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Effect of an Air Cleaner with Electrostatic Filter on the Removal of Airborne House Dust Mite Allergens

  • Interbational school
  • Catholic Kwandong University, College of Medicine

Abstract and Figures

The effects of air cleaners on the removal of airborne indoor allergens, especially house dust mites (HDM), are still controversial. The objective of this study is to evaluate the effect of an air cleaner with an electrostatic filter on the removal of airborne mite allergens. A dried HDM culture medium that contained mite body particles and excretions was dispersed in a chamber equipped with an electrostatic air cleaner. The number of airborne particles was recorded continuously by a dust spectrometer for 60 minutes. Airborne particles in the chamber were collected on a sampling filter at a flow rate of 10 L/min and the Der f 1 concentration in the filter extracts was measured by two-site ELISA. The air cleaner efficiently removed airborne HDM particles. The air cleaner removed airborne HDM particles (size 2-12.5 µm) 11.4 ± 2.9 fold (cleaner operating for 15 minutes), 5.4 ± 0.7 fold (cleaner operating for 30 minutes), and 2.4 ± 0.2 fold (cleaner operating for 60 minutes) more than the removal of HDM particles by natural settle down. Removal kinetics differed according to the particle size of the airborne particles. The air cleaner decreased the concentration of Der f 1 in the extraction of airborne particles collected on the air sampling filter by 60.3%. The electrostatic air cleaner can remove airborne HDM allergens and may be useful as a supplementary environmental control tool for HDM sensitized respiratory allergic patients.
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Yonsei Med J Volume 51 Number 6 November 2010
Indoor air quality has received increasing attention in recent years due to the global
increase in allergic diseases. Researchers have investigated the relationship
between the level of environmental exposure to house dust mites (HDM) and
sensitization to and development of certain allergic diseases.1,2 Although naturally-
occurring aeroallergens are difficult to avoid, exposure to these allergens may be
reduced, often substantially, by environmental control measures such as encasing
bedding materials and pillows, air cleaners, and ventilation.3Use of an high
efficiency particle arrest (HEPA) air cleaner to reduce airborne allergens in the
indoor environment may provide clinical benefits for patients with respiratory
allergies; these HEPA air cleaners have been demonstrated to reduce levels of
Original Article DOI 10.3349/ymj.2010.51.6.918
pISSN: 0513-5796, eISSN: 1976-2437 Yonsei Med J 51(6):918-923, 2010
Effect of an Air Cleaner with Electrostatic Filter on the
Removal of Airborne House Dust Mite Allergens
Santosh Rani Agrawal,1* Hak-Joon Kim,2* Yong Won Lee,1Jung-Ho Sohn,1Jae Hyun Lee,1
Yong-Jin Kim,2Sung-Hwa Lee,3Chein-Soo Hong,1and Jung-Won Park1
1Division of Allergy and Immunology, Institute of Allergy, Department of Internal Medicine, Yonsei University College of Medicine, Seoul;
2Aerosol Laboratory, Environmental Machinery Research Division, Korea Institute of Machinery and Materials, Daejeon;
3Digital Appliance Company Research Laboratory, LG Electronics, Changwon, Korea.
Purpose: The effects of air cleaners on the removal of airborne indoor allergens,
especially house dust mites (HDM), are still controversial. The objective of this
study is to evaluate the effect of an air cleaner with an electrostatic filter on the
removal of airborne mite allergens. Materials and Methods: A dried HDM
culture medium that contained mite body particles and excretions was dispersed in
a chamber equipped with an electrostatic air cleaner. The number of airborne
particles was recorded continuously by a dust spectrometer for 60 minutes.
Airborne particles in the chamber were collected on a sampling filter at a flow rate
of 10 L/min and the Der f 1 concentration in the filter extracts was measured by
two-site ELISA. Results: The air cleaner efficiently removed airborne HDM
particles. The air cleaner removed airborne HDM particles (size 2-12.5 µm) 11.4 ±
2.9 fold (cleaner operating for 15 minutes), 5.4 ± 0.7 fold (cleaner operating for 30
minutes), and 2.4 ± 0.2 fold (cleaner operating for 60 minutes) more than the
removal of HDM particles by natural settle down. Removal kinetics differed
according to the particle size of the airborne particles. The air cleaner decreased
the concentration of Der f 1 in the extraction of airborne particles collected on the
air sampling filter by 60.3%. Conclusion: The electrostatic air cleaner can remove
airborne HDM allergens and may be useful as a supplementary environmental
control tool for HDM sensitized respiratory allergic patients.
Key Words: House dust mite, electrostatic air cleaner, environmental control
Received: March 8, 2010
Revised: May 5, 2010
Accepted: May 18, 2010
Corresponding author: Dr. Jung-Won Park,
Division of Allergy and Immunology,
Institute of Allergy, Department Internal
Medicine, Yonsei University College of
Medicine, 250 Seongsan-ro, Seodaemun-gu,
Seoul 102-752, Korea.
Tel: 82-2-2228-1961, Fax: 82-2-393-6884
*These authors contributed equally to this
The authors have no financial conflicts of
© Copyright:
Yonsei University College of Medicine 2010
This is an Open Access article distributed under the
terms of the Creative Commons Attribution Non-
Commercial License (
licenses/by-nc/3.0) which permits unrestricted non-
commercial use, distribution, and reproduction in any
medium, provided the original work is properly cited.
Removal of Airborne Mite Allergens by an Air Cleaner
Yonsei Med J Volume 51 Number 6 November 2010
airborne pet allergens.4,5 However, the ability of air clea-
ners to remove HDM and other allergens is still controver-
sial.4,5,6-10 Electrostatic air cleaners remove airborne parti-
cles by charging the airborne particles and then trapping
them on oppositely charged metal plates. The efficiency of
the HEPA air cleaner has been reported to be better than
electrostatic air cleaners, and most clinical studies with
allergic patients were done with HEPA air cleaners. How-
ever, HEPA cleaners are noisy and require regular chang-
ing of expensive HEPA filters. The objective of this study
was to evaluate the ability of an air cleaner equipped with
electrostatic filter to remove airborne HDM particles.
Study design
We evaluated the efficiency of electrostatic air cleaner by
comparing the concentrations of HDM particles in the
closed chamber measured by spectrophotometer and the
amount of HDM particles in the glass filter collected by
constant flow air sampler in the chamber. We compared
these two parameters measured at the on and off statuses
of the air cleaner.
Air cleaner and sample preparation
To create a source of house dust mite particles, we dried 30
g of culture medium containing Dermatophagoides farinae
at room temperature for 24 hours. D. farinae was cultured
at 25 ± 3˚C and 75 ± 2% relative humidity. One gram of
the culture medium contained 9.6 µg of Der f 1. An LG air
cleaner with an electrostatic filter (LG Electronic, Chang-
won, Korea), embedded into an air conditioner, was used
in this study. However, the air conditioner was off during
the experiment. The airflow of the air cleaner was 10
m3/min and it was equipped with two different filters: an
electrostatic filter and a fine mesh mechanical pre-filter.
The dust removal efficiency of the electrostatic filter and
fine mesh mechanical pre-filter was about 50% and 35%,
respectively, as calculated by the one-pass method using a
particle counter (PM2.5) at the velocity of 5.3 cm/sec.
Measurement of airborne particles using dust
A closed stainless steel chamber (30.4 m3) was first cleared
by air circulation and a HEPA filter system and the particle
concentration was monitored until it reached a baseline of
less than 104 particle/m3for 2-12.5 µm size range. A total
of 150 mg of the allergen particles were loaded into a
suction type particle feeder and then dispersed at 30 L/min
spraying velocity for 1 minute. The number of airborne
particles during each experiment was measured using a
particle sampling unit installed 1 m above the chamber
floor at the center of the chamber. The unit was connected
to an optical particle counter (Portable Aerosol Spectro-
meter, Model 1.109, Grimm Aerosol Technik, Ainring,
Germany) and data was analyzed using a particle concentra-
tion analyzing program (Version 3.20 build 6, Grimm Aero-
sol Technik, Germany). Distribution of the particles imme-
diately after dispersion of 0.15 gm of the medium is shown
Fig. 1. The distribution was expressed as a volume (µm3/m3)
or number of particles (/m3) per volume of the chamber. The
number of the airborne particles was recorded every minute
for 60 minutes. These experiments were done triplicate.
Calculation of Clean Air Delivery Rates (CADRs)
The air cleaning performance with the LG air cleaner was
represented by a Clean Air Delivery Rate (CADR) which
determines how well an air cleaner reduces airborne part-
icles.11,12 The CADR of the air cleaner to remove the air-
bone particles is based on the below formula.
Ct,i = Cie-kt
Abbreviations; where Ct, = concentration at time t
(particles/m3), k= decay rate constant (min-1), and t = time
Volume concentration (µm3/m3)
0.1 1 10 100
Particle size (µm)
Particle number (×103/m3)
0.1 1 10 100
Particle size (µm)
Fig. 1. Distribution of airborne particles per size immediately after dispersion of the 150 mg of culture media at the closed chamber. It was
expressed with the volume of the particles (A) or number of particles (B) per m3according to the size of the particles.
The decay constant, k, is obtained by fitting a linear
regression line to the slope of-ln(C(t)/C0), which is the
negative of the natural log of the time-varying particle
concentration (C(t)) normalized by the initial concentration
at the time the incense was extinguished (C0), versus time
(h). The CADR of airborne particles was calculated by the
below formula.
CADR = V×k
Abbreviations; V = volume of test chamber (m3), k=
measured decay rate, min-1.
Measurement of the concentration of Der f 1 from the
air sampling filters
Airborne particles in the closed chamber were collected on
glass fiber filter paper (Model GF/C Φ47 mm, Whatman,
UK, pore size; 1.6 µm) at a flow rate of 10 L/min for 30 mi-
nutes after the concentrations of HDM particles reached
the maximum values. During the experiments, the air clea-
ner and the air sampler worked simultaneously. The HDM
allergens were extracted from the filter paper with borate
buffered saline (BBS) (170 mM boric acid, 125 mM NaCl,
pH 7.0) for 18 hr at room temperature with gentle shaking.
The Der f 1 concentrations in each extracted solution were
measured by 2-site ELISA (Indoor biotechnologies, Man-
chester, UK) following the manufacturer’s recommenda-
tions, except that 3,3’-5,5’-tetramethylbenzidine (TMB,
Kirkegaard & Perry Laboratories, Gaithersburg, MD,
USA) were used as a substrate instead of ABTS. The
detection limit for Der f 1 was 60 pg/mL. These experi-
ments were done triplicate.
Statistical analysis
The Mann-Whitney U-test was used to compare the Der f
1 level or CADR value between the samples. The Wilcoxon
Sign Ranked test was used to compare the fold-removal
rate in comparison with the baseline. Significance was set
up at p< 0.05. All statistical analysis was performed with
SPSS 12.0 (SPSS Inc., Chicago, IL, USA). Bars in the
figures indicate the standard error of the mean.
Removal efficiency of the air cleaner measured by dust
HDM airborne particles between the size ranges of 10.0-
12.5, 5.0-6.5, and 2.0-2.5 µm were rapidly removed by the
Santosh Rani Agrawal, et al.
Yonsei Med J Volume 51 Number 6 November 2010
0 10 20 30 40
AC on
AC off
50 60
Time (min)
010 20 30 40
AC on
AC off
50 60
Time (min)
0 10 20 30 40
AC on
AC off
50 60
Time (min)
0 10 20 30 40
AC on
AC off
50 60
Time (min)
Fig. 2. Removal efficiency of the split electrostatic and fine mesh mechanical pre-filter air cleaner. The size and number of HDM particles were
measured using a dust spectrometer. (A) HDM particles in the size range 10.0 - 12.5 µm. (B) HDM particles in the size range 5.0 - 6.5 µm. (C) HDM
particles in the size range 2.0 - 2.5 µm. (D) HDM particles in the size range 0.25 - 0.28 µm. The bars in these figures represent standard error of the
mean. HDM, house dust mites.
split electrostatic and fine mesh mechanical air cleaner (Fig.
2). Fig. 2 shows the efficiency of the cleaner at different
time points. The cleaner removed airborne HDM particles
(size 2-12.5 µm) in the chamber 11.4 ± 2.9 fold (cleaner
operating for 15 minutes), 5.4 ± 0.7 fold (cleaner oper-
ational for 30 minutes), and 2.4 ± 0.2 fold (cleaner operat-
ing for 60 minutes) more than particle removal by natural
settling. By running the cleaner for 30 minutes, 79.9 ±
2.6% of HDM particles (2-12 µm) were removed compared
to the 9.7 ± 5.0% of HDM particles removed by natural
settling. Approximately 64.3% of the HDM particles (size
2-12.5 µm) were still airborne 60 minutes after the concen-
tration of the HDM particles, sprayed from the particle
feeder; use of the air cleaner reduced this value to 10%
(Fig. 3).
Measurement of CADRs
Significant differences in removal kinetics were found ac-
cording to the size of airborne particles. The air cleaner com-
pletely removed larger particles between 10-12.5 µm within
30 minutes. The CADRs were particle size-dependent and
the difference between the CADRs with and without the
air cleaner operation dramatically increased as the size of
the particles were larger (Fig. 4). Particles larger than 15
µm settled down so rapidly that they could not be accurately
measured using the dust spectrometer (data not shown).
Measurement of airborne Der f 1 with or without air
The concentration of airborne Der f 1 in the chamber was
markedly decreased by the air cleaner as shown in Fig. 5.
The Der f 1 concentration in the extract solution of the air-
sampling filter was 47.2 ± 7.5 ng/mL when the air cleaner
was used, as compared to 118.75 ± 14.12 ng/mL when the
air cleaner was not used, indicating that the air cleaner
decreased the Der f 1 concentration by 60.3%.
Our study shows that the use of an air cleaner with electro-
static filters can reduce the exposure to HDM particles of
several sizes. Air cleaners can reduce airborne Can f 1 or
Fel d 1,4-5,10 but few studies have investigated the ability of
these cleaners to remove airborne or settle down HDM
allergens, and the clinical effects of air cleaners for reduc-
ing the exposure to HDM are questionable.3,6,7 In this study,
an air cleaner with electrostatic filters removed larger
HDM particles of sizes (10-12.5 µm) within 30 minutes
and markedly reduced the concentration of smaller particles
Removal of Airborne Mite Allergens by an Air Cleaner
Yonsei Med J Volume 51 Number 6 November 2010
% Removal
0.25 - 0.28
2.0 - 2.5
Particle size (µm)
5.0 - 6.5
10.0 - 12.5
2 - 12.5
100 Off
% Removal
0.25 - 0.29
2.0 - 2.5
Particle size (µm)
5.0 - 6.5
10.0 - 12.5
2 - 12.5
100 Off
% Removal
0.25 - 0.28
2.0 - 2.5
Particle size (µm)
5.0 - 6.5
10.0 - 12.5
2 - 12.5
100 Off
Fold increase for removal
0.25 - 0.29
2.0 - 2.5
Particle size (µm)
5.0 - 6.5
10.0 - 12.5
2 - 12.5
20 15 min
30 min
60 min
Fig. 3. Removal efficiency of the air cleaner expressed as the percentage removal of airborne HDM particles in the chamber. The baseline value is
the concentration of particles immediately after spraying. (A) Removal efficiency after 15 minutes. (B) Removal efficiency after 30 minutes. (C)
Removal efficiency after 60 minutes. Black bars in A-C represent the rate of settlement of HDM particles. D shows the fold-increase in HDM
removal by the cleaner compared to the natural settlement. The dotted line represents 1 fold indicating natural settlement. Bars in these figures
represent standard errors of the mean. *p< 0.05.
(2-10 µm) within 60 minutes. These results may suggest
that electrostatic air cleaners may reduce the exposure of
inhabitants to HDM allergens. As the removal of the air-
borne particles by an electrostatic cleaner is governed by
impaction, electrical charging and interception of the air-
borne particles on the filter, efficiency of removal of air-
borne particles depends on the physical size of the parti-
cles. Generally, removal of larger particles is easier than
that of smaller particles.13 The CADR values for airborne
particles in the ranges of 5-12.5 µm, critical ranges for
HDM allergens, were superior to the values of smaller
particles in this study. It is noteworthy that 60% of airborne
Der f 1 was removed by the air cleaner within 30 minutes.
More than 80% of airborne particles, including HDM
fecal material, are usually larger than 10 µm14-16 and the fecal
material has been recognized as the major sources of the
HDM allergen.17 These findings raise a question of how
HDM aggravates asthma in patients with HDM sensitiza-
tion. Aerodynamically larger particles (> 10 µm) usually do
not reach the peripheral airway, and encasings with pore
sizes less than 10 µm are effective in blocking dust mite
allergens.18 However, Svartengren, et al.19 demonstrated that
4% to 15% of inhaled large Teflon particles ranging from
8.2 to 13.7 µm were deposited in the alveoli, confirming
that large particles can be deposited in the peripheral air-
ways. Although the majority of HDM particles in homes
are larger than 10 µm, this does not decrease the signifi-
cance of small HDM particles in asthmatic patients. Cus-
tovic, et al.20 showed that group 2 dust mite allergens are
carried not only on large particles but also on small
particles (< 5 µm). Furthermore, HDM had more than 15
major allergens, and the aerodynamic characteristics of
these allergens may be different. HDM allergens have
been shown to be present in the lower airway of most HDM
sensitized asthmatic patients. HDM allergens have been
identified in BAL fluid at low concentrations.21 De Lucca,
et al.22 demonstrated that Der p 1 is associated with parti-
cles not ranging from feces, to fibers, to flakes, which are
all sources of inhaled mite aeroallergens. The particle sizes
of the culture medium used in this study was not that
different from the HDM particles in the house dust. The
median size of the particles (in the aspect of volume concen-
tration) used in this study fell in the range of 4-8 µm, and a
considerable amount of particles were larger than 10 µm.
Although the air cleaner cannot efficiently reduce ex-
posure to HDM particles during sleep, settled dust is likely
to be disturbed by various living activities of inhabitants
during the day, and under these circumstances, the air
cleaner may protect inhabitants from exposure to HDM.
The HEPA filter can remove 99.9% of airborne particles
0.3 µm in size by passing through the filter, but the perfor-
mance of the air cleaner is expressed as the CADR that
takes into account both the flow rate through the air clea-
ner and the filter efficiency.23,24 HEPA air cleaners have
some weak points. Noise is one drawback of the HEPA air
cleaner, and it requires high energy costs. Pressure drop is
another drawback. As the air particles are deposited into
the HEPA filter, the air flow rate decreased. In spite of a
lower filtration efficiency of the electrostatic filter than that
of the HEPA, the electrostatic filter may permit higher air
flow rates due to a smaller pressure drop than that with the
HEPA filter. Furthermore, we showed that the levels of
airborne Der f 1 could be reduced using an electrostatic air
cleaner in this study.
This study has several limitations. Measurements were
taken in a closed chamber in which HDM particles from
the culture medium had been sprayed. These experimental
conditions do not reflect a real living environment. Further-
more, we only measured indirect parameters to assess the
effects of the air cleaner. Further clinical studies are need-
Santosh Rani Agrawal, et al.
Yonsei Med J Volume 51 Number 6 November 2010
CADR (m3/min)
0 42 86 1210
Airborne particle size (µm)
Air cleaner on
Air cleaner off
Fig. 4. CADR values of the electrostatic and fine mesh mechanical pre-filter air
cleaner according to the size of airborne particles. CADR values are airborne
particle size dependent. The bars in this figure represent standard error of the
mean. *p< 0.05. CADR, Clean Air Delivery Rate.
Der f 1 (ng/mL)
Air cleaner
Off On
Fig. 5. Concentration of Der f 1 in extracts from the air sampling filters installed in
a closed chamber with or without air cleaner. The bars in this figure represent
standard error of the mean. *p< 0.05.
Removal of Airborne Mite Allergens by an Air Cleaner
Yonsei Med J Volume 51 Number 6 November 2010
ed to determine the clinical usefulness of the air cleaner.
In conclusion, an electrostatic air cleaner can remove
airborne HDM allergens and may be useful for the mana-
gement of respiratory allergies in patients sensitized to
HDM allergens.
This study was supported by grant from LG Electronics.
1. Sporik R, Holgate ST, Platts-Mills TA, Cogswell JJ. Exposure to
house-dust mite allergen (Der p I) and the development of asthma
in childhood. A prospective study. N Engl J Med 1990;323:502-7.
2. Wahn U, Lau S, Bergmann R, Kulig M, Forster J, Bergmann K,
et al. Indoor allergen exposure is a risk factor for sensitization
during the first three years of life. J Allergy Clin Immunol 1997;
3. Eggleston PA. Improving indoor environments: reducing allergen
exposures. J Allergy Clin Immunol 2005;116:122-6.
4. Wood RA, Johnson EF, Van Natta ML, Chen PH, Eggleston PA.
A placebo-controlled trial of a HEPA air cleaner in the treatment
of cat allergy. Am J Respir Crit Care Med 1998;158:115-20.
5. Green R, Simpson A, Custovic A, Faragher B, Chapman M,
Woodcock A. The effect of air filtration on airborne dog allergen.
Allergy 1999;54:484-8.
6. Reisman RE, Mauriello PM, Davis GB, Georgitis JW, DeMasi
JM. A double-blind study of the effectiveness of a high-efficiency
particulate air (HEPA) filter in the treatment of patients with
perennial allergic rhinitis and asthma. J Allergy Clin Immunol
7. Antonicelli L, Bilò MB, Pucci S, Schou C, Bonifazi F. Efficacy
of an air-cleaning device equipped with a high efficiency parti-
culate air filter in house dust mite respiratory allergy. Allergy
8. Reisman RE. Do air cleaners make a difference in treating aller-
gic disease in homes? Ann Allergy Asthma Immunol 2001;87:
9. Sheikh A, Hurwitz B, Shehata Y. House dust mite avoidance mea-
sures for perennial allergic rhinitis. Cochrane Database Syst Rev
10. Sulser C, Schulz G, Wagner P, Sommerfeld C, Keil T, Reich A,
et al. Can the use of HEPA cleaners in homes of asthmatic children
and adolescents sensitized to cat and dog allergens decrease
bronchial hyperresponsiveness and allergen contents in solid dust.
Int Arch Allergy Immunol 2009;148:23-30.
11. Waring MS, Siegel JA, Corsi RL. Ultrafine particle removal and
generation by portable air cleaners. Atmos Environ 2008;42:
12. Association of Home Appliance Manufacturers (AHAM).
AHAM method for measuring performance of portable house-
hold electric room air cleaners. ANSI/AHAM AC-1-2006; Revi-
sion of ANSI/AHAM AC-1-2002 and AHAM AC-1-2005. Wa-
shington DC: 2006. p.16-9.
13. Hinds WC, Aerosol technology: properties, behaviors, and
measurement of airborne particles. New York: Wiley-Interscience
14. Tovey ER, Chapman MD, Wells CW, Platts-Mills TA. The distri-
bution of dust mite allergen in the houses of patients with asthma.
Am Rev Respir Dis 1981;124:630-5.
15. de Blay F, Heymann PW, Chapman MD, Platts-Mills TA. Air
borne dust mite allergens: comparison of group II allergens with
group I mite allergen and cat-allergen Fel d 1. J Allergy Clin Im-
munol 1991;88:919-26.
16. Platts-Mills TAE, Heymann PW, Longbottom JL, Wilkins SR.
Airborne allergens associated with asthma: particle sizes carrying
dust mite and rat allergens measured with a cascade impactor. J
Allergy Clin Immunol 1986;77:850-7.
17. Tovey ER, Chapman MD, Platts-Mills TA. Mite faeces are a
major source of house dust allergens. Nature 1981;289:592-3.
18. Vaughan JW, McLaughlin TE, Perzanowski MS, Platts-Mills
TA. Evaluation of materials used for bedding encasement: effect of
pore size in blocking cat and dust mite allergen. J Allergy Clin
Immunol 1999;103:227-31.
19. Svartengren M, Falk R, Linnman L, Philipson K, Cammer P.
Deposition of large particles in human lung. Exp Lung Res 1987;
20. Custovic A, Woodcock H, Craven M, Hassall R, Hadley E,
Simpson A, et al. Dust mite allergens are carried on not only
large particles. Pediatr Allergy Immunol 1999;10:258-60.
21. Ferguson P, Brodie DH. Environmental and bronchoalveolar
lavage Dermatophagoides pteronyssinus antigen levels in atopic
asthmatics. Am J Respir Crit Care Med 1995;151:71-4.
22. De Lucca S, Sporik R, O’Meara TJ, Tovey ER. Mite allergen (Der
p 1) is not only carried on mite feces. J Allergy Clin Immunol
23. Fox RW. Air cleaners: a review. J Allergy Clin Immunol 1994;
24. Shaughnessy RJ, Sextro RG. What is an effective portable air
cleaning device? A review. J Occup Environ Hyg 2006;3:169-81.
... The primary health effects of high humidity are caused by the growth and spread of biotic agents, although humidity interactions with non-biotic pollutants, such as formaldehyde, may also cause adverse effects [22]. Humidity sensor should provide its level in the incubator in terms of relative humidity (%RH) in the range of 0-100% RH [23]. Few investigations have shown that the bodyweight and insensible water loss are inversely proportional. ...
An infant incubator in the neonatal intensive care unit (NICU) is a medical instrument of care that provides oxygen, warmth and moisture to a newborn baby. Due to environmental conditions affecting the infants foster babies may experience discomfort and pain at some point. Thus, this study aimed to assess ambient air quality in neonatal incubators to improve the environmental quality of neonatal intensive care units and safety. Air pollutants concentrations consisting of particulate matter (pm2.5, pm10), hydrocarbons (HOCH), volatile organic compounds (VOC), air quality index (AQI), humidity and temperature, were measured at four selected Baghdad hospitals (Al-Karkh and Rusafa) . The results showed that the increase in relative humidity (RH%) measured in all baby incubators and in all locations during the summer and winter seasons contributed to an increase in polluting gases emission inside the incubators, which was the highest measurement that gave significant difference for the incubator.The highest recorded measurement with significant difference for pm2.5 was 73.78 ± 0.096 in site2 in winter, pm10 was 106.73 ± 0.05 in site1 in winter, HCHO was 0.148 ± 0.005 in site3 in winter, VOC was 673 ± 0.005mg/m3 in site2 in winter, AQIwas 177.25 ± 0.5 in site2 in winter, RH% was 53.887 in the site3 during the winter and temperature was 32.25°C in site3 in summer. It can be concluded that there is a relationship between relative humidity and temperature that affectedthe gases concentrations as it showed variations in gases and particle matters concentrations which affected the air quality inside the neonatal incubatorsthat in turn affected the health of the neonates, especially increased severity of asthma attacks and low Intelligence quotient (IQ) in children in the future.
... Due to the fact that the amount of allergen used in the fabric exposure experiment could potentially affect the outcome 22,23 and due to a lack of a consistent ratio for amount of allergen to container size in the literature, four separate trials with differing pollen amounts were conducted, which were 25mg, 10mg, 5mg, and 2mg of pollen. 24 Fabric pollen exposure experiments were conducted inside a glass cylinder with a dimension of 11inch height and 5 inch diameter. A stereoscope (Swift Stereo-eighty 680962) was used to quantify the pollen count on fabrics. ...
The purposes of this study are to determine if there is a relationship between fabric parameters such as electrostatic charge and moisture regain on the attraction of Timothy grass pollen, and to determine the effects of various fabric treatments on their pollen attraction. Half of the fabrics were treated with acidic acid (vinegar), basic (ammonia), and fabric softener. Both untreated and treated fabrics were exposed to Timothy grass pollen. The pollen counts of both untreated and treated fabrics were then determined. The results showed that untreated wool was found to be the least attraction to Timothy grass pollen while nylon was the most attracted to this pollen. The relationship between electrostatic cling time and pollen attraction was not established in the fibers studied except nylon, which had the highest cling time and the highest pollen count. When examining the relationship between fiber moisture regain and their pollen attraction, the results showed that other than wool, which had the highest MR and the lowest pollen count, no consistency was found between the MR and pollen count for the rest of the fibers tested. For fabric treatments, the results showed that dilute acidic acid was found to be most effective in reducing the pollen counts in all fiber fabrics except made no change in wool. Fabric softener was found to be the least effective treatment in lowering the fabric pollen attraction on all fiber fabrics except nylon.
... Stillerman et al.71 tested the effects of air filtration on mite, cat and dog allergens, but focused on the breathing zone and pillow encasements were also included, whilst Punsmann et al.50 studied dog, cat and mites allergens, but that study was limited to one home. A study with electrostatic air cleaners was carried out by Agrawal et al.,72 who reported a 60.3% reduction for Der f 1. However, they used cultures of D. farinae and the experiments were performed in a chamber, whilst our study was done in real bedrooms.To our knowledge, the study presented here is the most extensive study focused exclusively on portable air filtration efficiency in bedrooms covering a wide range of airborne features: six fractions for PM, including UFP and four allergens. ...
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Indoor allergens (i.e. from mite, cat and dog) are carried by airborne particulate matter. Thus, removal of particles would reduce allergen exposure. This work aims to assess the performance of air filtration on particulate matter and thus allergen removal in 22 bedrooms. Indoor air was sampled (with and without air filtration) with a cascade impactor and allergens were measured using enzyme-linked immunosorbent assay (ELISA). Particulate matter (including ultrafine particles) was also monitored. The median of allergen reduction was 75.2% for Der f 1 (p < 0.001, n = 20), 65.5% for Der p 1 (p = 0.066, n = 4), 76.6% for Fel d 1 (p < 0.01, n = 21) and 89.3% for Can f 1 (p < 0.01, n = 10). For size fractions, reductions were statistically significant for Der f 1 (all p < 0.001), Can f 1 (PM>10 and PM2.5–10, p < 0.01) and Fel d 1 (PM2.5–10, p < 0.01), but not for Der p 1 (all p > 0.05). PM was reduced in all fractions (p < 0.001). The allergens were found in all particle size fractions, higher mite allergens in the PM>10 and for pet allergens in the PM2.5–10. Air filtration was effective in removing mites, cat and dog allergens and also particulate matter from ambient indoor air, offering a fast and simple solution to mitigate allergen exposome.
... Air purifiers relieves symptoms of asthma, eliminates harmful chemicals, neutralizes unpleasant odors, reduces the chance of air borne diseases and improves sleep 11 . HEPA filters are specialized filter which filters all types of allergens with an accuracy of 99.97% 10 . ...
Background: The allergic rhinitis is one of the most miserable disease and most of the sufferers waste a lot of time in place of earning and at home. The main treatment of this disease is avoidance of allergen. There are many different ways to avoid allergens but it is not acceptable for many patients in the long term. These are use of nasal filter and face mask, change of job, change of city etc. Aim: To assess the effectiveness of air purifier in the management of allergic rhinitis. Methods: As this is not possible in many cases, doctors prescribe different medical treatments. Systemic therapy includes different types of antihistamines, immunotherapy and steroids such as nasal sprays and topical decongestants. Every treatment modality has its own complications like topical decongestants cause rhinitis medicamentosa which is very difficult to manage. Steroids are prescribed in oral or injectable form. It causes hypertension, gastritis and peptic ulcers, re-distribution of body fat, Cushing’s syndrome and hirsuitism. Moreover patient has lack of concentration, sleepiness and dryness of nose. In long term follow up these medical interventions are not effective as many patients are non compliant due to side effects of these treatment modalities. Now in this new era it is possible to clean our room air from allergens and other agents by different methods, which can prevent the allergic rhinitis. Results: We are dealing with only nasal allergies and not vasomotor rhinitis, which is a completely different disease as it involves autonomic nervous system. We advised patient to purchase and use air purifier at home and during duty in the office. Patient may use special mask containing HEPA (High Frequency Particulate Air Filter) filter when they are working outside Conclusion: Air purifier machine contains HEPA filter, which sucks the air of room and remove the particles of size up to, even viruses and bacteria are also removed. Now patients feel comfortable in this healthy environment and sleep without any symptoms like itching, rhinorrhoea, blocked nose and eye problems resulting in improved quality of life. Keywords: Allergic rhinitis, Particulate matter, Air purifier
... Electronic air cleaners (EACs) are another type of portable room air purifier that apply an electrical charge to eliminate airborne particles. Electrostatic precipitators (charged-media filters) based on pairs of oppositely charged plates trap particles when air passes through them (Agrawal et al., 2010). The non-ionizing electronic air filter type installed in EACs can reduce the risk of respiratory irritation and asthma symptoms due to the absence of ozone (Gent et al., 2003;Hood, 2005) and so is suitable for animate spaces. ...
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Exposure to bioaerosol in hospitals is associated with health effects but in the field of veterinary hospitals there are limited articles on bioaerosol concentration especially in Southeast Asia. This investigation of aerosol bacteria (cross-sectional study) was conducted in two veterinary teaching hospitals located in Thailand. Airborne bacteria were collected from air samples in four room types (reception hall, intensive care unit, outpatient department and in-patient hospital department) in each hospital using a sieve impactor air sampler at different periods (8:00-10:00, 10:00-12.00 and 13:00-15:00). The results revealed high bacterial contamination in all collected air samples. The average levels of total viable bacteria count were >500 colony forming units (CFU)/m 3 in all rooms but at some periods aerosol bacteria were <500 CFU/m 3. Also, in the late morning and afternoon period, aerosol bacteria increased from early morning period. Further investigation on the experimental efficacy of two different types of air purifier (also called cleaners in some of the literature) was tested to identify an alternative apparatus requiring limited space and competent for a high concentration of odor and animal fur. The non-ionized air purifier for animate space and the ozone generator air purifier for inanimate space significantly (p<0.05) reduced the aerosol bacterial concentration.
... This finding is expected, given that the efficiency of HEPA filters is > 99.97% (American National -Conditioning et al., 2013). Electrostatic filters reduce PM by charging and trapping PM on oppositely charged plates (Agrawal et al., 2010). It would be interesting to distinguish between the effects of HEPA and electrostatic HVAC filters on indoor BC. ...
... The concentrations of Der p 1 and Der f1 collected with the static method (constant collection over a full month) significantly decreased after initiating the air purifier. This is consistent with the data of Agrawal et al, 12 which reveal that removal of dust mite from the air will diminish surface allergen by assuring that allergen sedimentation rate is decreased. It should be noted that our research used HEPA air filtration, while the one conducted by Agrawal et al 12 used electrostatic air cleaners. ...
Background: With the rising prevalence of allergic rhinitis, the utility of indoor environmental management deserves increasing scrutiny. This research aims at evaluating the ability of air purifiers to be a therapy of allergic rhinitis. Methods: 32 subjects (25±13.5 years old) diagnosed with allergic rhinitis were selected and HEPA air purifiers placed in their bedrooms for 4 months. Before the intervention and each month, dust samples were collected with a vacuum cleaner and the dust collector assessed for allergen content. Additionally, static dust collectors were left in place all month to collect dust by sedimentation. Particulate matter (PM) was assessed in terms of PMindoor/outdoor ratios. The Rhinitis Quality of Life Questionnaire (RQLQ) was used to assess symptoms. Results: Der p 1 (78 (30,82) ng/g) was the dominant dust mite allergen in air samples of patients' bedroom as well as static collections. Der f1 (444 (345,667) ng/g) was the dominant allergen in bedding. Der f1 levels in both air and bed sampling significantly decreased after initiation of HEPA air purifiers (P<0.05). PM1.0indoor/outdoor, PM2.5indoor/outdoor, PM10indoor/outdoor all decreased (P<0.001) with the HEPA filtration intervention. According to RQLQ data, HEPA filtration was associated with improvements in activity limitation, non-nasal-eye symptoms, practical problems, and nasal symptoms (P<0.001). Conclusion: HEPA air purifiers can effectively reduce PM and HDM allergen concentration in the indoor air, and thereby improve clinical manifestations of patients with AR.
... Most daycare centers in Korea are at risk of indoor pollution due to fluxes in external sources, especially during spring season when yellow dust or particulates are common. Using air purifiers seems to be one of the most effective ways to maintain a healthy indoor environment, as they reduce particulate pollutants in the air and remove contaminants such as allergens (Agrawal et al. 2010). In this study, the use of an air purifier also resulted in lower concentrations of PM 10 and AB than with no air purifier. ...
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The purpose of this study was to measure concentrations of PM10 and airborne bacteria (AB) to determine how each one of them correlated with particular indoor environmental factors and characteristics of daycare centers in Seoul, South Korea.PM10 and AB were sampled in 330 daycare centers in the middle of a classroom, along with measurements of temperature and relative humidity. Spearman’s correlation and Mann-Whitney analyses were used to examine the relationship among and differences between PM10 concentration, AB concentration, indoor environmental factors, and daycare center characteristics.There were significant correlations between PM10 concentration and AB concentration (r = 0.128, p < 0.05), temperature (r = 0.153, p < 0.01), and relative humidity (r = 0.185, p < 0.01). PM10 concentrations with two or more windows; a heating, ventilation, and air conditioning (HVAC) system; and the use of air purifier were lower than concentrations with one window, window ventilation only, and no air purifier. AB concentrations were significantly higher when daycare centers had only one window and used only window ventilation (p < 0.05).Though there are many uncontrollable outdoor environmental factors that influence air quality, we demonstrated that using an HVAC system and an air purifier significantly reduces PM10 concentrations and is a practical change that could be implemented to improve the indoor air quality of daycare centers.
Background: Allergic rhinitis (AR) is characterized by distinct clinical heterogeneity and allergic sensitization patterns. We aimed to quantify rhinitis symptoms in patients with Self-reported allergic rhinitis according to the potential sensitization patterns for relevant allergens in China. Methods: We used a subset of structural equation modeling (LCA) to independently cluster patients into different patterns of atopic sensitization in an unsupervised manner, based on sIgE tests. AR symptom severity was assessed by the visual analogue scale. We evaluated the association between the severity of AR and the allergen sensitization patterns. Results: LCA revealed 4 phenotypes of atopic sensitization among 967 patients with self-report AR. We labeled latent classes as: (1) weed pollens and indoor sensitization (n = 74 [7.7%]); (2) weed pollen with low indoor sensitization (n = 275 [28.4%]); (3) low or no sensitization (n = 350 [36.2%]); (4) HDM dominated sensitization (n = 268 [27.7%]). AR was more severe in Class 2 compared to the other 3 classes, indicating that upper respiratory symptoms are more severe among patients with isolated seasonal rhinitis. Conclusion: We have identified 4 sensitization patterns in Patients with Self-reported AR, which were associated with different clinical symptoms and comorbidities. This article is protected by copyright. All rights reserved.
This study involved a seasonal exposure assessment in a hospital environment using several air quality indicators including carbon monoxide (CO), carbon dioxide (CO2), particulate matter (PM10 and PM2.5), and total volatile organic compounds (TVOCs). We examined the distribution of and variation in the indoor and outdoor pollutant concentrations in 12 working areas across three hospitals, with an emphasis on capturing seasonal variations. We assessed correlations between measured indoor and outdoor levels to quantify the importance of indoor sources on air quality relative to outdoor sources. Our results indicated that while indoor and outdoor CO levels were below air quality standards/guidelines, measured PM2.5 and PM10 concentrations at several locations exceeded the standards 2- to 3.5-fold. We generally recorded higher indoor PM levels during the warm season, particularly during regional desert storm events. The ingress of particles from the outdoor to indoor environment was evident with high correlations between indoor and outdoor PM2.5 (r between 0.83 and 0.92) and PM10 (r between 0.74 and 0.86) levels, particularly during the warm season. Indoor to outdoor (I/O) ratios of PM2.5 and PM10 were mostly < 1. In contrast, indoor levels of CO2 and TVOCs exceeded outdoor concentrations during both the warm and cold seasons with I/O ratios >1 across all sampling locations. Our paper concludes with implications of high PM exposure and a suggested management framework for limiting such exposure in hospitals.
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Exposure and sensitization to pet allergens are associated with allergic asthma in children. Conflicting data have emerged regarding the potential benefit of air cleaners with respect to a reduction of indoor pet allergens and bronchial hyperresponsiveness (BHR). In a randomized controlled trial 36 asthmatic children with sensitization to cat and/or dog and significant exposure to cat and/or dog allergen (Fel d 1 and/or Can f 1 >500 ng/g of carpet dust) were included in order to study the effect of high-efficiency particulate arresting (HEPA) air cleaners placed in the living room and bedroom compared with the effect of sham air cleaners. Patients were allocated to two groups: group 1 exposed to active filters and group 2 exposed to sham filters. At month 0, after 6 months and after 12 months, pulmonary function testing and cold air challenge were performed, serum eosinophil cationic protein (ECP) and specific IgE to seven aeroallergens were determined, and carpet dust samples and filters were collected. Major pet allergen concentrations (Fel d 1, Can f 1) were determined in filters and bulk dust samples. Thirty patients completed the study. After 6 and 12 months, there was no significant change in delta FEV(1) after cold air challenge, or in the use of medication and serum ECP levels. However, there was a trend in the active group towards an improvement of bronchial hyperresponsiveness, whereas the sham filter group showed a deterioration of BHR. Although HEPA air cleaners retained airborne pet allergens, no effect on disease activity or allergen concentrations in bulk dust samples was observed in this study.
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Twenty-four nonsmoking males, all without history of pulmonary disease, were randomly divided into four groups of six subjects each. The subjects in each group inhaled monodisperse Teflon particles labelled with 111In (half-life 2.83 days); 8.2, 11.5, 13.7 and 16.4 micron aerodynamic diameter, respectively. Radioactivity in head and throat, lung and stomach was determined after 0, 3 and 24 hrs using a profile scanner. For some subjects radioactivity was also determined using a whole-body scanner at 3.5 and 24 hrs. After the 24-hr determination the subjects inhaled labelled Teflon particles again, this time with a filter in front of the mouth. Average values for total deposition in the body, obtained using a profile scanner, whole-body scanner and filter measurements, agreed fairly well. Lung retention values obtained by whole-body and profile scanning also agreed well. The average deposition in the lung, expressed as a percentage of total deposition, was 49, 31, 21 and 13% for the four particle sizes (8.2-16.4 micron). Alveolar deposition, determined as retention at 24 hrs and expressed in percent of total deposition, was 15, 4, 4 and 1%. For the smallest particle sizes the deposition values agreed with earlier investigations. However, for the larger particles the two deposition values were higher than expected when compared to earlier studies.
Portable air cleaners can both remove and generate pollutants indoors. To investigate these phenomena, we conducted a two-phase investigation in a 14.75 m3 stainless steel chamber. In the first phase, particle size-resolved (12.6–514 nm diameter) clean air delivery rates (CADR) and efficiencies were determined, as were ozone emission rates, for two high-efficiency particle arresting (HEPA) filters, one electrostatic precipitator with a fan, and two ion generators without fans. The two HEPA air cleaners had count average CADR (standard deviation) of 188 (30) and 324 (44) m3 h−1; the electrostatic precipitator 284 (62) m3 h−1; and the two ion generators 41 (11) and 35 (13) m3 h−1. The electrostatic precipitator emitted ozone at a rate of 3.8±0.2 mg h−1, and the two ion generators 3.3±0.2 and 4.3±0.2 mg h−1. Ozone initiates reactions with certain unsaturated organic compounds that produce ultrafine and fine particles, carbonyls, other oxidized products, and free radicals. During the second phase, five different ion generators were operated separately in the presence of a plug-in liquid or solid air freshener, representing a strong terpene source. For air exchange rates of between 0.49 and 0.96 h−1, three ion generators acted as steady-state net particle generators in the entire measured range of 4.61–157 nm, and two generated particles in the range of approximately 10 to 39–55 nm. Terpene and aldehyde concentrations were also sampled for one ion generator, and concentrations of terpenes decreased and formaldehyde increased. Given these results, the pollutant removal benefits of ozone-generating air cleaners may be outweighed by the generation of indoor pollution.
Airborne particles are present throughout our environment. They come in many different forms, such as dusts, fumes, mists, smoke, smog, or fog. These aerosols affect visibility, climate, and our health and quanlity of life. This book covers the properties, behaviour, and measurement of aerosols. This is a basic textbook for people engaged in industrial hygiene, air pollution control, radiation protection, or environmental science who must, in the practice of their profession, measure, evaluate, or control airborne particles. It is written at a level suitable for professionals, graduate students, or advanced undergraduates. It assumes that the student has a good background in chemistry and physics and understands the concepts of calculus. Although not written for aerosol scientists, it will be useful to them in their experimental work and will serve as an introduction to the field for students starting such careers. Decisions on what topics to include were based on their relevance to the pratical application of aerosol science, which includes an understanding of the physical and chemical prinicples that underlie the behaviour of aerosols and the instruments used to measure them. (from preface)
The properties and behavior of suspended particles (dust, smoke, clouds), and the physical principles underlying their behavior are covered. Applications such as filtration, respiratory deposition, sampling, and the production of test aerosols are discussed. Physical analysis rather than mathematical analysis is emphasized.
Background: The purpose of the study was to investigate the influence of environmental allergen exposure on allergic sensitization in infancy and early childhood. Methods: A cohort of 1314 newborns was recruited and followed up prospectively at the ages 12, 24, and 36 months. The levels of major mite (Der p 1 and Der f 1) and cat (Fel d 1) allergens were determined from domestic carpet dust samples by sandwich ELISA. Specific serum IgE antibodies to mite and cat allergens were determined by CAP fluoroimmunoassay (Pharmacia). Logistic regression was used to assess the effects of allergen exposure, age, family history, and cord blood IgE simultaneously on the risk of sensitization. Results: Children, who had been found to be sensitized at least once during the first 3 years of life, were found to be exposed to significantly higher house dust mite (median, 868 ng/gm vs 210 ng/mg; p = 0.001) and cat (median, 150 ng/gm vs 64 ng/gm; p = 0.011) allergen concentrations in domestic carpet dust compared with the group without sensitization. In homes with low (< or = 25th percentile) dust concentrations, the risk of sensitization to mite (1.6%), and cat (2.0%) is low, compared with 6.5% for mite and 6.3% for cat if the domestic exposure is above the 75th percentile. The dose-response relationships between allergen levels and sensitization indicate that the increase in sensitization risk at low allergen levels is more pronounced in cat allergy (p = 0.002) than in mite allergy (p = 0.026). In the group with a positive family history, lower mite and cat allergen concentrations are needed to achieve specific sensitization compared with the group with a negative family history. Conclusion: Our data indicate that avoidance measures in the domestic environment aimed at the primary prevention of allergen-driven sensitization should be introduced at the earliest possible stage, if possible during infancy.
The form in which allergens become airborne is important because it may influence both symptoms caused by allergen exposure and methods used to reduce exposure. The group I allergens from dust mites only become airborne during disturbance and fall rapidly, which is in keeping with their being carried on fecal pellets. Their mean size is approximately 20 microns in diameter. By contrast, the cat-allergen Fel d I is airborne on particles varying from greater than 10 to less than 2 microns in diameter, some of which remain airborne even without disturbance. A second group of mite allergens, molecular weight 14,000, are equally important and are associated predominantly with mite bodies. With a monoclonal antibody-based assay and a cascade impactor, we have investigated the form in which group II mite allergens become airborne. The results reveal that these allergens only become airborne during disturbance and that they fall within 15 minutes. However, the mean size of particles carrying group II allergens appears to be slightly smaller than the mean size of particles carrying group I allergens. In addition, the quantities of group II allergen becoming airborne during disturbance (mean, 26 ng/m3) could not be explained by the quantity found in fecal particles. Thus, group II mite allergens become airborne in a form quite distinct from cat allergens and very similar to group I mite allergens; however, it appears unlikely that fecal particles are the main form in which group II allergens become airborne.
The efficacy of an air-cleaning device equipped with a high efficiency particulate air (HEPA) filter (without further avoidance measures) was studied in patients allergic to house dust mite. The effects of the air-cleaner on indoor Dermatophagoides sp. levels, symptom score and bronchial hyperresponsiveness in nine mite-allergic patients were assessed using a cross-over controlled study. No significant effect was demonstrated on indoor Dermatophagoides sp. levels when comparing the period of air-cleaner activity (2 months) with the control period (2 months). The Dermatophagoides sp. levels in the houses studied were lower than the risk level for asthmatic attacks, making it difficult to assess any effect on asthma; however, neither bronchial hyperresponsiveness nor rhinitis symptom score were changed by air-cleaner activity. During the trial period, however the mean level of Dermatophagoides sp. allergen in the houses changed spontaneously from 4.4 micrograms/g (mean level in the first 2 trial months) to 1.75 micrograms/g of dust (second 2 months) (P less than 0.05). Owing to this change, the mean rhinitis symptom score also decreased (P less than 0.05), even if no significant correlation was demonstrated (r = 0.4 P = 0.089). HEPA filter air-cleaners appear insufficient as substitutes for standard avoidance measures in mite allergic patients.
This study was designed to assess the effectiveness of a high-efficiency particulate air (HEPA) filter in alleviating allergic respiratory symptoms. Thirty-two patients were studied who had symptomatic perennial rhinitis and/or asthma during the fall and winter months and had a positive skin test with house dust or house dust--mite extract. An ENVIRACAIRE room air cleaner was placed in the bedroom for 8 weeks. In a random manner, the active filter was used for 4 weeks and a blank filter for 4 weeks. There was an average 70% reduction in the particulate matter greater than or equal to 0.3 micron with the HEPA filter. In a double-blind design, results were assessed by analysis of the patients' symptom/medication scores and subjective evaluation. For the total study, there was no difference in the total symptom/medication scores or individual symptom scores during the placebo and active-filter periods. Analysis of the last 2 weeks of each filter period in which respiratory infection was absent demonstrated definite differences in total and individual symptoms, suggesting active-filter benefit. Patients' subjective responses also suggested benefit from the filter. The overall impression is that the HEPA filter can reduce allergic respiratory symptoms.
Children with asthma commonly have positive skin tests for inhaled allergens, and in the United Kingdom the majority of older children with asthma are sensitized to the house-dust mite. In a cohort of British children at risk for allergic disease because of family history, we investigated prospectively from 1978 to 1989 the relation between exposure to the house-dust mite allergen (Der p I) and the development of sensitization and asthma. Of the 67 children studied in 1989, 35 were atopic (positive skin tests), and 32 were nonatopic. Of the 17 with active asthma, 16 were atopic (P less than 0.005), all of whom were sensitized to the house-dust mite, as judged by positive skin tests and levels of specific IgE antibodies (P less than 0.001). For house-dust samples collected from the homes of 59 of the children in 1979 and from 65 homes in 1989, the geometric means for the highest Der p I exposure were, respectively, 16.1 and 16.8 micrograms per gram of sieved dust. There was a trend toward an increasing degree of sensitization at the age of 11 with greater exposure at the age of 1 (P = 0.062). All but one of the children with asthma at the age of 11 had been exposed at 1 year of age to more than 10 micrograms of Der p I per gram of dust; for this exposure, the relative risk of asthma was 4.8 (P = 0.05). The age at which the first episode of wheezing occurred was inversely related to the level of exposure at the age of 1 for all children (P = 0.015), but especially for the atopic children (r = -0.66, P = 0.001). In addition to genetic factors, exposure in early childhood to house-dust mite allergens is an important determinant of the subsequent development of asthma.