Presented
at the “Indoor Air Quality 2001: Moisture, Microbes,
and Health Effects” Conference, sponsored by the
American Society of Heating, Refrigerating, and Air conditioning
Engineers, Inc., San Francisco, California, November 4-7
2001.
Preventing House Dust Mite Allergens
in New Housing
Subrato Chandra, Ph.D. David Beal
Member ASHRAE
ABSTRACT
House
dust mite allergens are a major cause of asthma. The
objective of this study was preventing dust mite allergens
in new houses with wall to wall carpets, NOT eradicating
dust mites from infested homes.
Five
pairs of homes, located in the southeastern USA, were
studied for an average of 12 months. Each pair consisted
of a newly built home with a central dehumidifier and
an existing home without a central dehumidifier. Three
new homes and 2 existing homes had thorough vacuum cleaning
practices. Dust samples were taken monthly from a minimum
of 4 locations in each home and analyzed for mite allergen
levels. Carpet level relative humidities and temperatures
were measured.
The
dehumidification systems maintained the average carpet
relative humidity (RH) below 60% in all 5 new homes for
all months. The 3 new homes with dehumidification
and thorough vacuum cleaning practices did not
have any dust mite allergens at the beginning of the
study and continued allergen free. The 2 new homes with
a dehumidifier and the 3 existing homes with poor cleaning
practices had high allergen levels. The 2 existing homes
with thorough vacuum cleaning practices had high allergen
levels on one occasion only.
INTRODUCTION
House
dust mite allergens are a major cause of asthma in humid
climates1. In U.S. homes, carpets, mattresses
and upholstered furniture are major reservoirs of mites2. Current
recommendations1 to reduce allergen exposure
are to encase mattresses and pillows and box springs; weekly
washing of bedding with hot water (>55 oC);
and regular use of vacuum cleaners with allergen trapping
features, such as dust bags with two layers, or
whose exhaust air is ducted outside (central vacuums),
or filtered with HEPA, or electrostatic filtration systems.
Available data on the efficacy of vacuum cleaners indicate
that while they reduce the allergen burden, they are unlikely
to control exposure from carpets and furnishings1.
A 1982 study, presumably using vacuum cleaners with no
allergen trapping features, found that vacuum cleaners
had little effect in reducing mites in carpets3.
A current recommendation1 is to remove carpets
from houses in humid climates. However, carpets are desired
by many consumers. Thus, the present study was motivated
by the need to eliminate dust mite allergens in houses
with wall to wall carpets. Please note that the objective
of this study is to prevent dust mite allergens in new
houses which start off allergen free. The objective is
NOT to eradicate dust mites from infested houses, a more
difficult problem.
Laboratory
studies of fasting female Dermatophagoides farinae dust
mites indicate that their bodies continue to dehydrate
when kept below 58% relative humidity (RH) at 25oC
or 52% RH at 15oC4. Other dust mite
species have higher RH requirements to prevent dehydration5.
While dehydration does not necessarily lead to death, it reduces fecal and allergen production5.
Several
studies have investigated the usefulness of dehumidifiers
in reducing mite allergens. Cabrera et al. investigated
the use of bedroom dehumidifiers in 10 houses in the humid
Canary Islands6. Allergens were reduced significantly
in the mattresses from bedrooms in 8 of the 10 homes, although
the final allergen level was greater than 2 μg/g of
dust, considered relevant for asthma7, in 9
out of the 10 houses. Another study, conducted in the U.K.,
found that a single portable dehumidifier placed centrally
in the house was not effective in reducing mite counts, when
comparing data from six houses with dehumidifers and six
control homes without dehumidification8. Niven
et al. conducted a study in the U.K., using central dehumidifiers,
and they also failed to find a positive benefit in terms
of reducing mite allergens despite achieving control of
temperature and humidity9. All these studies
were done on existing, not new homes.
Thorough
vacuuming should eliminate the food source for dust mites, and
central dehumidification should control the relative humidity
(RH) to below that required for dust mite mobility. As
a result, new houses with negligible dust mite allergens
should not experience an increase. The literature does
not have data on the effectiveness of this combined strategy,
the subject of this study.
METHODS
Description of monitored houses
Five
pairs of homes, all with central cooling and heating systems, were
investigated in four cities of the Southeastern U.S. Table
1 provides the details of the homes. Each pair of homes
consisted of a new home with a central dehumidifier and
an older existing home without a central dehumidifier.
The existing homes differed from the new homes in many
ways as shown in Table 1. The main benefit of studying
the existing homes was to determine the level of dust mite
allergens in typical homes without central dehumidification.
The existing homes were located in the same city as the
new homes, and the occupants agreed to participate
in the study. The different types of vacuum systems and
the frequency of vacuuming is indicated in Table 1. The
dehumidification system was a whole house dehumidifier
with a mechanical ventilation system. This system is installed
next to the air handler and ducted to dehumidify the whole
house. It is controlled by a dehumidistat so the RH inside
the house can be controlled independent of the temperature.
One new house, in New Orleans, initially used an energy
recovery ventilation system with a desiccant wheel. The
desiccant wheel in this system passively lowers the RH
in the house when the outside air absolute humidity is
lower than that inside the house. The
homeowner chose to deactivate this system in mid June,
1997 and replace
it with a mechannical dehumidification system, as described
above, in August, 1997. The carpets installed in these
new houses were of the conventional variety, with heat
set Nylon filaments with a 1.3 cm pile height, except for
the New Orleans house, which was made in the Berber
style, with closed loop Nylon. The Jacksonville new house
did not have wall to wall carpets, only area rugs.
Environmental monitoring
Each
home was monitored on average for 12 months (minimum 8
, maximum 16 months). Interior temperatures and RH were
measured at the carpet level by small battery powered data
loggers, which recorded the instantaneous data every
hour. The measurement uncertainty is estimated to be +/-
2.5% RH and +/- 0.5oC.
Dust
sample collection. Dust samples were collected in
each home at a minimum of four locations – master
bedroom mattress, master bedroom carpet in front of the
bed, family room sofa, and the family room carpet in
front of the sofa. For dust sample collection, the entire
top surface of the mattress was vacuumed after removing
the bed sheet. The entire surface of the sofa was also
vacuumed for sample collection. Carpet dust samples were
collected by vacuuming an area of approximately one m2 for
one minute. Dust samples were collected with a portable vacuum cleaner whose inlet was modified to accept
a standard paper coffee filter which collected the dust.
The paper filter was folded and taped and stored in separate
air tight plastic bags and shipped to the laboratory
for analysis. Dust sampling was performed by the same
students in all Orlando area homes. The other homes were
sampled by the homeowner who were each given a similar
set of dust collection equipment and instructed in their
use. Those homeowners were paid a small monthly fee for
their cooperation whereas the Orlando area homeowners
cooperated voluntarily. Dust samples were collected monthly
during the study period.
Dust
sample analysis. Analysis of the dust samples was
performed by the same technician throughout the study.
After receiving the dust samples in the laboratory, they
were stored at 4oC until analysis. Twenty-five
mg were extracted at 1:40 w/v in 0.2 mM ammonium bicarbonate
buffer, pH 9, overnight at 4oC. The supernatant
was isolated and filtered through a 0.45 μm nitrate
acetate filter. Der p1 and Der f1 allergen levels were
measured by ELISA, according to the protocol described
by Chapman et al.10,11.
RESULTS
Figure
1 shows the measured RH and temperature data for the five
existing homes. The data collected for each house is aggregated
for each calendar quarter and displayed. Except for house
#5, the summer time average temperature is close to 25oC.
As
all homes used central air conditioning, the average RH
during the summer months ( second and third quarters of
every year) are below or near 58% RH. During the winter
months ( first and fourth quarters), the outside temperatures
are lower and, as a result, the central
air conditioner is not used much. Due to occupant activities
and infiltration of outside humid air, the average
carpet level RH for all existing houses during the winter
quarters are higher than the summer quarters, while
the average carpet level temperature goes down during the
winter. This promotes dust mite growth in the winter months
in the Southeastern U.S.
Figure
2 presents the measured allergen levels in the existing
homes. Existing house #2 has an allergen trapping vacuum
cleaner, but it is not used often. Consequently, the
house has high allergen levels. Existing houses #4 and
#5 have regular vacuum cleaners, and, despite
frequent usage, have significant allergens. Existing
house #1 has a central vacuum cleaner, which was
used regularly. Despite not having a central dehumidifier,
this house had low allergens for the first three quarters.
Only in the last
quarter of sampling (Q1'97), after a period of prolonged
high humidity, the maximum allergen
level in the living room carpet location exceeded the threshold
value. In existing house #3, a housekeeper vacuumed
two or three times a week with an allergen trapping vacuum
cleaner. This house never had measurable allergens. However,
due to lack of home owner cooperation, the sampling period
did not last as long as desired. Existing house #1 and
#3 indicate the dust mite control potential of thorough
vacuum cleaning practices with a good equipment.
Figure
3 shows the measured carpet level RH and temperatures in
the five new homes. Except for new house #2 in the last
quarter, where the dehumidifier system was not ducted optimally,
the average measured RH values are lower than 58%. The
central dehumidification systems performed well in controlling
the winter time humidity. The average temperatures are
comparable to those in the existing homes.
Figure
4 presents the measured dust mite allergens. New house
#1 had a regular vacuum cleaner, and, despite
RH control, had significant dust mite allergens. The fact that this new house
had high allergen levels is consistent with other studies6,8,9 which
found that RH control alone was ineffective in controlling
mite allergens.
New
houses #2, 4 and 5 all had wall to wall carpets and thorough
vacuum cleaning practices. The highest measured group 1 allergen level at any location in these
three homes was only 1.04 μg/gram of dust. Although
the new house #3 did not have wall to wall
carpeting, just
area rugs, it had allergen levels occasionally exceeding
the threshold value. It used a regular vacuum cleaner.
CONCLUSIONS
The
results of this study show that thorough vacuuming (defined
as at least twice weekly with an upright vacuum cleaner
with an allergen trapping dust bag, or once weekly
with a central vacuum cleaning system exhausting to outside
of the conditioned living area) and a central dehumidification
system installed in conjunction with central cooling and
heating can maintain dust mite allergens below clinically
significant levels in new homes with wall to wall carpeting
even in humid climates. The measurements were taken
on average for 12 months, which is long enough to show promise
for this strategy, as new homes can be infested
with dust mites in a few weeks, as shown by the
data from new house #1, and also reported in other
literature12.
The
houses studied are larger than median sized new U.S. homes.
Census data indicates that in 1997, 15% of the current
new home construction was in this category of houses having
279 square meters or larger amount of conditioned area 13.
The number of occupants did not exceed four. The
sample size is also small. Future studies should
look at smaller new and existing homes with higher occupant
densities, lower cost central dehumidification systems
and allergen trapping vacuum cleaners to determine the
range of housing types that can be positively affected
by this promising dust mite control strategy. Future
studies should also quantify the benefit of thorough vacuum
cleaners alone in preventing dust mite allergens.
ACKNOWLEDGMENTS
This
research was sponsored by the U.S. Department of Energy
(DOE), Office of Building Technology, State and Community
programs. We thank Mr.
George James and Mr. Richard Karney of the U.S. Department
of Energy for
their encouragement.
The
dust mite allergen analysis was performed under subcontract
by the University of South Florida College of Medicine.
The technician was Rosa Codina, Ph.D. who worked under
the guidance of professor Richard F. Lockey, M.D. They
made significant contributions to experiment design and
reviewed and commented on several drafts of this paper
for which we are thankful.
We
appreciate the assistance of student assistants, Mr. Andrew
Downing and Mr. Brian Fuehrlein in data collection and
processing. We thank Dr. Laila Alidina, M.D., Ph.D and
Mr. Ken Gehring for the time they took in discussing this
research effort with us. Special thanks go to the Central
Florida and Minnesota chapters of the American Lung Association
who spearheaded the construction of Health
Houses® and got us involved in the design and research
of such houses. The first new house in Orlando and the
new houses in Jacksonville, New Orleans and Huntsville
were Health Houses. Last
but not the least, we thank the home owners for their cooperation.
REFERENCES
