in the Aspergillus glaucus
group (A. amstelodami, A. repens, etc.), A versicolot;
and Penicillium brevicompactum.
o Secondary colonizers, which are able to grow at an aw
of
0.80-0.90, e.g, Cladosporium cladosporoides and C.
sphaerospermum.
o Tertiary colonizers, which are only able to grow at an a
w
more than 0.90, e.g., Alternaria alternata, Phoma her-
barum, Ulocladium consortiale, and Stachybotrys atra.
Where there is ingress of water over a restricted area, e.g.,
as a result of rain water’ penetrating via a structural fault in a
wall, tertiary colonizers rnay be found at or near the site of
ingress and primary colonizers, such as A. versicolor and Peni-
cillium, at the less wet margins of the affected area (Grant et al.
1989). Pasanen et al. (1992) found that Aspergilli and Peni-
celia (primary colonizers) grew under all conditions when
samples of timber, plywood, gypsum board, fiberboard, and
wallpaper were incubated in atmospheres of 75-76% RH and
above, but species of Cladosporium (secondary colonizers) and
Stachybotrys and Trichoderma (tertiary colonizers) only devel-
oped at the highest RH, where the substrate aw would be 0.96-
0.98. The degree of dampness determines what species are able
to grow and sporulate, and therefore strongly influences the
composition of the spora in indoor air.
This statement agrees with the findings of Kalliokoski et
al. (1993), who carried out a controlled-chamber study
fungal growth on a number of moisture-damaged building
200 ASHRAE Transactions: Research
materials. Based on this study, the authors suggest that fungal
growth is dependent on temperature, composition, and hygro-
scopicity of materials and fungal species and is likely when the
ERH exceeds 76% to 96%. The growth rate of a xerophilic
fungi from a number of different ecological sites and contami-
nated materials was studied in the laboratory by Avari and
Allsopp (t983). Optimum ranges for growth were found to
between a~,, levels of 0.97 and 0.90. For all of the species
studied, no growth was observed after one month at au, levels of
approximately 0.80. This study is in agreement with the recom-
mendation by the International Energy Agency (Annex 14) that
the monthly average water activity of a material surface should
not exceed 0.8 (IEA 1991). This recommendation recognizes
the importance of the surface microclimate and was developed
in response to the request from building professionals for a
simple criterion by which to .judge the likelihood for mold
growth.
Field Studies
The most common cause of fungal spore contamination
within residences is condensation on surfaces and reoccurring
spills or leaks (Seltzer 1995). Besides superficial condensation,
interstitial condensation within porous building materials (such
as concrete, brick, and/or plaster) may provide a reservoir for
fungal growth. Interior dampness problems are usually related
to construction faults, such as inadequate insulation and
thermal bridges, in combination with inadequate ventilation
and/or the pattern of use within homes. For example, in a study
of 86 newly built energy-efficient residences in the Pacific
Northwest, Tsongas (1991) found that one-third had mold
growing on indoor wall surfaces and one-third had mold on
window frames and/or sills. Although homes were well insu-
lated, condensation still occurred due to internal sources of
humidity that were not properly ventilated. Nevalainen et al.
(1991) studied residences in Finland, where microbial prob-
lems in buildings are relatively uncommon due to the heating
and insulation requirements of a cold climate. They found that
most of the houses with mold problems had improperly weath-
erproofed outside walls, which allowed rainfall into the insula-
tion material, and/or inadequate drainage that allowed moisture
to penetrate in through the floor. Becker (1984) conducted
post-occupancy evaluation of 200 homes in a coastal region of
Israel, which has mild winters. He concluded that condensation
was the main source of fungal growth and that the major factors
contributing to mold problems were location and orientation of
the dwelling (affecting wall surface temperatures), occupant
density, cooking habits, and the type of paint or wall covering.
Abe (1993) developed a biosensor method, using a xerophilic
fungus, to study the potential for growth within different
microenvironments of a Japanese apartment. A significant vari-
ation among and within rooms was evident, with lower poten-
tials for growth observed in spaces with walls adjacent to
internal spaces and highest potentials observed at cold corners
of northern and eastern walls adjacent to the outside.
The most extensive fungal contamination problems occur
in hot, humid climates; control of indoor humidity in these
regions is an important factor. Bayer and Downing (1992)
observed fungal contamination in schools in a climate where
outdoor humidities ranged from 75% to 90% for most of the
year. High indoor humidities resulted in visible mold growth on
ceiling tiles, fan-coil units, papers, and books. In one case,
carpeting adhesive was not able to cure in the highly humid
conditions and provided a medium for microbial growth. Effec-
tive remediation procedures focused on removing and cleaning
contaminated materials and controlling indoor humidity levels.
Within commercial buildings, microbiological contamination
is frequently a result of the absence of’ or inadequate preventive
maintenance of conditioning systems. Morey (1988) evaluated the
occupied space and heating, ventilating, and air-conditioning
(HVAC) systems of 21 commercial buildings for the presence
microbiological reservoirs and amplification sites. Microbiolog-
ical growth was detected in 18 of the 21 buildings. In nine of the
buildings, contamination was found in the porous duct insulation.
Other significant sources included stagnant water in drain pans (10
buildings), excessive relative humidity (6 buildings), flood damage
(6 buildings), and the location of outdoor air’ intakes near external
bioaerosol sources (6 buildings). Ezeonu et al. (1994) demon-
strated that the fiberglass duct liners and boards from eight build-
ings whose occupants complained of unacceptable or moldy odors
were heavily colonized by fungi, particularly by Aspergillus versi-
color.
Remediation
The most effective remediation procedures depend on the
source of the contamination and regional climatic conditions.
Control of indoor humidity is an important factor in hot,
humid climates; however, for other climatic types other
means of controlling moisture, such as insulation to keep
interior surfaces above the dew point, proper placement of
vapor barriers to control vapor and airflow between indoors
and outdoors, and control of external rain and groundwater
penetration into the building, may be more critical Proper
maintenance of HVAC equipment is also an important factor,
particularly in commercial buildings. This may require
design innovations focused on improving accessibility and
maintenance procedures. These issues are discussed in
greater detail in part two of this paper.
Once a material has become contaminated, it is almost
impossible to completely eliminate the fungi and removal is
often the only option. In a recent study of microbial growth in
chipboard, Thogersen et al. (1993) found that water damage
resulted in massive growth and that, even after drying, the
material still contained spores. Use of biocides is usually
discouraged, since most are toxic and continuous use may
increase corrosion and encourage the development of resis-
tant strains (Nevalainen 1993). Bleach treatment has been
unsuccessful in cleaning contaminated duct liners (Morey
1988).
ASHRAE Transactions: Research 201
BACTERIA AND VIRUSES
Introduction
Most viral and bacterial respiratory infections are trans-
mitted among human hosts. This may occur by touching an
infected person or an object they have infected or by inhaling
contaminated airborne droplets expelled from the nose and
mouth during sneezing, coughing, and talking. Most of these
airborne droplets are large enough to fall to the ground within a
meter. Smaller" droplets quickly shrink through desiccation to
form "droplet nuclei," which are small enough (between 0.5
and 5 gm in diameter’) to remain suspended in the air for" long
periods (LaForce 1986). Droplets within this range are of the
greatest important when considering health effects since they
are small enough to penetrate deep into the lungs. If these drop-
lets contain viable infectious organisms and are in sufficient
number’s, they will cause infection in susceptible hosts. Rela-
tive humidity can affect the desiccation process of droplets,
which, in turn, affects droplet size and the viability and infec-
tivity of the airborne pathogens (Burge et al. 1991). For’ more
information concerning specific types of infectious disease, see
Burge (1989, 1995).
Airborne Viability
Most of the information concerning the viability of
airborne pathogens has been determined through in vitro
studies. Evidence from these studies suggests that the humidity
level has complex effects on the viability and virulence of
airborne pathogens that vary from organism to organism, while
the effect of temperature is not significant within the range of
interest for’ conditioned environments. In many cases certain
bacteria were found to have a window of relative humidity at
which they died more quickly (Anderson et al, 1968; Cox 1966;
Brundrett 1990). For’ example, Cox (1966) found that Escheri-
chia coli strain jepp had minimal viability in the range of 70% to
80% RH and that the viability increased at RH values above and
below this window. The results of a study by Hambleton et al.
(1983) suggest that Legionella pneumophila has minimal
viability at two humidity levels---between .50% and 60% RH and
at 30% RH. Hambleton et al. also demonstrated that at the
optimum humidity of 65% RH, only about 20% of the cells are
viable after’ one hour; Experiments on the bacterium Pneu-
moccus suggest a shatp decrease in viability in a narrow band at
.50% RH (Brundrett 1990). In a study of the survival of Strepto-
coccus, Flynn et al. (1971) found that the change in the viable
count was insignificant for the RH range of 0% to 92%.
hz vitro studies of viruses suggest that particular strains,
including mengovims 37A, polio virus, foot-and-mouth
disease virus, and encophalomyocarditis virus, are unstable as
aerosols in atmospheres below about 70% RH (Cox 1989).
contrast to these viruses, other’ strains, including vesicular
stomatitis, vaccinia, and influenza, are least stable at high RH.
Cox suggests that this difference can be attributed to specific
structural differences in the viruses~ Mbithi et al. (1991) studied
the survival of hepatitis A virus on surfaces at RH levels of
25%, 55%, 80%, and 95% and found that the survival of the
virus was inversely proportional to the level of RH and temper-
ature.
Expelled droplets and skin flakes that settle out may
survive in dust and transmit disease if re-entrained when
surfaces are disturbed. There is evidence that outbreaks of
bacterial infections in hospitals have been associated with
cleaning processes (Brundrett 1990). Studies of viability
bacteria in dust suggest that there is a trend of decreasing
viability with increasing relative humidity (Brundrett 1990).
addition, dust is less likely to become airborne at higher" humid-
ities.Field studies of airborne pathogen survival at high humid-
ities at’e limited. Studies at lower humidities suggest a higher’
survival rate for airborne viruses at humidities below 30%
(Green 1985). In general, considering the results from in vitro
studies, there is little evidence to suggest that for humidities in
the upper range (> 50% RH) one specific level is better than
any other’ in reducing the viability or number of suspended
microorganisms~
Building-Related Sources
A few microbes, pathogenic to humans, are able to
flourish in nonhuman environments. These can be introduced
into building systems from outside sources and proliferate if
conditions are favorable. The most important example of such
a contaminant is Legionella, which can lead to fatal pneumonia
in susceptible hosts. Aside from Legionnaire’s disease, no
specific infections have been documented to be of great clinical
importance in commercial buildings (Hodgson 1989).
Hypersensitivity pneumonitis has been directly associated
with microorganisms (pat’ticularly thermophilic actino-
mycetes) cultured fi’om poorly maintained humidification and
air-conditioning systems (Hodges et al. 1974; Fink et al. 1976;
Burge et al. 1980). Patients often report a relief of symptoms
upon avoidance of the environment containing the offending
contaminant (LaForce 1986). Outbreaks of bacterial infections
in hospitals and flare-ups of asthma have also been associated
with humidification systems (Covelli et al. 1973; Airoldi et al.
1972; Smith et al. 1977; Solomon 1974; Bencko et al. 1993). In
all these cases the offending humidifiers have been of the spray
or mist types that form aerosols in the airstream.
Use of direct evaporative cooling is a potential concern
because poor’ maintenance, which is not uncommon in residen-
tial systems, can result in microbial growth within sump water
(Macher and Girman 1990; Stetzenbach et al. 1990; Macher
1994). However, it is not apparent that this could lead to an
outbreak of disease. One study of homes in the Las Vegas area
cooled by direct evaporative systems traced the presence of
gram-negative bacteria to a fouled sump in one of the homes,
although none of the dweller’s was infected (Stetzenbach et aL
1990). In a laboratory study using tracer’ organisms, Macher
(1994) found "minimal transfer" from the fouled sump into the
air under normal operating conditions. Conversely, direct evap-
orative cooling may help to reduce human-to-human spread of
infectious disease because of the relatively high supply rate of
202 ASH RAE Transactions: Research
outside air required. Increased ventilation has been shown to
lead to decreased rates of viral respiratory infections (Burge et
al. 1991) and is often a recommended means of reducing
indoor air contaminants (ASHRAE 1989).
NONBIOLOGICAL AIR POLLUTANTS
Formaldehyde
Formaldehyde is found in numerous building materials,
including plywood, particleboard, and other pressed wood
products; fi~rnishings; carpets; and textiles. It is also a ma.jor
component of urea-formaldehyde foam insulation, which is
now banned in the U.S. Formaldehyde is highly water soluble
and causes irritation of the mucous membranes within the eyes
and upper respiratory tract at concentrations starting at 0.1 ppm
but is most frequently reported at or above 1 ppm (Berglund et
al. 1992). Formaldehyde is also classified by the U.S. Environ~
mental Protection Agency (EPA) as a probable carcinogen.
Both ASHRAE and the World Health Organization (WHO)
have set maximum guidelines of 0.1 mg/m
3 to ensure suffi-
ciently low formaldehyde concentrations in indoor air
(Puhakka and Karkkainen 1993; Gammage 1990). The effect
of these standards, along with the ban on urea-formaldehyde
foam insulation, has been an overall decrease in indoor formal-
dehyde concentrations within the last decade (Marbury and
Krieger 1991).
The rate of formaldehyde offgassing from pressed-wood
products decreases exponentially with age and is sensitive to a
number of factors, including the initial properties of the mate-
rial, temperature, and humidity (Gammage 1990; Meyer 1986).
Laboratory and field studies agree that temperature is the most
significant environmental effect (van Netten et al. 1989;
Godish and Rouch 1986; Arundel et al. 1992). In general, since
formaldehyde within binding resins is water soluble, higher
humidity levels also tend to increase the offgassing rate. In a
controlled environment study within mobile homes, a decrease
in humidity from 70% to 30% resulted in an approximate 40%
reduction of formaldehyde levels (Godish and Rouch 1986).
a study of formaldehyde offgassing from chipboard within a
controlled chamber, Anderson et al. (1975) found that the
formaldehyde concentration within the air was directly propor-
tional to temperature and air humidity. A change in relative
humidity from 30% to 70% doubled the equilibrium formalde-
hyde concentration, while a 7°C rise in temperature in the
range of 14°C to 35°C caused the formaldehyde concentration
to double. In a study of 20 homes referred to by Arundel et al.
(1992), a significant correlation was found between the indoor
relative humidity and the formaldehyde concentration in the
air.
Seasonal fluctuations have also been observed, with the
highest rates of offgassing occurring in summer months, when
marked increases in temperature, solar gains (which can cause
localized increases in wall temperature), and humidity occur
(Marbury and Krieger 1991). Puhakka et al. (1993) studied
apartments in Finland and found that the concentration of
formaldehyde in the air increased from 0.08 mg/m
3 to 0.20 mg/
m
3 when the relative humidity increased from 34% to 70%
during a period of 24 hours. In addition, they found that short-
term increases in relative humidity within a period of one to
five hours, as occur when using a sauna or drying clothes, also
caused increases in formaldehyde levels. The offgassing rate of
chipboard within a chamber was also examined in this study
and it was found that sealing chipboard on all sides with "reac-
tive paint" significantly reduced offgassing rates.
Ozone
Ozone is well recognized as a respiratory irritant and a
significant problem in urban areas of southern California,
where the EPA standard for outdoor ozone concentration is
often exceeded. Ozone is formed as a result of reactions
between photochemically reactive hydrocarbons and oxides of
nitrogen under the influence of sunlight. Because of its strongly
oxidizing properties and low water solubility, ozone can pene-
trate deep into the alveoli of the lungs and affect lung function.
It is also an irritant to the mucous membranes of the eyes.
The primary source of ozone indoors is infiltration/venti-
lation of polluted outdoor air. Indoor sources of ozone, such as
photocopiers and air-cleaning equipment, exist; however,
under normal living and working conditions there is no
evidence that these would reach levels high enough to be of
concern. Once indoors, ozone decomposes through heteroge-
neous reaction with indoor surfaces. Mueller et al. (1973) esti-
mate the half-life of ozone in a typical bedroom to be less than
6 minutes, while Weschler et al. (1991) suggest a half-life esti-
mate of 11.1 minutes for a typical office environment with a
surface-to-volume ratio of 2.9 m
-1. Using this general knowl-
edge, authorities have typically advised the public to remain
indoors and reduce infiltration as much as possible during
episodes of high outdoor ozone levels. However, this recom-
mendation is not usefifl for those buildings that depend on
natural ventilation or evaporative cooling as a means of
cooling. This condition is made more problematic by the fact
that ozone episodes often correspond with high outdoor
temperatures.
In the study by Weschler et al. (1991), ozone concentra-
tions were measured for three office buildings with different
ventilation rates. The indoor values closely tracked those of the
outdoor values and, depending on the ventilation rate, were
20% to 80% of those outdoors. Weschler et al. also cite a
number of previous studies in which the concentration of ozone
in the indoor environment was measured to be 10% to 80% of
outdoor concentrations. Considering this information, along
with the fact that people spend more than 90% of their time
indoors, Weschler suggests that exposure (concentration
time) to ozone indoors is a more significant issue than outdoor
exposures.
There is little information regarding the effect of humidity
on ambient ozone concentrations within indoor environments.
One controlled-chamber study found that the rate of ozone
decay increased as either temperature or humidity was
increased (Mueller et al. 1973). However, in the case of direct
ASHRAE Transactions: Research 203
evaporative cooling, this may not be a significant effect
compared to that of the higher ventilation rate. The impact of
direct evaporative cooling vs. refrigerated air conditioning was
studied by Stock et al. (1993) in homes in El Paso and Houston,
Texas. They found that the average indoor/outdoor ratio of
ozone concentrations was much higher in homes with evapora-
tive cooling compared to those with refrigerated air-condi-
tioning systems (0.7 vs. 0.1).
The most promising remediation method available is the
use of activated carbon filters, which have been shown to be
successful in significantly reducing ozone levels (Mueller et al.
1973; Weschler et al. 1991). It may be possible that these could
be incorporated into evaporative cooler’s. However, such factors
as engineering practicality, cost, efficiency, and service life
would need to be tested under actual conditions to determine if
this option is viable.
Nitrogen and Sulfur Oxides
As with ozone, nitrogen and sulfur’ oxides are produced
primarily from outdoor sources~ However, nitrogen dioxide
and nitrous and nitric acids are also combustion by-products of
gas cooking stoves and heaters and can accumulate indoors as
a result of improper ventilation. Nitrogen and sulfur oxides
react with water on indoor surfaces to form acid aerosols,
which are generally found in higher concentrations indoors
(Leaderer et al. 1993). Although nitrogen and sulfur oxides are
common pollutants, surprisingly little work has been done thus
far to determine the respiratory toxicity of their acid aerosols.
Their acidic nature, reactivity, and aqueous solubility, however,
suggest that respiratory damage is possible (Brauer et al. 1993)
and increased indoor humidity does seem to increase the heter-
ogeneous reaction on surfaces. In one chamber study it was
found that at the highest relative humidity tested (80%), nitrous
acid (HONO) concentrations were approximately 8% of the
observed NO
2 level, while 30% and 4.5% relative humidities
resulted in HONO/NO
2 ratios of 0.9% and 2o7%, respectively
(Brauer et al. 1993). In te~xns of direct evaporative cooling,
again the issue of high ventilation rates may be a significant
factor to consider if outdoor air levels of nitrogen and sulfur
oxides are high.
DISCUSSION
To the maximum extent possible, building standards
should reflect the knowledge available when they are written.
The subject of humidity limits is a complex one and many
questions remain at this time. Nonetheless, there is a wide
range of research under way through numerous disciplines,
much of which can inform the preparation of standards. This
paper summarizes this research and suggests a format for
putting new information into a building standards context.
A paper’ by Sterling et al. (1985) also addresses the topic
humidity and health in buildings and is the only cited reference
pertinent to humidity in ASHRAE Standard 62-1989. This
paper includes a figure that has received wide circulation
within the HVAC engineering profession. It graphically depicts
humidity impact zones using bars that decrease in width,
suggesting a decrease in the effect for each of the eight environ-
mental health factors addressed. These bars converge for all of
the eight categories into a nat~’ow recommended "optimum"
zone between 40% and 60% RH; both low- and high-humidity
effects are addressed. There is clearly a need for’ such a
summary because it has appeared in numerous journals and
conferences (Arundel et al. 1986, 1992). It is also arrestingly
drawn and easy to grasp, which adds to its appeal. This figure is
the basis of the recommendation in ASHRAE Standard 62-
1989 that humidity in the occupied space should be between
30% and 60% RH.
There are issues that may be raised with a figure that
attempts to combine the influences of so many factors. One
concern is that it does not assign relative weights (severities)
for the different health factors. Another is that the practicality
of the recommended humidity limits in various climates and
building types is not assessed. In practice, these issues need to
be addressed. As an example, ASHRAE Standard 62 ove~’ides
(without explanation) the recommended lower limit of 40%,
lowering it to 30% although, based on the figure, the impact of
this action is not much different than that of raising the upper
limit from 60% to 70%. This may have represented a value
judgment by ASHRAE about the relative severity of the
different health effects--differences that are not expressed in
the figure. Conversely, the change may have been forced by
practical reality in conditioning the indoor environments of
buildings.
In defense of the figure, it is ultimately necessary for
designers to make decisions combining disparate elements
even without complete justification. Someone needs to draw a
line in the sand. The figure does this for them.
A more severe criticism is based on the factual substantia-
tion of the health impact zones presented. As the figure is
drawn and captioned, these zones imply linear relationships
between humidity and health effects that are not supported by
the literature. In addition, the focus on ambient RH to the
exclusion of other’ environmental conditions is misleading in
that it suggests that RH is the controlling environmental factor
for all of the pollutants listed without regard to climate and the
conditions of the building system.
In addition, the specific points at which these zones begin
and end are not consistently supported by the references
provided. The recommendations for bacteria (RH below 60%),
viruses (RH below 70%), and fungi (RH below 60%) are
supported by the discussion or the references given in the orig-
inal paper or its more recent versions. The literature cited for
the mite recommendation (below 50% RH) is incomplete, and
seasonal effects, which may be a significant factor, are not
addressedl The limits for allergic rhinitis are presumably based
on the potential for mites and fungal growth (for upper
humidity limits), although most of the discussion concerns
problems of mist humidifiers and low humidities. The chem-
ical interactions category (RH below 30%) appears to be based
entirely on two studies of formaldehyde offgassing rates (the
204 ASHRAE Transactions: Research
only other chemical interaction mentioned in the text is the
conversion of sulfur and nitrogen dioxides to acids, for which
no specific limits were cited). In light of the recent reductions
in the formaldehyde levels within materials, this limit may be
outdated. The use of the term "ozone production" as a pollutant
category is confusing and possibly based on the incorrect
assumption that ozone production from office lighting and
equipment is a significant pollutant source.
For the purpose of setting humidity standards, the figure is
clearly inadequate. To promote good building design it is
important to identify the specific physical causes and solutions
to health hazards and to regulate design practice to avoid them.
Table 1 summarizes the results of this review. It addresses
the humidity requirements, the actual site of contamination,
and the means of control for each of the biological pollutants.
CONCLUSIONS
¯Most of the identified biological health agents grow on the
surfaces of the building, its systems, and its furnishings, or
in standing water within or outside the building. None of the
agents grows in the air of the occupied space or the mechan-
ical system. Their growth is therefore only indirectly related
to the atmospheric humidity measured in the occupied
space or the ducts of its mechanical system. To control
these, one needs to ensure that the surfaces remain dry.
There are a number of ways to achieve this in the design,
furnishing, and operation of buildings. It is also necessary to
avoid producing aerosols of water from the mechanical
system or humidifiers. How this is done is independent of
the level of indoor air humidity.
¯The single exception to this is dust mite contamination
(particularly D. farinae), which appears to be directly
related to ambient RH. Controlled laboratory studies
suggest that optimal conditions for growth are 70% to 80%
RH at 25°C. A number of field studies have found mite
contamination in residences with ambient RHs as low as
50%. Rather than ambient RH, however, the more rele-
vant factor may be the RH within the microhabitat of the
mite (within a few millimeters of the horizontal surfaces
on which they lie). In the laboratory, surface and air
temperatures can be controlled to provide equilibrium
conditions; however, in actual environments equilibrium
conditions seldom exist. In one field study, the RH within
carpets was found to be more than 9% higher than the
space RH, suggesting lower temperatures or local mois-
ture production that may benefit mites. This difference
between surface and ambient RH may possibly explain
the discrepancy between the laboratory and field find-
ings. Remedial methods are designed to address contam-
ination at the source. For example, one of the most widely
recommended remedial methods is to encase mattresses
with a semipermeable polyurethane cover. This discour-
ages moisture from getting into the bedding where the
mites live. Researchers have also suggested that electric
blankets, which can lower the RH within bedding, are
effective in reducing mite levels. In terms of seasonal
effects, it is not yet known whether use of evaporative
cooling a few months out of the year, sometimes only
during the day, leads to increased mites. In general, since
mite contamination occurs primarily in residences and
affects only a subset of the population, it may be that,
when necessary, they should be controlled by other
means such as cleaning and covering bedding, treatment
or removal of carpets, and insulation of cooled floor slabs
under carpets.
¯Fungal contamination occurs primarily as a result of
condensation on surfaces and/or water damage. Field and
laboratory studies suggest that fungal growth does not
become an issue below 70% or even 80% RH unless there
are other factors influencing their growth on building
surfaces. Studies that reported problems at lower RH values
appeared to have problems that could be corrected other-
wise. In setting a maximum limit to air humidity in the
space, there is little, if any, evidence from field studies that
provides a reason for distinguishing 60% relative humidity
from 70%.
¯The health impacts of nonbiological health agents are hard
to assess at this time. Formaldehyde generation is exacer-
bated in some materials by higher humidity. Because of the
greater awareness of the adverse health effects, new build-
ing products and furnishings generate far less formaldehyde
than before. The effect of this change will need to be eval-
uated. For a given ventilation rate, indoor ozone concentra-
tions may decrease as humidity increases due to an
increased rate of surface reactions. However, at the high
ventilation rates associated with direct evaporative cooling,
the level of ozone will not be significantly offset by the
higher humidity levels. Oxides of nitrogen and sulfur are
primarily of outdoor origin, but the severity of their effects
on health may increase with higher humidity levels. At this
point there is little evidence in the literature to suggest that
this is a significant health effect.
The other significant source of biological health agents is
humans harboring infectious diseases. This source (prima-
rily the respiratory tract but also the skin) is largely inde-
pendent of the humidity level in the space. However, the
spread of infectious disease agents depends somewhat on
atmospheric humidity in the space, in that aerosol evapora-
tion rates and deposition rates may affect viability of anti-
gens, bacteria, and viruses enclosed in water droplets.
Space humidity may also affect the settling rate of dust
particles to which bacteria are attached. The viability of
these dustborne organisms also varies with humidity, with
viability optima occurring throughout the range of RH. For
each of the above considerations, there is little evidence to
suggest that any humidity between 50% and 90% is signif-
icantly better than any other in reducing the viability or
number of suspended infectious disease organisms, as well
as the susceptibility of the human receptor.
ASHRAE Transactions: Research 205
o Direct evaporative cooling through porous media appears to
be benign in that field and laboratory studies suggest that
biological organisms in the cooling water appear not to be
aerosolized or transmitted downstream. The wet pads may
have benefits over dry filters in removing incoming pollut-
ants. However, this needs to be experimentally investigated.
In addition, the once-through ventilation requh’ed by such
systems should act to dissipate the concentration of infec-
tious organisms in the ai~; since such organisms are almost
always internally generated. This process also needs to be
systematically evaluated.
¯Finally, very little of the literature on health effects is
expressed in terms of risk to the occupant: first, the likeli-
hood of humidity-influenced pollutants occur~’ing in the
building and then the likelihood of the pollutant affecting
the occupant.
ACKNOWLEDGMENTS
The authors would like to thank Alison Kwok, a Ph.D.
candidate at the University of California at Berkeley, for help
with the literature search. We also wish to thank William Fisk
of the Indoor’ Environment Program at Lawrence Berkeley
Laboratory and Professor Wayne Wilcox of the University of
California Forest Products Laboratory for their detailed
reviews of the manuscript. Tile research reported here was
funded by the California Institute for Energy Efficiency
(CIEE), a research unit of the University of California. Publi-
cation of the research does not imply CIEE endorsement of or
agreement with these findings, nor that of any CIEE sponsor.
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206 ASHRAE Transactions: Research
TABLE 1 BIOLOGICAL POLLUTANTS IN INDOOR AiR
Summary of Environmental Requirements and Recommended Remediation Procedures
Health Implications Environmental Common Sites of Frequently Recommended
Requirements Contamination Remediation Procedures
Dust
Mites Allergic reactions: asthma,
rhinitis, dermatitis
Dust mite allergens: mite
body parts and fecal matter
are the most common source
of allergic reactions within
house dust
Species most commonly
associated with disease:
Dermatophagoides
Do pteronyssinus- more
common in humid
regions
D. farinae - more
common in relatively
dry regions
Euroglyphus - gener-
ally rare, found only
in humid regions
Source of nutrients: shed ¯ Mattresses/bedding ¯
human and animal skin ¯ Thick carpeting
scales ¯ Heavily used upholstered "
Source of water: furniture
¯ Uptake water vapor (Dust mites are more ¯
directly from air commonly associated with
optimal for growth
Critical relative humidity:
55% at 15°C to 75% at 35°C
(For more detailed informa-
tion on the humidity require-
ments for mites see Arlian,
1992)
RH of 70-80% at 25°C is residences rather than
commercial buildings)
Removal of contaminated carpet-
ing, bedding, and/or furniture
Frequent hot-water cleaning of
bedding
Encasement of mattresses with
semi-permeable vinyl casing
Special vacuuming procedures
(e.g., HEPA filters, central
vacuum system with outside
equipment)
Surface treatments (e.g., benzyl
benzoate)
Reduction of ambient humidity or
specifically within the microhabi-
tat of mites (e.g, through the use of
electric blankets, radiant heating
of carpeted floor surfaces, etc.)
Fungi Allergic reactions: asthma, Source of nutrients -
rhinitis, dermatitis (most organic debris, dirt, organic
common: Alternaria, building materials
Aspergillus, Cladosporium,
Sourceofwater:moisture
and Penicillium) on and within surfaces
Hypersensitivity Pneu- ¯ Specific water require-
monitis (e.g., Cladospo- ment varies from species
rium, Sepula) to species.
Mycotoxicosis ¯ Growth of xerophilic
(e.g., Aspergillus, Penicil- species such as Aspergil-
lus is likely to begin at an
lium, Stachybotrys atra, ERH of 75% to 80%.
Trichoderma virde ) (For more detailed informa-
Infectious disease tion on humidity require-
(e.g.,Aspergillusfi~migams, ments see Flannigan 1992
Cryptococcus) and IEA Annex 14 guide-
lines.)
Moisture damaged build- ¯
ing materials (walls,
carpeting, books, etc.)
Within fiberglass duct ¯
lining in which condensa-
tion has occurred ¯
On wall surfaces with
high ERH or on which
condensation has
occurred
Within poorly main-
tained conditioning o
systems containing water
(e.g., humidifiers, cool-
ing coil drip pans)
Removal of damaged material
where possible (i.e. carpeting, duct
liners, wallpaper, etc.)
Cleaning of water resistant materi-
als with chlorine bleach
Proper maintenance and operation
of conditioning systems (eog.,
cleaning and disinfecting cooling
coils and drain pans, continual
operation of forced air systems to
avoid condensation)
Proper construction techniques to
avoid water damage (e.g., proper
placement of vapor barriers to
avoid condensation within walls,
design of drainage systems to
avoid flooding and water incur-
sion)
Adequate ventilation to reduce
internal humidity loads
Bacteria
and
Viruses
Infectious disease: person- ¯
to-person (e.g., common
cold, flu, measles, TB, etco)
Infectious disease: build-
ing related (e.g., Legionella
pneumophila)
Hypersensitivity pneu-
monltis (most commonly
associated with thermo.
philic actinontycetes)
Virulent bacteria and ¯
viruses are transferred to ¯
hosts through droplet
nuclei expelled when
coughing/sneezing or
formed within contami-
nated building systems
that form aerosols
¯Bacteria and viruses that
grow outside of the
human host require liquid
water for growth
(For detailed information,
see Cox 1989.)
Infected humans Person-to-person spread of
Improperly maintained airborne infection:
building systems that ¯ Isolation of contaminated persons
have the potential to form ¯ Increase fresh air exchange rate
aerosols: (e.g., spray-type
humidifies, cooling Growth within building systems:
towers etco) ¯ Remove source of contamination
(e.g., replace system, biocide treat-
ment, etco)
¯ Routine cleaning of water systems
and/or filters
¯ Relocate intake vents (in the case
of cooling tower contamination)
ASHRAE Transactions: Research 207
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