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There is a long-standing dispute about indoor air humidity and perceived indoor air quality (IAQ) and associated health effects. Complaints about sensory irritation in eyes and upper airways are generally among top-two symptoms together with the perception "dry air" in office environments. This calls for an integrated analysis of indoor air humidity and eye and airway health effects. This overview has reviewed the literature about the effects of extended exposure to low humidity on perceived IAQ, sensory irritation symptoms in eyes and airways, work performance, sleep quality, virus survival, and voice disruption. Elevation of the indoor air humidity may positively impact perceived IAQ, eye symptomatology, and possibly work performance in the office environment; however, mice inhalation studies do not show exacerbation of sensory irritation in the airways by low humidity. Elevated humidified indoor air appears to reduce nasal symptoms in patients suffering from obstructive apnea syndrome, while no clear improvement on voice production has been identified, except for those with vocal fatigue. Both low and high RH, and perhaps even better absolute humidity (water vapor), favors transmission and survival of influenza virus in many studies, but the relationship between temperature, humidity, and the virus and aerosol dynamics is complex, which in the end depends on the individual virus type and its physical/chemical properties. Dry and humid air perception continues to be reported in offices and in residential areas, despite the IAQ parameter "dry air" (or "wet/humid air") is semantically misleading, because a sensory organ for humidity is non-existing in humans. This IAQ parameter appears to reflect different perceptions among other odor, dustiness, and possibly exacerbated by desiccation effect of low air humidity. It is salient to distinguish between indoor air humidity (relative or absolute) near the breathing and ocular zone and phenomena caused by moisture-damage of the building construction and emissions therefrom. Further, residential versus public environments should be considered as separate entities with different characteristics and demands of humidity. Research is needed about particle, bacteria and virus dynamics indoors for improvement of quality of life and with more focus on the impact of absolute humidity. "Dry (or wet) air" should be redefined to become a meaningful IAQ descriptor.
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International Journal of Hygiene and
Environmental Health
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Indoor air humidity, air quality, and health An overview
Peder Wolko
National Research Centre for the Working Environment, NRCWE, Lersø Parkallé 105, Copenhagen Ø, Denmark
Air quality
Inuenza virus
Sensory irritation
Sleep quality
There is a long-standing dispute about indoor air humidity and perceived indoor air quality (IAQ) and associated
health eects. Complaints about sensory irritation in eyes and upper airways are generally among top-two
symptoms together with the perception dry airin oce environments. This calls for an integrated analysis of
indoor air humidity and eye and airway health eects. This overview has reviewed the literature about the
eects of extended exposure to low humidity on perceived IAQ, sensory irritation symptoms in eyes and airways,
work performance, sleep quality, virus survival, and voice disruption. Elevation of the indoor air humidity may
positively impact perceived IAQ, eye symptomatology, and possibly work performance in the oce environ-
ment; however, mice inhalation studies do not show exacerbation of sensory irritation in the airways by low
humidity. Elevated humidied indoor air appears to reduce nasal symptoms in patients suering from ob-
structive apnea syndrome, while no clear improvement on voice production has been identied, except for those
with vocal fatigue. Both low and high RH, and perhaps even better absolute humidity (water vapor), favors
transmission and survival of inuenza virus in many studies, but the relationship between temperature, hu-
midity, and the virus and aerosol dynamics is complex, which in the end depends on the individual virus type
and its physical/chemical properties. Dry and humid air perception continues to be reported in oces and in
residential areas, despite the IAQ parameter dry air(or wet/humid air) is semantically misleading, because a
sensory organ for humidity is non-existing in humans. This IAQ parameter appears to reect dierent percep-
tions among other odor, dustiness, and possibly exacerbated by desiccation eect of low air humidity.
It is salient to distinguish between indoor air humidity (relative or absolute) near the breathing and ocular
zone and phenomena caused by moisture-damage of the building construction and emissions therefrom. Further,
residential versus public environments should be considered as separate entities with dierent characteristics
and demands of humidity. Research is needed about particle, bacteria and virus dynamics indoors for im-
provement of quality of life and with more focus on the impact of absolute humidity. Dry (or wet) airshould be
redened to become a meaningful IAQ descriptor.
1. Introduction
Yaglou (1937) concluded that Articial humidication, about
which so much is heard on connection with winter air conditioning,
was shown in the rst part of this paper to be relatively unimportant
from the standpoint of comfort and, so far is known, not essential from
the standpoint of health. While a relative humidity of between 40 and
60 percent would probably be more normal and perhaps more healthful
than between 20 and 30 percent, it is practically impossible to maintain
this high range in cold weather because excessive condensation and
freezing on the windows and sometimes inside the exposed walls.
Indoor air humidity, in terms of perceived dry air (dryness) and
potentially associated health eects is an important parameter (relative
(RH) or absolute (AH)) both in the aircraft and oce environment. A
long-standing dispute continues about the health relevance of RH and
the cause(s) of perceived dry air, a very common and abundant
complaint about perceived indoor air quality (IAQ)inoce-like en-
vironments. Further to this, causation of perceived sensory reactions in
eyes and upper airways, among top-two reported symptoms in oces,
continue to be a puzzle, despite several identied risk factors that in-
uence the development of eye symptoms have been identied
(Wolko, 2017); the risks of symptoms in the upper airways remain
largely unexplained. Furthermore, there is an increasing recognition of
the impact of humidity, e.g. on virus survival and transmission and
sleep quality, regarding derivation of a safe limit for indoor air hu-
midity (Derby et al., 2016).
Nagda and Hodgson (2001) reviewed the indoor air literature and
concluded that slightly elevated RH would have a benecial eect on
perceived IAQ; in part based on the conclusion that experimental out-
comes appeared to be strongly dependent upon the experimental
Received 17 October 2017; Received in revised form 28 December 2017; Accepted 29 January 2018
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International Journal of Hygiene and Environmental Health 221 (2018) 376–390
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design. A conclusion that was further supported by Wolkoand
Kjærgaard (2007). It was unclear, however, whether the conclusion
reached by Nagda and Hodgson (2001) would include typical sensory
and CNS-related symptoms as commonly encountered in oce en-
vironments. Contrary to this, the pollutant hypothesis was reintroduced
partially based on short-term assessments of the emissions from
building materials (Fang et al., 1999) and by Sundell and Lindvall
(1993) and Fang et al. (2004). These authors concluded that indoor air
pollutants, like volatile organic compounds (VOCs), were the most
likely cause of reported dry air by exposure to sensory irritants. Fur-
thermore, it was concluded that high RH as well as high temperature
were detrimental to the immediately perceived IAQ (a snapshot of
perception) by sningthe emission from building materials (Fang
et al., 1998). In that context, Fanger (2000) concluded that IAQ should
be perceived as dry and coolin oce environments, i.e. low RH and
not too warm. Sun et al. (2009) and Qian et al. (2016) further ad-
vocated this stating that the perception of dry air is more likely related
to sensory irritants, despite lack of a clear scientic rationale.
Thus, there is a need for a balanced and integrated analysis of the
impact of indoor air humidity on associated health eects as opposed to
the well-known problems associated with moisture-damaged buildings
(World Health Organization, 2009). As pointed out the relationship
between health, indoor air humidity and pollution is complex and re-
mains a challenge (Davis et al., 2016a;Derby et al., 2016). Thus, the
focus of this overview is eects in the public domain of perceived IAQ,
sensory irritation in eyes and airways, work performance, infection by
virus, sleep quality, and the voice.
2. Method
This overview integrates and analyzes studies about how ex-
tendedexposure to low relative (absolute) humidity impacts health,
IAQ, work performance, sensory eects in the eyes and airways (sen-
sory symptoms), transmission and survival of inuenza virus, sleep
quality, and the vocal cord. Searches in PubMed and Google Scholar
were carried out for humidityin combination with: airways,
asthma,eyes,indoor air quality,particles,pungency,
mucociliary clearance,sensory irritation,throat irritation,ocular
surface,sleep quality, and voice or vocal cords, and combined
with own selection of literature compiled during the last decade up to
September 2017, cf. Arundel et al. (1986),Derby et al. (2016),Nagda
and Hodgson (2001),Wolkoand Kjærgaard (2007). Health eects of
moisture damage (dampness) of construction products and mold-re-
lated issues (e.g. microbiological contaminants) and dust mites are
excluded from this overview, cf. Hurraß et al. (2017) and World Health
Organization (2009).
2.1. Absolute and relative humidity denition
Humidity is usually measured by a hygrometer and reported as re-
lative % water vapor in the room air relative to the total amount of
vapor in the same room air may contain at given temperature; absolute
humidity amounts the water in grams per kg of air (g/kg) at a dened
pressure. Thus, indoor AH appears to correlate better with outdoor AH
than outdoor and indoor RH in some regions, but not in others, in part
depending on season, building style, and ventilation (Nguyen et al.,
2014;Zhang and Yoshino, 2010).
3. Results of overview
3.1. Oces symptoms, perception of IAQ (VOCs, particles), and work
3.1.1. Symptoms in oces
Irritation in eyes and upper airways are among top-two reported
symptoms in oce questionnaire studies (Brightman et al., 2008;
Bluyssen et al., 2016;Wolko, 2013). There are minor dierences be-
tween the studies which in part reect the recall period, usually one
symptom per week during the last four weeks. The reported prevalence
is generally from 20 to 40%.
Several questionnaire studies in oces have shown associations
between low RH (530%) and increased prevalence of complaints about
perceived dry and stuy air and sensory irritation of the eyes and upper
airways, see Table 1. However, many intervention studies have shown
Table 1
Studies in oce environments, homes, schools, and hospitals.
Study Observation
Angelon-Gaetz et al. (2016)
Teachers (n = 122) reported daily symptoms in 412 weeks diaries. Modest, but not signicant, increase in respiratory (asthma-like)
symptoms over 5 days, both at low (< 30%) and high RH (> 50%) in
comparison to referent teachers (3050% RH). No eects of RH on
cold/allergy symptoms.
Azuma et al. (2015, 2017) Oces - Workers (n = 3335) in 320 oces responded to questionnaire. Both studies showed strong correlation between perceived air dryness
and report of eye irritation.
- Workers (n = 3024) in 489 oces responded to questionnaire. General symptoms were also associated with perceived humidity in
the summer season.
Bakke et al. (2007,2008) Oces Four university buildings, 2 complaint and 2 control buildings.
Questionnaire and examination of the precorneal tear lm (PTF)
stability, nasal patency and inammatory markers in nasal lavage
uid in university stamembers.
Stuy or dry air was signicantly associated with low RH (1535%
RH). Otherwise no signicant exposure dierences between complaint
and control buildings and no signicant dierence in objective
signs.PTF stability (NBUT/SBUT) was improved at higher RH and
perception of air dryness was reduced.
Brasche et al. (2005) Oces Data from oce workers (n = 814) No clear conclusion about RH and reported eye symptoms or PTF
stability. Indication that high RH might be protective, and particles
associated with epithelial damage of the PTF.
Hashiguchi et al. (2008) Hospital Temperature and RH measured for 3 months in hospital in
sickrooms and wardens during winter. Symptoms and comfort was
reported once a week by sta(n = 45) and patients (n = 36).
Humidiers were installed after 2 months in half of the rooms.
Humidication from 33 to 44% RH, on average, resulted in decrease
of thermal discomfort and perceived air dryness among the sta, but
not among the patients.
Lindgren et al. (2007) Aircraft Cabin attendants (n = 58) and pilots (n = 22).
Double blinded 310% increase of RH by ceramic humidier during
long-haul ights.
Signicantly lower concentration of respirable particles at elevated
RH from 6 to 1 μg/m
; similar observation for mold and bacteria.
Cabin air quality signicantly improved at elevated RH by being
perceived less dry and fresher.
(continued on next page)
P. WolkoInternational Journal of Hygiene and Environmental Health 221 (2018) 376–390
that an increase of RH may alleviate the perception of dry air and
symptoms of dry eyes and upper airways, i.e. the longer-term IAQ re-
duces the symptom reporting; for example, see Hashiguchi et al. (2008)
and other intervention studies shown in Table 1.
3.1.2. Perceived air quality
Perceived IAQ is an umbrella of reported descriptors like tempera-
ture, draft, odor/smell, stuy air, and dry and wet (humid) air, and
where dry airis a common and among the most abundant, e.g.
Bluyssen et al. (2016). The perception of dry aircan be associated
with mucous membrane irritation of the eyes (e.g. dry eyes) and upper
airways in presence of strong sensory irritants (Doty et al., 2004), which
is an important component included in the classic sick-building syn-
dromein non-industrialized buildings. For further discussion of the
semantic validity of this terminology, see Wolko(2013).
There is some inconsistency about the perceived IAQ by humidi-
cation. Thus, the immediately (snapshot) perceived IAQ, not to be
confused with symptoms, appears more acceptable in laboratory set-
tings at low RH and low temperature from the assessment of VOC
emission from building materials (Fang et al., 1998). However, the
thermodynamic condition (i.e. the inuence of temperature and RH
(Zhou et al., 2017)) and altered VOC emission proles inuences the
Table 1 (continued)
Study Observation
Lukcso et al. (2016) Oces Oce workers (n = 7637; response rate 49%) in 12 buildings.
Subset wore personal sampling equipment and underwent medical
examination. Symptoms experienced over the last 4 weeks.
Low RH was signicantly associated with lower respiratory and sick-
building syndrome-type symptoms, thus suggesting that low RH may
exacerbate upper and lower airway symptoms.
Nordström et al. (1994) Hospital Blinded steam air humidication to 4045% RH in two units and
compared with two control units of 2535% RH in a 4 months
period. Air quality and symptoms were reported before and after
intervention in hospital sta(n = 104).
Signicant decrease of perceived air dryness and airway symptoms.
Weekly sensation of air dryness was 24% in humidied units contrary
to 73% in the non-humidied units. Perceived IAQ was unchanged in
control unit.
Norbäck et al. (2000) See also
Nordström et al. (1994)
Longitudinal 6 weeks study with blinded steam humidication in
hospital with two units with independent ventilation systems,
outside of pollen season. Sta(n = 26, 100% female; 14 in
humidity group and 12 in non-humidity control group) were
investigated before and after humidication applied in one of the
units for a period of 6 weeks. Questionnaire and medical
examination before and after.
The perception of air dryness was reduced signicantly (p = 0.04)
from 73 to 36% in the humidication unit by increase of RH from 35
to 43%, while only slightly reduced in the control group (9081%).
Perception of dustiness and stuy air remained unchanged.
No changes in the PTF stability (SBUT), nasal patency (rhinometry),
and inammatory markers in nasal lavage uid.
Cannot be excluded that outdoor RH may have inuenced, also, although
exposed subjects and controls were investigated on the same days.
Reinikainen et al. (1992) Oces Oce workers (n = 290) and cross-over trial, in two wings. Slight
increase of temperature during humidication.
Dryness symptom score (dryness, irritation or itching of the skin and
eyes; dry throat and nose) was signicantly smaller (p < 0.01) during
humidication (3040% RH) compared with the non-humidication
phase (2030% RH). However, the perception of stuy air increased
signicantly during humidication, which also included unpleasant
odor and dustiness perceptions (not signicant).
Reinikainen et al. (1997) Oces Steam humidication up to 3040% RH compared with non-
humidied units.
Humidication caused a decrease of the perceived IAQ, strongest
among women.
Cross-over trial, use of naïve panel (n = 20) to assess the perceived
IAQ, weekly.
Reinikainen and Jaakkola (2001)
Same oce workers as in 1992 study. Cross-over trial in two wings.
One wing humidied and the other non-humidied for one week
(constant temperature), then switch for a total of 6 weeks. Daily
High temperature conditions increased dryness symptoms and sick-
building syndrome symptoms during non-humidied conditions.
Increase of RH from about 2535% resulted in fewer sick-building
syndrome symptom complaints.
Synthesis of studies: high temperature conditions increased sick-
building syndrome symptoms in 4 out of 7 studies; high temperature
resulted in an increase of perceived dryness. Humidication
reportedly decreased sick-building syndrome symptoms or dryness in
5 out of 11 studies and in 3 studies an increase.
Present study showed lower sick-building syndrome symptoms than in
non-humidied conditions and alleviation of perceived dryness during
humidication. Dryness increased more acutely under non-humidied
Reinikainen and Jaakkola (2003)
Oce workers (n = 368; 71%) returned baseline questionnaire and
diaries with information about symptoms or perceptions; 342
diaries from non-humidied (2526% RH) and 233 from humidied
conditions (2149% RH). Temperature from 21 to 26 °C.
Eye dryness was alleviated, but not signicant. Humidication
decreased nasal dryness. High temperature increased nasal congestion
signicantly (especially for AH). Odor perception increased at elevated
RH; slightly stronger for AH. Stuness seemed to be associated with
humidication. Humidication alleviated nasal congestion.
Sato et al. (2003) Factory Comparison workers (n =12) in ultra-low RH (2.5%) with workers
(n = 143) at normal RH.
33% versus 18% reported eye symptoms in ultra-dry and normal RH,
respectively, but not signicantly. Skin complaints were signicantly
higher at ultra-low RH.
Singh and Jaiswal (2013)
Cochrane review
Only two selected double-blind studies. Conclusion: Little (scanty evidence) benet from use of
dehumidication by use of mechanical devices on the clinical status of
asthma patientssensitive to house dust mites.
Wiik (2011) Oces Comparison of responses from oce workers (n = 484) and green
house workers (n = 21).
Deteriorated productivity in oces at dry conditions based on
calculating the indoor productivity indexfrom questionnaire data.
Wright et al. (2009) Homes Double-blind placebo control (ventilation) study; intervention in
about half of the homes of adults (n = 120) with asthma (dust
mite): 54 active with mechanical-heat-recovery-ventilation and 47
placebo. RH went from 45% to 21%.
Carpets were steam-cleaned, new bedding and mattress before
activation of the mechanical heat-recovery-ventilation. The addition
of mechanical-heat-recovery-ventilation to house dust mite
eradication strategies did not reduce mite allergen levels, but did
improve evening peak expiratory ow.
AH = absolute humidity. NBUT = non-destructive break-up time of PTF (same as SBUT). RH = relative humidity. SBUT = subjective break-up time of PTF. PTF = precorneal (eye) tear
P. WolkoInternational Journal of Hygiene and Environmental Health 221 (2018) 376–390
perception of IAQ (Cain et al., 2002;Fang et al., 1998). For instance,
the emission prole of polar VOCs from building materials may alter
markedly by increase of the RH, e.g. Fang et al. (1999),Fechter et al.
(2006),Huang et al. (2016),Markowicz and Larsson (2015),Wolko
(1998), but also the temperature including competitive adsorption
mechanisms and water solubility (Zhou et al., 2017).
The immediate perception of odor and stuy airincreased slightly
upon increase of RH in oces (Reinikainen et al., 1997;Reinikainen
and Jaakkola, 2003), see Table 1. Thus, the perceived stuness may, in
part, be caused by the altered VOC (odor) emission prole combined
with the thermodynamic eect. However, at the same time, alteration
of the inhalable particle chemical composition, the deposition and re-
suspension that occur from surfaces may dier at dierent RH, see
below. This is especially for particles that predominantly deposit in the
nostrils and upper airways, i.e. particle sizes > 2.5 μm, that also may
inuence the perception of the IAQ, cf. Bottcher (2001). The particle
surface-active properties may also inuence the nature of perception of
IAQ, if the mucus membranes are susceptible, i.e. desiccated or dena-
turized; for instance, one may speculate whether reporting of dry air
reects slightly irritatingair that is triggered by odor, while desic-
cated mucus membranes or eyes may interact more readily with active
particle surfaces or sensory irritants.
3.1.3. Particles
Particles impact the IAQ and health by their chemical and physical
properties, but they may also be carriers of inuenza virus. Particle
concentration, chemical composition, particle size (diameter) and
shape, particle deposition and resuspension, and hygroscopic growth
appear to depend on RH, however, the picture is far from complete; the
mechanisms behind resuspension are by nature complex that depend on
many factors and their interaction (Qian et al., 2014). For instance,
larger particles, in general, show larger resuspension by walking at low
RH in laboratory settings (Kivistö and Hakulinen, 1981;Qian et al.,
2014) and low deposition of particles < 1 μm(Han et al., 2011). This
agrees with a 6.4 μg/m
decrease of PM
per 10% increase of RH in
schools during winter; however, an opposite trend was observed during
summer time (Fromme et al., 2007). Increase of RH 310% from very
low RH in long-haul ights signicantly lowered the concentration of
respirable particles from 6 to 1 μg/m
; similar observation was seen for
mold and bacteria (Lindgren et al., 2007). Modeling indicates that
shorter people may be exposed to higher concentrations of resuspended
particles and pathogens, than taller people, but experimental con-
rmation is needed (Khare and Marr, 2015).
The resuspension of particles depends on walking style and density;
further, particle-substrate interaction and substrate interactions like the
microscopic surface roughness (Qian et al., 2014;Qian and Ferro, 2008;
Tian et al., 2014). For example, Tian et al. (2014) observed that course
particles (310 μm) resuspend with a 24 higher concentration from car-
pets in comparison with hard oorings, while no dierence was observed
for ne particles; however, elevated humidity caused enhanced re-
suspension on high-density carpets, while hard surfaces showed the op-
posite eect. Furthermore, the water solubility of the particles is im-
portant, thus, resuspension was higher for hydrophobic (e.g. cat and dog
fur) respirable particles than hydrophilic ones (e.g. dust mites) (Salimifard
et al., 2017). This is in some contrast to observations reported about
human emissions of course particles from frictional interaction between
humanskinandclothing.Forinstance,Bhangar et al. (2016) reported an
increase of emissions of uorescent particles from human subjects at ele-
vated RH in a climate chamber; however, dierentiation between re-
suspension from the oor and body envelope emissions was not possible.
This is further complicated by use of skin moisturizer, where mean
emission rates of uorescent particles diered insignicantly at low and
high RH, but lower humidity was associated with smaller emission peak
amplitudes of the uorescent particles (Zhou et al., 2016).
Overall, laboratory experiments indicate more resuspension of large
particles and less for smaller particles at low RH, however, surface
dependent. More experience from eld studies is warranted to under-
stand the mechanisms and chemical nature of the particles that inu-
ence the resuspension by indoor air humidity.
3.1.4. Work performance
The indoor productivity indexin normaland well-ventilated
oces in buildings, characterized as not sick, has been assessed by a
questionnaire study of 484 oce workers and 21 green house em-
ployees (Wiik, 2011). The self-assessed productivity depended equally
on psychosocial and environmental factors, and RH was identied as
important. This agrees with the observation of a few percent reduced
visual data acquisition for certain oce tasks among young students
that were exposed to low RH for 4 h (Wyon et al., 2006); an eect that is
expected to become more pronounced among elderly oce workers, cf.
Overall, the immediately perceived IAQ appears more acceptable at
low RH and low temperature, which reects altered VOC emission
proles from material surfaces or altered surface reactions with oxi-
dants. The common reported stuy or dry airmay thus be aected
not only by alteration in the VOC emission prole, but also at the same
time by alteration of the dynamics, composition, deposition and re-
suspension of inhaled particles; possibly in concert with susceptible
eyes or mucus membranes in the upper airways at low RH. This con-
trasts the outcome of many intervention studies which show the ben-
ecial eects by increase of the indoor air humidity from low RH as
shown in Table 1.
3.2. Sensory irritation in eyes and upper airways and odor
3.2.1. Rodents exposed to sensory irritants at dierent relative humidity
The only standardized and validated animal bioassay, which pre-
dicts sensory irritation in the airways in humans from airborne che-
micals, is the Alarie test (Nielsen and Wolko, 2017). It is a mice
bioassay, which uses the trigeminal reex-induced decrease in the re-
spiratory rate, where the 50% decrease (RD
) has been correlated with
occupational exposure limits (threshold limit values) caused by sensory
irritation in the upper airways; furthermore, no-observed-adverse-ef-
fect-levels may be predicted according to Kuwabara et al. (2007). Since
sensory irritation in the eye and upper airways is mediated by the same
nerve system (Trigeminus), the predicted limit is similar for both tar-
gets, although eyes may generally show a slightly lower limit as shown
in human exposure studies (Doty et al., 2004).
Data from animal inhalation studies about sensory irritation in the
upper airways are summarized in Table 2. The studies indicate that
sensory irritation in the upper airways is unaected by low RH; how-
ever, o-albumin-sensitized mice appeared to be less aected than
normal mice regarding bronchoconstriction at very high formaldehyde
levels (Larsen et al., 2013). This observation is compatible with slightly
less sensory irritation in nose and throat in asthmatic subjects in com-
parison to healthy subjects, when exposed to a steady-state reaction
mixture of ozone (max 37 ppb) and limonene (36 ppb) [resulting
in < 10 ppb formaldehyde (Atkinson and Arey, 2003)] for 3 h in a
controlled and blinded chamber study (Fadeyi et al., 2015). It has been
proposed that the excess mucus in the airways of asthmatics and in the
sensitized animals has a scrubbing (protective) eect, thus explaining
the dierence between the healthy and asthmatic subjects and similarly
in normal and sensitized mice exposed to formaldehyde or a reaction
mixture of ozone and limonene (Hansen et al., 2016;Larsen et al.,
No major inuence on sensory irritation was observed in mice ex-
posed to ammonia at dry versus humid conditions; however, a minor
eect was seen in rats (Li and Pauluhn, 2010). This eect should be
considered cautiously due to the breathing parameter, RD
used for
comparison, which reects the combined eect of sensory irritation and
time of inspiration, while time of break(not reported) before
P. WolkoInternational Journal of Hygiene and Environmental Health 221 (2018) 376–390
initiation of exhalation is a more specic measure of sensory irritation,
cf. (Wolkoet al., 2012). The animal data agree with statistically un-
altered lateralization thresholds for sensory irritation at humid and dry
conditions among eight volunteers (Monsé et al., 2016).
Many rodent studies have shown adverse eects on the eye physiology
by exposure to low RH, see Table 2. Thus, less tear production and dry spot
formation in the (precorneal) eye tear lm has been observed (Barabino
et al., 2005;Chen et al., 2008)andepithelialdamage(Xiao et al., 2015).
All in all, the studies show that low RH aggravates the stability of the eye
tear lm, which becomes more susceptible and consequently initiates a
cascade of adverse inammatory reactions (Wolko,2017). For instance, a
destabilized eye tear lm relative to a stable one may be more susceptible
to inammatory reactions by exposure to titanium dioxide nanoparticles
as shown in a rat model (Han et al., 2017).
3.2.2. Sensory irritation and odor in humans
Induction of sensory irritation in the upper airways (nose) by strong
sensory irritants (chemicals) appears independent of the RH in animal
studies, see above. This is contrary for odor thresholds in humans. For
instance, a lower threshold was found for butanol than at dry conditions
(80% vs 30% RH), at least in comparison at hypobaric conditions
(Kuehn et al., 2008), while no eect on RH was found for pyridine
(Callado and Varela, 2008), which in part agrees with clinical experi-
ence (Philpott et al., 2007). The studies, however, are not directly
comparable and the contradictory data does not allow for a general-
One study at three dierent geographical locations and humidity
indicated a trend of a higher overall skin irritation level at dryer cli-
matic conditions for both positive (0.1% sodium lauryl sulfate) and
negative controls (0.9% saline) (Trimble et al., 2007). Asthmatics may
be less sensitive to inhalation of strong water-soluble irritants, like
formaldehyde, than non-asthmatics. However, individuals with allergic
diseases (e.g. allergic rhinitis) may perceive IAQ and dry airdierently
than normal subjects and possibly react dierently to other pollutants,
e.g. unpleasant odors. Thus, more secure and standardized information
is warranted about the inuence of humidity on odor thresholds.
Table 2
Animal studies: airway (sensory irritation) and ocular surface eects.
Study Method Rel. hum.
Barabino et al.
Normal mice (5 for each condition) exposed to dry air in controlled
environmental chamber and compared with control mice (n =30) at
room temperature. Ocular surface examined after 3, 7, 14 and 28 days
of exposure.
Low RH caused a decrease of tear production and increase of
uorescein corneal staining in normal mice compared to control mice
exposed at 5080% RH. A signicant drop ingoblet cells after 7 days
was observed in mice exposed to low RH.
Chen et al. (2008) Mice (5 for each condition)) were exposed to controlled dry
environment. The ocular surface was examined after 3, 7, 14, 28, and
42 days of exposure at room temperature.
15 Aqueous tear production decreased, while an increase was observed for
corneal uorescein staining, thinning, and accelerated desquamation of
the apical corneal epithelium.
Upregulated apoptosis was observed on the ocular surface.
Larsen et al.
Mice (5 for each condition) were exposed to formaldehyde
(45.7 ppm) for 1 h.
Respiratory parameter, time of break, was measured.
Sensitization of mice did not cause increased sensitivity to sensory
irritation of formaldehyde at dry conditions.
Non-sensitized mice < 5 At humid conditions, sensitized mice were more sensitive to pulmonary
eects at high formaldehyde concentrations, while under dry
conditions the non-sensitized animals were more sensitive.
Non-sensitized mice 85
Sensitized (ovalbumin) mice < 5
Sensitized (ovalbumin) mice. 85
Han et al. (2017) Normal and evaporative dry eye induced rats (6 in each condition)
were exposed for 24h to titanium dioxide particles (< 75 nm;
0.5 mg/ml) by installation and compared with sham condition;
corneal clarity and tear samples were measured.
30 Evaporative dry eye induced rats were more susceptible (e.g.
inammatory cell inltration on the ocular surface) to titanium dioxide
particles than normal rats.
Li and Pauluhn
Mice and rat (male) bioassay. Animals (4 per condition) were exposed
to ammonia and respiratory rate RD
was measured.
Reduction of RD
0 582ppm, mice
95 732 ppm, mice
0 972ppm, rat
95 905 ppm, rat
Lin et al. (2009) Guinea pigs (79 for each condition) exposed to hot (40.5 °C)
humidied air for 4 min via a tracheal tube to the lung and compared
with control group exposed to humidied room air. Pulmonary
resistance was measured.
(Expiratory airway temperature is signicantly higher in asthmatics,
2.7 °C).
Elevated tracheal temperature from 36.4 to 40.5 °C induced immediate
transient airway constriction mainly mediated through cholinergic
reex, probably elicited by the activation of the TRPV1 temperature-
sensitive receptor water is believed to be a critical factor in delivery
of the heat load. Further, increase of total pulmonary resistance. In
contrast, hyperventilation with humidied air at room temperature did
not alter pulmonary resistance.
Nakamura et al.
Female rats were exposed in a swing to dry air for 6 h daily period.
Lacrimal function and morphology were evaluated after undergoing
10 days of the swing procedure.
25 It was shown that not only excess evaporation of tear uid but also
hypofunction of the lacrimal gland contributes to the pathogenesis of
visual display unit-associated dry eye in humans.
Suhalim et al.
Mice (n = 10) exposed to a controlled drafty dry air environment.
After 5 and 10 days eye samples were analyzed.
3035 Dry air environment has a direct eect on the Meibomian gland function
a 3-fold increase in basal acinar cell proliferation after 510days and
abnormal meibocyte dierentiation and altered lipid production.
Wilkins et al.
Mice (4 at each condition). Respiratory rate (RD
) reduction by
exposure to ozone/limonene mixture (16 s old) and ozone/isoprene
mixture (90 s old) at dierent RH.
Reduction of RD
0 By ozone/limonene: 33
32 22
0 By ozone/isoprene: 56
32 42
Xiao et al. (2015) Mice (60 for each condition) were exposed for 1, 2, 4, and 6 weeks to
controlled dry environment at room temperature, and air velocity
2.2 m/s. Further, mice were housed in normal laboratory conditions.
The ocular surface was analyzed.
13 vs 60 Dry environment induced corneal epithelium damage (apoptosis) and
stimulated inammatory cytokine production in conjunctiva and
lacrimal gland. Further, lacrimal gland structural alterations were
= concentration that causes 50% reduction of the respiratory rate.
* Statistically signicant.
P. WolkoInternational Journal of Hygiene and Environmental Health 221 (2018) 376–390
3.2.3. Ophthalmological investigations of the precorneal tear lm and
ocular comfort in humans
An intact and stable eye tear lm is essential for visual acuity and
ocular comfort, in general. The prevalence of external eye symptoms
continues to be high in European and Japanese oces (Bluyssen et al.,
2016;Yokoi et al., 2015). Retrospectively, the prevalence has not de-
clined substantially during the last decades, despite lower emitting
building materials and modern buildings (Bluyssen et al., 2016;
Wolko, 2013).
Many ophthalmologic studies have demonstrated how fast low RH
aggravates the stability of the eye tear lm, i.e. break-up or thinning of
the eye tear lm resulting in less tear production or exacerbation of
water loss, see Table 3. This leads to desiccation and hyperosmolarity in
the eye tear lm and initiation of a cascade of inammatory reactions
Table 3
Human exposure and eld studies at dierent humidity conditions ocular surface.
Study Approach Rel. hum.% Observation
Abelson et al.
Dry eye patients (n = 33) exposed for 1½ h Low Decrease of mean break-up area induced by low RH; correlation with
other measures of dry eye diseases and demonstration of
compensatory mechanisms in dry eye patients.
Abusharna and
Pearce (2013)
Healthy subjects (10 men, 2 women) were exposed for
1 h followed by tear and ocular measurements
40 vs 5 Evaporation rate of water, lipid layer thickness, ocular comfort, low RH
signicantly adversely aected precorneal lm stability and production.
Tear lm parameters became like dry eye patients after 1 h at low RH.
Alex et al. (2013) Normal subjects (n = 15) and dry eye patients (n = 10)
were exposed for 1½ h.
1525 Signicant increase of corneal and conjunctival dye staining (dry spot
formation) in both groups, but greater staining in superior cornea in
dry eye patients. Eye blink frequency between 30 and 90 min was
higher in dry eye patients.
et al. (2007)
Contact lens-wearers (n = 10) with minimum of
symptoms were exposed for 2 hours without contact
lens and with contact lens at 2 RHs. Dry eye signs were
evaluated before and after each exposure
19 (22 °C)
35 (24 °C)
Without contact lens: Signicant changes were observed in comfort,
noninvasive BUT, conjunctival hyperemia, and phenol thread test at
low RH as opposed to normal conditions (no changes).
With contact lens: Same changes were observed in both conditions.
These returned to normal after about 1 month, i.e. reversible.
Galor et al. (2014) United States veteran study. All patients seen in a
veteran administration eye clinic between 2005 and
2011; retrospective analysis
National Climatic
Data Center and
NASA adm
The most important risk factors of dry eye symptoms were shown to
depend primarily on air pollution (optical measurement of aerosols)
and pressure (high altitude). Furthermore, higher RH and wind speed
was inversely associated with the risk of dry eye symptoms.
Hirayama et al.
Dry eye patients (n = 10) were exposed during
minimum 4 h daily visual display unit (VDU) work to
Moist Cooling Air Device (MCAD; 100300 μm
droplets) for 5 working days; similarly, patients
(n = 10) carried VDU work without MCAD
+/MCAD in
MCAD signicantly improved functional visual acuity, lowered BUT
and symptom score (less dryness). Strip meniscometry and
evaporation rate of water signicantly improved. No signicant
changes in lipid layer stability or corneal staining between the 2
groups. Blink frequency was signicantly increased without MCAD.
Lan et al. (2011) Subjects (n = 12; male/female = 1:1) doing oce
work at 23 °C and 30 °C
2122 Ferning test showed an increase of type III and IV patterns that
indicate substantial alteration of the precorneal tear lm
composition, i.e. lower tear lm quality, at the high temperature.
López-Miguel et al.
Mild to moderate dry eye patients (n = 19) and
asymptotic controls (n = 20) were exposed in climate
chamber for 2 hours. Single-item score dry eye
questionnaire and diagnostic tests were performed
before and after the exposure period
5 Signicant increase in corneal staining and signicant decrease in
uorescein break-up time were observed in patients and controls.
Also, a signicant increase in matrix metallopeptidase.
In controls: signicant decrease of epidermal growth factor and
signicant increase of interleukin-6 levels were observed after
Melikov et al.
Subjects (n = 30) were exposed for 4 h by personalized
ventilation (PV) or without at dierent temperatures
and 15 min video recording and analysis of eye blink
frequency and Ferning test of tear liquid
+PV 7026 °C, 28 °C
-PV 4023 °C
Increase of T and RH without PV reduced blink frequency. Only
decreased blink frequency signicantly in comparison with 26 °C/70%
RH, indicating that temperature perhaps is more important than RH.
Ferning test showed a decreasing trend in precorneal lm quality, by
disappearance in Grade I quality going from neutral condition to
higher temperature and RH. Use of PV slightly improved the quality of
the precorneal tear lm. Data, however, should be interpreted with
caution. Ferning test should be compared at comparable conditions,
thus interpretation of data is dicult.
Madden et al.
Dry eye patients (n = 3) and normal subjects (n = 3)
were exposed in controlled climate chamber. Tear
evaporation rate was measured after 0, 5, 10, 15, 20 and
25 min. No ninvasive BUT and tear evaporation rate w ere
determined at 5% to 70% R H in dry ey e patients (n = 10)
and normal subjects (n = 10); T = 72 °F (22.2 °C)
40 Ten min required for reaching steady-state of the tear evaporation rate
(peak after 5 min) and no chance in noninvasive BUT. Dry eye patients
had higher evaporation rate and shorter noninvasive BUT than normal
subjects at 5% and 40% RH, but not at 70%. Emulsion drops helped.
Nakamura et al.
Cross-sectional survey of tear lm characteristics in
1025 oce workers during VDU work.
VDU users have less tear secretion (impaired lacrimal function), less
the more VDU use, both on a yearly duration and daily basis. Dry
conditions cause less tear secretion.
Human and rat studies provided the evidence that not only excess
evaporation of tear uid but also hypofunction of the lacrimal gland
contributes to the pathogenesis of VDU-associated dry eye.
The study suggests that a proper number of eye blinks is required for
healthy lacrimal gland function to occur. Since VDU use suppresses
the blink frequency, modications, such as the use of bigger and
clearer characters, should be considered when trying to increase the
blink frequency, in addition to modifying daily working conditions or
Norbäck et al.
Aircraft cabin crew (n = 7079) were exposed to low or
elevated (blind) humidity for 8 hours transatlantic ight.
Signicant improvement of precorneal tear lm stability (i.e. longer
subjective BUT) and decrease of perceived eye dryness and fatigue.
(continued on next page)
P. WolkoInternational Journal of Hygiene and Environmental Health 221 (2018) 376–390
(Wolko, 2017). For example, one-hour low RH exposure to healthy
subjects resulted in tear lm parameters like in dry eye patients
(Abusharna and Pearce, 2013).
The instability of the eye tear lm may increase the formation of
(local) dry spots, which enhances direct exposure of the corneal epi-
thelium to pollutants; thus, the eyes may possibly become more sus-
ceptible and react faster to external stimuli like sensory irritants (e.g.
formaldehyde) and other aggressive pollutants, e.g. oxidants like ozone
and particulate matter (Wolko, 2017).
Attempts have been carried out to reduce dry symptoms among
oce workers by various techniques that locally elevate the humidity.
For example, even a modest increase from 45% to 50% RH for one hour
showed a signicant increase of the eye tear lm stability by increase of
the non-invasive break-up time followed with a signicant dry eye re-
lief (Wang et al., 2017). Similar positive eects have been shown for the
use of glasses with moist inserts, e.g. Korb and Blackie (2013);Ogawa
et al. (2017);Waduthantri et al. (2015).
Overall, the human eye tear lm is susceptible to low RH, which ag-
gravates its stability, and likely the susceptibility to aggressive chemicals
and particles, and which potentially may initiate a cascade of reactions
like hyperosmolarity leading to inammatory reactions; however, even
modest local increase of RH may be benecial by dry eye relief. Further
studies are necessary to explore the interplay between low humidity, tear
lm stability and exposure to aggressive indoor air pollutants.
3.3. Human climate chamber studies airway eects and sleep quality
3.3.1. Airway eects
The major function of the nose and nasal cavity is to humidify and
warm inhaled air; thus, the anterior part of the nasal cavity contributes
within a short nasal passage to air conditioning of inspired air (Keck
et al., 2000). The temperature of the nasal cavity strongly depends on
feet temperature; for instance, the conditioning capacity in response to
cold-dry-air is signicantly higher at 40 °C feet temperature than 30 °C,
i.e. the ability of the nose to condition inspired air without concomitant
change of volume of the nasal cavity (Naclerio et al., 2007).
Mucous membranes lose water by evaporation and heat in the hu-
midication and warming processes (Cruz and Togias, 2008;Naclerio
et al., 2007). Thus, the mucosal function depends strongly on the hu-
midity and heat in the inhaled air, the exposure time, and the health of
the individual (Williams et al., 1996). Hundred percent RH at core
temperature is moisture neutral, and preserving maximum mucociliary
transport velocity(Williams et al., 1996); either lower or higher RH
will alter the mucous viscosity and the mucociliary activity.
The respiratory epithelium plays an important role by evaporation
of water from its surface (desiccation). This continuous need for eva-
poration may lead to a hyperosmolar environment on the surface of the
epithelium. Increased ventilation may result in a larger hyperosmolaric
surface that moves more distal, and this may stimulate the epithelial
cells releasing inammatory mediators. Cold-dry-air led to signicantly
higher osmolarity than methacolin or histamine, thus conrming that
the osmolarity in nasal secretions has increased after cold-dry-air
challenge (288 to 306 mOsm/kg H
O) (Naclerio et al., 2007). De-
siccation (dehydration) of the epithelium includes desquamation, leu-
kocyte inltration, vascular leakage, and mast cell degranulation, all of
which may worsen inammation. Thus, the epithelial cells may be
stimulated to release inammatory mediators if the hyperosmolaric
surface is not restored (Naclerio et al., 2007). Further, hyperosmolar
challenge may cause histamine and leukotriene (C4) release. It is con-
cluded that the histamine release is probably caused by hyperosmolar
Table 3 (continued)
Study Approach Rel. hum.% Observation
Paschides et al.
Three geographically and climatically dierent groups
(n = 5557, each) were tested for eye tear lm
Dry, warm and
heavy pollution
Dry, warm, and
low pollution
Cool, humid and
low pollution
The precorneal tear lm stability (Schirmer-1 test and BUT) was
inuenced by low RH and high temperature. The outdoor pollution
(trac) may have impacted the tear lm quality dierently.
Sunwoo et al.
Healthy students (n = 16) were exposed to dierent
RH for 90 min.
10, 30, 50 Signicant increase in eye blink frequency below 30% RH.
Takahashi et al.
Eye steaming after VDU work at dierent humidity. Increase of RH at the periocular region improved subjective amplitude
of accommodation and near vision.
Tesón et al. (2013) Dry eye patients (n =20; 6 males) and dry eye control
patients (n = 15: 5 males; 45% RH) were exposed in
simulated in-ight condition for 2 hours at 23 °C.
Tear IL-6 and matrix metalloproteinase increased signicantly, while
epidermal growth factor decreased signicantly. Dry eye patients
suered signicantly by lower BUT, tear volume, and an increase of
corneal staining. A mild increase of corneal staining was seen in the
control patients.
Uchiyama et al.
Dry eye patients (n = 18) and healthy subjects
(n = 11) were exposed to dierent RH.
Evaporation rate increased 100% from normal to low RH in both dry
eye patients and healthy subjects.
Um et al. (2014) Korean adults (n = 16431; age > 30) were analyzed
for the spatial epidemiological pattern of dry eye
disease prevalence.
Lower RH, sunshine exposure, and degree of urbanization (air
pollution) were suggested to be associated with increase of dry eye
Walsh et al. (2012) Cross-sectional design assessment of patients (n =111;
56 males; age = 77 ± 8) admitted to acute unit. Dry
eye questionnaire, dryness (VAS), noninvasive BUT,
hydration, and tear osmolarity.
Dry eye patients showed higher plasma osmolarity, thus indication of
suboptimal hydration in comparison to non-dry eye patients. Whole-
body hydration appears to be important.
Wang et al. (2017) The RH was randomly elevated by use of a desktop
USB-powered humidier in a masked crossover study
with VDU users (n = 44) for 1 hour. The eye tear lm
quality was measured, and the eye comfort was
assessed by the users.
4550 BUT increased from 6.4 to 9.0 sec at 50% RH. The lipid layer thickness
and tear meniscus were unaltered. The users (36%) of humidier
reported a signicant improvement in eye comfort versus 5% without
humidier at 50% RH. 7% of the users reported less comfort at 50%
RH and 48% reported less comfort at 45% RH.
Wyon et al. (2006) Young subjects (n = 30; 13 males) were exposed for
5 hours to dierent RH.
5, 15, 25, 35 Increased eye blink frequency and eye discomfort and reduced visual
data acquisition at low RH.
BUT = break-up time. MCAD = moisture cooling air device. NBUT= noninvasive break-up time. PV = personal ventilation. RH = relative humidity. T = temperature. VDU = visual
display unit.
P. WolkoInternational Journal of Hygiene and Environmental Health 221 (2018) 376–390
stimuli in mast cells and the release is greater among those responding
to cold-dry-air (e.g. asthmatics) than non-responders. Furthermore, in
the end, dryness of the epithelium may increase bacterial adherence
and allows for greater penetration of foreign species, like particles
(Naclerio et al., 2007).
Table 4 shows studies about the impact on lung function in normal
and asthmatic subjects exposed to dierent humidity. For example,
asthmatic patients appear to be more sensitive to cold dry air than
normal subjects (Hanes et al., 2006;Naclerio et al., 2007). However, for
thermally induced asthmatics the issue of airway desiccation, i.e. hy-
perosmolarity, per se, does not appear to be important; further, there is
indication that the cooling-rewarming gradient, rather than desicca-
tion is important(McFadden Jr. et al., 1999). The observation that
humidity is of less importance among asthmatics is compatible with the
studies by Larsen et al. (2013) and Fadeyi et al. (2015). These studies
showed that ovalbumin-sensitized mice (asthmatic) and asthmatics
were less aected than non-sensitized mice or normal subjects, re-
spectively, from exposure of formaldehyde or an ozone-limonene in-
itiated reaction mixture.
Nasal mucociliary transport (an epithelial function) is an important
factor in exchange of heat and water, and protection of the mucosal
interface; this requires an periciliary uid layer of a certain height
(thickness) for an ecient mucociliary transport (Naclerio et al., 2007).
Thus, the saccharin mucociliary clearance time in the upper airways
was signicantly lower in elderly subjects at low RH in comparison with
younger subjects, which appear to be less sensitive (Sunwoo et al.,
2006a, 2006b). This could be interpreted that the upper airways of
elderly are less ecient in achieving moisture neutrality and maximum
mucociliary transport. However, young subjects also showed longer
clearance time below 30% RH, experienced sensation of dry eyes at
entering the exposure chamber, while the sensation of dry nose and
throat became signicant after 90 min (Sunwoo et al., 2006a). Sub-
jectively, the elderly group had diculty in feeling dryness in the upper
airways (nose), despite longer clearance time, but the young subjects
did feel dryness after 180 min. In general, subjects felt greater dryness
in the throat. This agrees with a previous study that showed a sig-
nicant increase in clearance time from 11.9 min at 4043% RH
breathing air for 30 min to 18.5 min at 0.1% RH among healthy subjects
(Salah et al., 1988). Sunwoo et al. (2006a, 2006b) recommended
RH > 30% to avoid dry eyes and RH> 10% to avoid nasal dryness. At
the same time, nasal patency is lower at dry and/or cold air in com-
parison to room air (Zhao et al., 2011); furthermore, the forced ex-
piratory volume within 1 s (FEV
) was shown to reduce by increase of
water loss from extended dry air exposure (McFadden Jr. et al., 1999).
Overall, except for longer saccharine mucociliary clearance time
among elderlies, asthmatics appear to be more susceptible to cold dry
air and at the same time more robust to exposure to strong (water-
soluble) sensory irritants than non-asthmatics at low RH. While it is
well established that RH less than 30% aggravates the eye tear lm
leading to eye symptoms like dry eyes, the sensation of dry nose and
throat also occurs in the nose and throat after some latency and without
pollution, but more pronounced at RHs below 10%. Further studies are
necessary to clarify the interplay between clearance time, humidity,
and indoor pollution, e.g. particles.
3.3.2. Continuous positive airway pressure in patients
Table 5 shows studies with patients suering from obstructive sleep
apnea. Humidied (heated) air appears to reduce nasal symptoms in the
patients, but not quality of life and sleepiness (Ryan et al., 2009;Nilius
et al., 2008, 2016), but methodological issues may obscure the result,
e.g. sleepiness and quality of life, according to Ruhle et al. (2011) and
Ugurlu and Esquinas (2016).
Benets of an increase of RH in bedrooms is controversial in view of
the consensus that elevated humidity (water activity) in moisture-da-
maged building constructions is associated with adverse health eects,
e.g. by increase of the exposure risk to fungi, mildew, dust mites, etc.
(World Health Organization, 2009); however, increase of bedroom RH
has been proposed to have a benecial eect (Myatt et al., 2010).
Overall, based on the studies, humidied breathing air appears to be
benecial for high-risk patients with nasopharyngeal complaints (Ryan
et al., 2009;Nilius et al., 2008, 2016). However, eects on sleepiness
and quality of life are unclear and need further documentation. More
controlled eld studies are necessary to identify the eects of tem-
perature and humidity in bedrooms and associated ventilation for fur-
ther substantiation.
3.4. Inuenza virus survival and transmission
Table 6 shows inuenza virus survival and transmission studies at
dierent air humidity. Cold temperature and low RH has been asso-
ciated with increased occurrence of respiratory tract infections, in line
with increased survival and transmission eciency of inuenza virus,
e.g. from coughing. Thus, RH > 40% greatly reduces the infectivity of
virus, e.g. Lowen et al. (2007);Mäkinen et al. (2009);Noti et al. (2013);
Myatt et al. (2010). For instance, Myatt et al. (2010) estimated that an
increase of RH to 47% reduced the inuenza survival by 1732% with
an operating humidier in a bedroom. Furthermore, it has been mod-
eled that low temperature and low AH prevents disruption of the in-
uenza virus as opposed to higher temperature and humidity (Koep
et al., 2013;Ud-Dean, 2010).
Several studies indicate favorable survival conditions for some in-
uenza viruses at cold and low RH (see Table 6). Experimental in-
adequacy of the studies should be considered carefully together with
the overall complexity of transmission and survival, and associated
mechanisms of infection (Memarzadeh, 2012;Yang and Marr, 2012). At
least three mechanisms have been proposed, cf. Memarzadeh (2012);
RH interacts with the hosts airways, i.e. desiccation of mucus mem-
branes in nose and upper airways may cause epithelial damage and
reduced mucociliary clearance (an important defense mechanism), thus
the airways may become more susceptible to viral infection. Second,
RH impacts the virus-aerosol stability that depends on the physical-
chemical properties; thus, virus with a lipid envelope are more stable in
dry air as opposed to a non-lipid virus envelope (Morawska, 2006). For
instance, high RH decreases the survival of lipid-enveloped virus, like
inuenza A and inuenza b (Schaer et al., 1976;Tang, 2009;Teller,
2009). Third, RH impacts virus/droplet dynamics, i.e. size, surface
properties, water content, and consequently transmission and deposi-
tion, etc. Probably, all three mechanisms act in a concerted manner.
Perhaps, more important is the strong association identied between
AH and inuenza survival and transmission as reviewed by (Lipsitch
and Viboud, 2009). Finally, it has been hypothesized that disease
transmissions could depend on resuspension of oor dust, thus shorter
people may be exposed to higher levels of infectious particles than taller
ones (Khare and Marr, 2015); for resuspension of particles, see the
subsection Particles.
Overall, many studies have shown that survival and transmission
potential of inuenza viruses are inversely associated with AH rather
than RH in wintertime, e.g. Lipsitch and Viboud (2009);Metz and Finn
(2015);Shaman and Kohn (2009);Shaman et al. (2010). This, in part,
agrees with a large cross-over study among military conscripts
(Jaakkola et al., 2014). Thus, indicating that cold and low RH condi-
tions favor survival and transmission for some inuenza virus, which
also include viruses like RS virus, human rhinovirus, and avian inu-
enza virus, e.g. Ikäheimo et al. (2016) and Davis et al. (2016b). How-
ever, the opposite has been observed for other virus types (Morawska,
2006;Weber and Stilianakis, 2008). Thus, the mechanisms of survival
and transmissions are far from fully understood and generalization
about viral transmission and survival, due to the complexity, is not
applicable, but should be dealt with virus-by-virus, cf. (Morawska,
2006;Weber and Stilianakis, 2008). Clearly, ventilation rates of fresh
and adequately humidied air and temperature play an important role
that needs more research attention for substantiation about the
P. WolkoInternational Journal of Hygiene and Environmental Health 221 (2018) 376–390
Table 4
Human exposure studies at dierent relative humidity conditions the airways.
Study Approach Number of
Baroody et al. (2008) Double-blind, placebo- controller, cross-over, clinical trial of patients. 20 Nasal allergen challenge probably initiates nasal and a nasal ocularreex.
Cruz and Togias (2008) Review paper about upper airway reactions to cold dry air in context of cold-air
It is proposed from that cooling and water loss/hyperosmolarity are key candidates for a
clinical response. However, it is argued that water loss/hypertonicity is more important, but
the two stimuli work in concert. Hyperosmolarity stimulates the sensory nerves that generate
a central reex (contra-lateral secretory response), but can also release inammatory
biomarkers (neuropeptides) by the same nerves.
Freed and Davis (1999)
Canine exercise-induced model to investigate hyperventilation at warm humidied
air vs dry air.
Dry air challenge increased peripheral airway resistance, the airway surface volume, and the
surface osmolarity. Data support that changes in airway osmolarity during hyperventilation
initiate peripheral airway constriction.
Hanes et al. (2006) Comparison of the response of subjects with allergic rhinitis (AR) and asthma (ARA)
and subjects with only AR to cold dry air.
24 (ARA) vs 17
Patients with ARA were more responsive to cold dry air than subjects with AR alone.
Hashiguchi et al.
Young healthy male subjects were exposed for 6 h to 10% and 60% RH and pressure
(sea level and 2000 m altitude (=low pressure), independently.
14 23 Body uid loss was signicant at low pressure, but combined low pressure and low RH
increased the loss even further. Blood viscosity increased also, but low RH alone did not alter
the blood viscosity signicantly; this, however, cannot be excluded as a possibility, i.e. more
subjects required.
Kalho(2003) Review paper about patients with asthma and chronic bronchitis and dehydration. Still unclear about mild dehydration as risk factor of broncho-pulmonary disorders.
Khosravi et al. (2014) Allergic rhinitis (AR) patients and healthy subjects (HS) were exposed for 4 min to
hot (49 °C; 7580% RH) humid air and room air (21 °C; 6575% RH), respectively,
during hyperventilation.
7 (AR)
6 (HS)
Coughs/min in AR patients increased from 0.1 before challenge to 2.4 during and 1.8 rst
8 min after end of challenge, for hot air exposed, only. The hot air challenge also caused
respiratory discomfort (throat irritation) among AR patients. No eects seen among healthy
subjects. Bronchoconstriction was not seen in both groups. Upper airways seem to be
triggered by hot air.
Kim et al. (2007) Prospective trial of 2 groups of adult patients under general anesthesia: controls did
not receive warm and humidied air (27 °C; 76% RH), while another group received
warm and humidied air (36 °C; 99.5% RH).
200 No appreciable dierences in complaints after active warming and humidication of inspired
gases (air) after 2 h.
Kuehn et al. (2008) Male volunteers exposed to 30% or 80% RH and butanol. 27 22 ± 6 High RH showed lower odor threshold for butanol, i.e. enhanced sensitivity.
Lindemann et al.
Nasal airway resistance was measured in healthy subjects by active anterior
rhinomanometry and compared with intranasal RH at dierent locations.
15 30 (2542) Degree of water saturation did not correlate with active anterior rhinomanometry, i.e. no
correlation between nasal resistance and water vapor saturation at dierent anterior nasal
segments during the nasal daily cycle.
McFadden Jr. et al.
Thermally induced asthma was investigated by mucosal dehydration in subjects
carrying out isocapnic hyperventilation of dry air at constant level for max 8 min.
Lung functions (FEV
) were investigated at cold (12.5 °C) and ambient (24.3 °C) T.
Water loss in intrathoracic airways was calculated.
Less water loss at cold temperature and FEV
decreases as water loss increases.
The eect of water loss increases with time.
= 30%, water loss = 4.7 mg
= 16%, water loss = 7.1 mg
The issue of airway dehydration, i.e. hyperosmolarity, per se, does not appear to be of major
importance for thermally induced asthma. When respiratory heat exchange increases in
asthmatics the intensity of obstruction follows suit;acooling-rewarming gradient, rather
than airway desiccation.
Melikov et al. (2013) Students (n = 30) were exposed to high temperature (26 or 28 °C; 70% RH) for 4 h
and compared with baseline (23 °C, 40% RH) and with additional PV (personal
ventilation: 24 °C, 40% RH).
15 male
15 female
The exposure to high T and RH results in lower acceptability of the thermal climate. The
subjectscontrolled use of a PV improved the thermal sensation and acceptance of climate.
Performance appeared to decrease at high temperature and RH. Use of PV improved, i.e. the
ability to work was higher during PV conditions.
Salah et al. (1988) Saccharin mucociliary clearance time and nasal breathing were measured after
30 min exposure of non-smoking subjects (n = 11) to dry air (0.1% RH) or room air
(4043% RH).
6 males
5 females
1738 Saccharin mucociliary clearance time was signicantly longer at breathing dry air (18.5 min)
versus room air (11.9 min).
Sunwoo et al. (2006b) Saccharin mucociliary clearance time, hydration state of skin, transdermal water
loss were measured in non-smoking elderly and students. Rating of thermal, dryness
and comfort.
8 students
8 elders
22 ± 1
71 ± 4
Saccharin mucociliary clearance time was signicantly longer in the elderly group at 10% RH
after 90 and 180 min of exposure. No change in saccharin mucociliary clearance time in the
students at the dierent RH.
Experimental conditions:
Precondition: 25 °C/50% RH (50 min); exposure for 180 min: 25 °C at 10%, 30% and
50% RH.
Sunwoo et al. (2006a) Saccharin mucociliary clearance time, hydration state of skin, transdermal water
loss were measured in non-smoking male students. Rating of dryness and comfort.
Experimental conditions:
16 students 23 ± 3 The eyes and the skin become dry below 30% RH; below 10% RH nasal dryness as well as eyes
and skin. 10% RH, but not 30%, decreases saccharin mucociliary clearance time.
Skin hydration state aected at 10% and 30% RH, only.
(continued on next page)
P. WolkoInternational Journal of Hygiene and Environmental Health 221 (2018) 376–390
Table 4 (continued)
Study Approach Number of
Precondition: 25 °C/50% RH (50 min); exposure for 120 min: 25 °C at 10%, 30% and
50% RH.
Zhao et al. (2011) Nasal patency was rated by subjects (n = 44) when breathing under controlled
conditions from 3 dierent boxes:
Room air 24 °C, 49% RH
Dry air 25 °C, 27% RH
Cold air 12 °C. 59% RH
20 males
24 females
Both temperature and humidity contributed signicantly to the perception of nasal patency
rating. Nasal mucosal cooling (heat loss) is the underlying stimulus in the individuals
perception of patency and trigeminal input.
cHH = controlled heated breathing tube humidier. PV = personal ventilation. T = temperature.
Table 5
Human chamber studies and sleep quality.
Study Approach Number of
Nilius et al.
Patients (n = 19) with obstructive sleep apnea. Eect of controlled heated breathing tube
humidier (cHH) on nasal symptoms and quality of life during continuous positive airway
14 male
5 female
55 ± 10 cHH improves subjective rating of nasal and pharyngeal symptoms (dry nose, dry mouth, dry
throat) during continuous positive airway pressure. After 4 weeks at home the use of cHH
also showed lower symptoms, but no eect was seen on sleep quality.
In summary, cHH improves side eects (symptoms) of continuous positive airway pressure,
but not quality of life, cf. (Ruhle et al., 2011). cHH might be benecial for patients preferring
a cool bedroom temperature.
Nilius et al.
Patients with obstructive sleep apnea were divided in high risk complaint group (n = 35)
and low risk complaint group (n = 37). Eect of controlled heated humidication on nasal
symptoms, improvement of sleep, quality of life during continuous positive airway pressure.
72 52 ± 8 Heated humidication in breathing air showed a positive eect by reduction of nasal
symptoms, and improvement of sleepiness and quality of life among the high-risk complaint
Ryan et al.
Patients with obstructive sleep apnea syndrome exposed to dry air, humidied air with or
without nasal steroid application in nasal continuous positive airway pressure therapy for 4
125 Humidied air decreased the reported frequency of nasal symptoms in unselected
obstructive sleep apnea syndrome patients (28%), but not in the other groups; compliance,
sleepiness, and quality of life remained unchanged.
cHH = controlled heated breathing tube humidier.
P. WolkoInternational Journal of Hygiene and Environmental Health 221 (2018) 376–390
inuence of humidity and associated mechanisms of infection, cf. Yang
and Marr (2012);Yang et al. (2012), especially in the eld. Jaakkola
et al. (2014) hypothesized that Higher temperature approaching 0 °C
may favor transmission and survival of the virus itself, but a decline in
temperature and humidity may make the host more susceptibility
through body cooling and/or drying of the respiratory tract.Itis
suggested the combination relatively warmer temperature and higher
AH followed by a sudden decline in these meteorological parameters
have the strongest impact on the risk of inuenza. Air-conditioned
cold and dry air in oces, thus would favor survival and transmission
for certain airborne viruses, but not others (Morawska, 2006;Tang,
2009;Teller, 2009).
For bacteria, the situation about survival and transmission is even
more complex than viruses, thus, likewise requiring individual assess-
ment (Tang, 2009).
3.5. Vocal cord eects
Table 7 shows studies about eects on the vocal cord by humidity.
Desiccating challenge may be detrimental to voice production in in-
dividuals with vocal fatigue, even in young and vocally healthy males
(Hemler et al., 1997;Tanner et al., 2016); further, it has been shown
that isotonic saline nebulization decreases the self-perceived eort
among the males (Tanner et al., 2016). However, extended daily ex-
posure to high RH should be considered cautiously, because it may for
still unknown reason increase the risk of respiratory symptoms
(Angelon-Gaetz et al., 2015); this, in contrast to the positive observa-
tions about improved sleep quality, see above. Teachersasthma-like
symptoms increased modestly, but not signicantly, at both low and
high RH in schools (Angelon-Gaetz et al., 2016).
Overall, there are indications that both too high or too low RH may
be associated with adverse eects of virus survival and asthma-like
Table 6
Survival of inuenza virus.
Study Approach Observation
Jaakkola et al.
Case cross-over study among military conscripts (n = 892); 66 inuenza A
(57) and B (9) episodes in a cold climate.
The risk of contracting inuenza was positively associated with mean T and
AH. A decrease in both temperature and AH (max change) during the three
days prior to seeking medical consultation increased the risk of inuenza.
According to these results, a 1 °C decrease and 0.5 g/m
decrease in AH
increased the estimated risk by 11% (OR1.11; 95%, CI 1.031.20).
Khare and Marr
Model study of resuspension of dust and inuenza virus. Shorter people (children) may be more exposed to higher levels of
resuspended particulate matter than would taller ones. It is hypothesized
that particle resuspension could be a mode of disease transmission.
Koep et al. (2013) Automated sensors for humidity and CO
levels in two schools and
humidication. Estimation of virus survival.
Strong association between outdoor and indoor AH. Estimated decrease of
virus survival at elevated AH. Classroom humidication may be an
approach to increase indoor AH to levels that may decrease inuenza virus
survival and transmission.
Lowen et al.
Inuenza virus transmission studied in guinea pigs as model host in
environmental chamber at dierent temperature and RH.
Both cold temperature (5 °C) and dry conditions (20%) favor spreading of
inuenza virus (transmission), while no transmission at 30 °C and 35% RH or
80% RH.
Transmission eciency depends on RH and is inversely correlated with
temperature. Transmission sensitivity to RH is largely due to virus stability.
Cold temperature does not appear to impair the innate immune response.
Hypotheses about mechanisms in variation in transmission:
1. Breathing dry air may desiccate nasal mucosa, leading to epithelial
damage and/or reduced clearance, render the host more susceptible to
virus infection.
2. Stability of virus in aerosols shown to be maximal at low (2040% RH) and
minimal at 50%, and high at 6080% RH. Stability appears to be a key
determinant (except at high RH where transmission is absent).
3. Low RH enhances evaporation from exhaled bioaerosols leading to small
droplet nuclei that remain airborne for extended period, thus increasing
the opportunity for transmission of pathogens; conversely, high RH causes
water uptake in droplet nuclei, increase in size and increase of deposition.
Mäkinen et al.
Population study (n = 892) where diagnosed respiratory tract infections
were compared with outdoor temperature and RH.
Cold outdoor temperature and low RH were associated with increased
occurrence of respiratory tract infection. Upper tract infection was associated
with AH and 1 g/m
of AH increased the risk of infections by 10%. A
decrease in temperature and RH preceded the onset of infection.
Myatt et al. (2010) Model study of survival of aerosolized virus in single-family residences by
moisture control. Estimation of emission rates for virus was particle specic.
Sleep quality included tidal breathing and coughing.
Output of 0.16 kg/h water increased median sleeping hours AH/RH levels of
1119% compared to without a humidier present. The associated decrease
in inuenza virus survival was 1732%. Distribution of water through a
whole residence increased RH 312% and reduced inuenza virus survival
Noti et al. (2013) Infectivity of aerosolized virus was studied in a chamber with a manikin and
a coughing simulator emitting inuenza virus.
The infectivity was ca. 75% at RH 23%, but only 1522% at RH 43%.
Maintaining indoor air at RH > 40% will signicantly reduce the
infectivity of aerosolized virus.
Shaman and Kohn
Reanalysis of previous studies. AH provides a coherent physical explanation for variability of inuenza virus
survival and transmission. The transmission of virus decreases with vapor
Silva et al. (2014) Correlation of 11953 hospitalizations (adults and children) with respiratory
22% of infections in adult patients admitted to emergency departments were
caused by respiratory viral infections. Inuenza-like illness was associated
with AH, use of air conditioning, and presence of mold in home. Severe
acute respiratory infection cases were found to be negatively related to RH.
Ud-Dean (2010) Model work and prediction of survival (persistence) and transmission of
inuenza virus.
Example shows that at lower temperature low AH prevents disruption of the
virus. On the other hand, higher temperature and higher RH prevent
desiccation of the virus.
AH = absolute humidity. RH = relative humidity.
P. WolkoInternational Journal of Hygiene and Environmental Health 221 (2018) 376–390
symptoms. This should be seen in view some of the reported benecial
eects of reduced virus survival at elevated humidity in contrast to dry
and cold conditions; however, generalization is not possible. There are
anecdotal reports about dry airand problems among art performers
(e.g. singers); however, the few studies (Table 7) identied indicate that
extreme dry conditions may be detrimental in conjunction with vocal
fatigue and saline nebulization may be benecial. It should be con-
sidered that vocal comfort among young healthy subjects may dier
from elderly subjects. Furthermore, other pollutants, e.g. with de-
siccating properties, should be considered.
4. Discussion and conclusion
Reporting of dry airor drynesscontinues to be a major com-
plaint in oce-like environments, e.g. (Bluyssen et al., 2016;Brightman
et al., 2008;Reijula and Sundman-Digert, 2004) and anecdotal reports
about detrimental voice performance in dry conditions ourish in the
artistic milieu. This is surprising in view of the continued eort to de-
velop and use low emitting building materials and consumer products
during the last decades, e.g. by implementation of national labeling
schemes for emission testing (Wolko, 2003), a change to lower room
concentrations of VOCs by lower material emissions (Tuomainen et al.,
2003), and use of less volatile organic compounds (Weschler, 2009), if
the complaints are associated with indoor VOCs, as re-advocated by
Sundell and Lindvall (1993) and Fang et al. (2004). For instance, Qian
et al. (2016) argued that parentsperception of both dry and humid air
is associated with the presence of sensory irritants, especially dry air,
despite an organ of sensing humidity by inhalation is non-existing in
humans (Nagda and Rector, 2003;Wolkoand Kjærgaard, 2007). Thus,
from a semantic point of view dry airor humid air, that is dierent
from the symptom sensing of dryness(e.g. dry eyes), appears to be
composed of dierent perceptions and associated causes, e.g. rhinitis
sicca (Hildenbrand et al., 2011). Further, it is unclear whether per-
ceived humid or wet aircould be confused by the body sensation of
feeling humid (sweaty). Trigeminal nerve endings are known to re-
spond to innocuous cooling via activation of TRPM8 receptors
(Lumpkin and Caterina, 2007).
The Qian et al. study does not present data nor RH measurements
that support their statement about dry air(versus humid air) caused
by sensory irritants. From a toxicological point of view, however, re-
ported concentrations of VOCs in both oce and residential environ-
ments, in general, are orders of magnitude below thresholds for sensory
irritation in the eyes and airways, perhaps with the exceptions of for-
maldehyde and acrolein (e.g., Huang et al., 2017), known emitters from
construction products, combustion, and ozone-initiated reactions
(Salthammer et al., 2010).
The prevalence of dry airmore than doubled from one-man cel-
lular oce to open space oce; further, both eye and nose irritation
doubled (Pejtersen et al., 2006). Pejtersen et al. conclude: It seems that
perceived dry air is something dierent from humidity and there is a
need to validate this questionand Wiik (2011) conclude real cause of
the sensation of dry air is dusty air. One may speculate about the
sensation of odor (pungent or moldy) might trigger the sensation of
dry air, possibly in concert with the physiological eects of low RH on
the eyes and upper airways. Further, unrecognized pollutants could
play a role by themselves or in (synergistic) combination with the
former loads, also see below. Furthermore, if the sense of drynessis
caused by stimulation of trigeminal nerve endings, it is fair to speculate
that irritated or dry eyes may cross interact with nerve endings from the
nose and vice versa, cf. Baroody et al. (2008). On the other hand,
several epidemiologic studies have shown associations between low RH
and complaint rates and intervention studies have demonstrated the
benecial eect of elevating the humidity (Table 1). Furthermore, ag-
gravation of the eye tear lm stability by exposure to low RH results in
desiccation, hyperosmolarity and inammatory reactions in the eye
(Table 2 and Wolko(2017)). Thus, the merged information about the
impacts of VOCs and particles versus low RH favors the latter as an
important parameter to consider for assessment of eye and upper
airway complaints in oce-like environments.
The overall mechanistic picture is that dry (and cold) air desiccate
the airways leading to hyperosmolarity, which stimulates the sensory
nerves generating a reex response and possibly release of in-
ammatory biomarkers (Cruz and Togias, 2008). Further, dry air
challenge may increase peripheral airway resistance, airway surface
volume, and increase the osmolarity in the airways, which may initiate
airway constriction in canine, in case of no moisture neutralization
(Freed and Davis, 1999), and reduced mucociliary clearance time. This
may cause dryness of the mucocilia thus compromising its defense
mechanism from inuenza virus, which for inuenza viruses and others
has a greater survival time at low humidity and low temperature.
Furthermore, the defense mechanism may also be compromised by
aggressive air pollutants.
Synergistic eects may occur between low RH and air pollutants.
For instance, in a large cross-sectional study low RH and ozone were
associated with dry eye symptoms and dry eye diseases (Huang et al.,
2016). Further, evaporative dry eye rats were shown to be more sus-
ceptible to titanium dioxide nanoparticles than normal rats (Han et al.,
2017). It is reasonable to hypothesize that low RH has aggravated the
eye tear lm stability, thus becoming more susceptible to aggressive
chemicals like ozone or its reaction products with chemically reactive
VOCs or particles. Surface active compounds like benzalkonium
chloride and particles like quartz may also cause compositional changes
of mucus membranes (Zhao and Wollmer, 2001), thus becoming more
susceptible to low RH and aggressive pollutants and mimic dry air.
In conclusion, elevated RH may reduce complaint rates and favor
work performance in oces in comparison with very dry conditions,
but more information is needed to understand how humidity inuences
symptom reporting and the performance, especially among the elderly
Table 7
Eects on the vocal cord and voice.
Study Approach Observation
Hemler et al. (1997) Subjects (n = 8) exposed to dierent RH, dry (2%), normal (45%), and high
(100%) for 10 min at 2324 °C. Analysis of voice perturbation during
producing repeatedly a sustained/a/of controlled pitch and loudness.
Dry conditions increased voice perturbation compared to normal and humid
air. No dierence was observed between normal and humid air exposure.
Sivasankar et al.
Subjects (n =8) reporting vocal fatigue and (n = 8) matched controls were
tested. Phonation threshold pressure was measured during oral breathing in
humid environments.
Drying challenge may be detrimental to voice production in individuals with
vocal fatigue. It is suggested that short-term oral breathing may cause
dehydration to impair compensation.
Tanner et al. (2016) Young (n =10; 22 years) male singers and male non-singers (n = 10)
underwent double-blinded exposure to oral breathing laryngeal desiccation
challenge for 30 min using medical grade dry air (< 1% RH) followed by
nebulized isotonic saline (3 or 9 ml).
Self-perceived eort and dryness increased (worsened) after challenge and
decreased after the saline nebulization. No consistent changes were
observed for phonation threshold pressure and cepstral spectral index of
dysphonia for sustained vowels and connected speech, self-perceived vocal
eort, mouth and throat dryness. Young, vocally healthy men may not
experience physiologic changes in voice production associated with
laryngeal desiccation.
P. WolkoInternational Journal of Hygiene and Environmental Health 221 (2018) 376–390
population. Low RH aggravates the eye tear lm stability and phy-
siology, and the osmolarity of the upper airways; even slightly elevated
humidity may be benecial and relieve dry eye symptoms. Thus, per-
sonal adjustment of humidity and temperature appears to be the way
forward towards a satisfactory workplace. Furthermore, elevated hu-
midity may improve sleep quality and reduce eects on the vocal cord,
but more substantiation is required. Low and cold RH favors the sur-
vival and transmission of many inuenza viruses, but the issue is
complex for generalization of associated mechanisms, thus, more con-
trolled indoor eld experiences is warranted. Furthermore, better un-
derstanding is required how humidity inuences particle dynamics,
resuspension, and their physiological impact on the eyes and the air-
ways as function of their surface chemistry.
There is an increasing trend to apply AH rather than RH as a
parameter for comparison and identication of associations, also con-
sidering sometimes better correlation between outdoor and indoor AH
than between RHs. However, most of all, it is pertinent to distinguish
between elevated moisture (activity) in construction materials and
behind, elevated RH resulting in condensation on surfaces, and RH in
the breathing and ocular zone. Furthermore, there is a need to re-
consider the causes and physiological meaning of the semantic in-
correct and confusing dry/wet air parameter by identication of its
Declaration of interest
The author declares no conicts of nancial interest. No-one has
seen the manuscript before submission.
This work was supported by an internal grant from the National
Research Centre for the Working Environment Denmark and in part by
Realdania under the project CISBO (Centre for Indoor Climate and
Diseases in Dwellings, 20092015).
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