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| Journal of the New Zealand Medical Association,
02-July-2002, Vol 115 No 1157
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House dust mite allergen levels in University student
accommodation in Dunedin
WM McRae, EM Flannery, JO Cowan, CR McLachlan, GP Herbison,
RW Siebers, J Crane, CS Wong.
House dust mite allergen,
Dermatophagoides pteronyssinus (Der p1)
exposure is a major risk factor for the development and maintenance of
asthma in genetically susceptible individuals. Around one third of the
population sufficiently exposed to house dust mite will become sensitised,
particularly families with a history of atopy.1
The suggested threshold for sensitisation to
Der p1 is 2 μg/g house dust, with
the threshold for exacerbation of symptoms being 10
μg/g.2
The temperate New Zealand climate with high year round
outdoor humidity has been found to be highly conducive to house dust mite growth
and proliferation. As a result, New Zealanders are exposed to higher levels of
Der p1 in their domestic environments
than individuals in other countries.3,4
Relatively low levels of Der p1 have
been documented in Canada,5
Denmark6 and
Sweden,7 with relatively higher levels in the
United Kingdom8 and the
Netherlands.9 However the highest
Der p1 allergen levels have been
measured in coastal Australia (mattress levels 22.5 μg/g, bedroom floor
levels 21.6 μg/g in Sydney)10 and in New
Zealand (mattress levels 46.6 μg/g, bedroom floor levels 26.4 μg/g in
Wellington).3
Otago university students make up a relatively large and
unique subsection of the Dunedin population. Many live in low cost, poorly
maintained rental accommodation. Others live in newer-style but often
underventilated flats. Furnishings, curtains and carpets are generally old
and/or dusty in these flats. Students living in the close confines of the halls
of residence have no choice in the types of bedding or flooring used or the
frequency of bed linen changes, and may not have access to a vacuum cleaner.
Previous studies have identified such environments as promoting house dust mite
proliferation.9,11-13 Assessment of the
Der p1 levels in the various forms of
accommodation may give us some insight into risk factors for asthma morbidity in
the university student population.
This study aimed to quantify the levels of
Der p1 in different university student
accommodations in Dunedin, and to assess the relationship between allergen
levels, housing characteristics and housekeeping practices. Levels of
Der p1 had not previously been measured
in the city. Whereas the New Zealand environment differs from other countries,
the Dunedin climate is not comparable to other New Zealand cities hence data on
Der p1 levels from earlier studies
could not easily be extrapolated.
MethodsSubjects.
A database of 1730 Otago University first year students was established
from a short questionnaire administered at the commencement of the 1998 academic
year. Student dwellings (n=178) were randomly selected from this database,
following grouping according to the type of accommodation. All students were in
their first year of University study in Dunedin and had lived in their
accommodation for at least six weeks prior to the study. A history of asthma or
allergy was not required.
Design. The study was cross-sectional, involving the assessment and measurement of Der p1 levels in three different forms of student accommodation: family homes (including boarding arrangements), student flats and halls of residence. Visits took place between May-September 1998. In each dwelling, reservoir dust samples were collected from two different sites, the bedroom floor and the bed. Floor samples were taken from the area adjacent to the bed. Samples were collected from 1m2 area of carpeted floor or 2m2 of uncarpeted floor. Rugs and other removable floor coverings were removed prior to sampling. Bed samples were taken from 1m2 area of the top of the mattress. Sampling was performed using an Hitachi CV-2500, 1100 watt vacuum cleaner, with a small furniture head attachment. A 15 cm length collection sock, made of 25μm pore-sized nylon mesh (nytel HP25), was attached to the furniture head. Each area was vacuumed for one minute using firm pressure. The floor sample was obtained before the bed sample and a new vacuum cleaner head was used for the collection of each sample. The sock containing the sample was tied then frozen at –20 °C until further preparation in the form of sieving and weighing of the sample was carried out. Weighed fine-sifted (425 μm) dust samples were extracted in phosphate-buffered saline at room temperature for 30 minutes, and Der p1 levels in the centrifuged extracts were estimated by double-monoclonal antibody ELISA.14 At the time of dust sampling, details of housing characteristics, occupancy and housekeeping practices were obtained by questionnaire. Analysis. Floor and bed samples of Der p1 were analysed separately. Der p1 levels were log-transformed and compared by site and between dwelling groups by one-way analysis of variance. Multivariate analyses were done by linear regression of the log-transformed Der p1 levels. The study was approved by the Otago Regional Health Authority Ethics Committee. ResultsBedroom dust samples and
questionnaire data were obtained from all 178 students enrolled in the study.
Accommodations were grouped into family homes (n=61), student flats (n=43) and
halls of residence (n=74).
Der p1
concentrations were significantly higher in dust from beds than from bedroom
floors in the three forms of student accommodation (Table 1). There were
significantly lower levels of Der p1 in
halls of residence compared with family homes and student flats.
Table 1. Geometric mean (95% confidence intervals) Der
p1 allergen levels according to type of accommodation.
Univariate analysis showed significantly higher floor levels
of Der p1 associated with the type of
accommodation (p=0.0005), fewer than fifteen people sharing the accommodation
(p=0.0005), occupancy of their current accommodation for longer than two years
(p=0.0005), the presence of a washing machine or clothes drier inside the
accommodation (p=0.0005), pets inside the accommodation (p=0.0005), the use of
electric or central heating (p=0.0005), infrequent vacuuming of the bedroom
floor (p=0.0005), failure to use a hot wash for sheets (p=0.0005), a history of
condensation or mold inside the accommodation (p=0.005), the sharing of the
accommodation with a smoker (p=0.004) and failure to vacuum the mattress
(p=0.03).
Univariate analysis showed significantly higher bed levels
of Der p1 associated with a mattress
greater than one year of age (p=0.0007), the type of accommodation (p=0.0005),
fewer than fifteen people sharing the accommodation (p=0.0005), occupancy of
their current accommodation for longer than two years (p=0.0005), the presence
of a washing machine or clothes drier inside the accommodation (p=0.0005), pets
inside the accommodation (p=0.0005), the use of electric or central heating
(p=0.0005), infrequent vacuuming of the bedroom floor (p=0.0003), failure to use
a hot wash for sheets (p=0.0001), a history of condensation or mold inside the
accommodation (p=0.005), a history of asthma in the occupying student (p=0.026)
and the sharing of the accommodation with a smoker (p=0.01). There was no
association between bed Der p1 levels
and vacuuming of the mattress.
The type of accommodation that the student was living in had
a major influence on the measured allergen levels. When the type of
accommodation was adjusted for by multivariate regression analysis, higher floor
levels of Der p1 were associated with
fewer people sharing the accommodation (p=0.035), the presence of a washing
machine inside (p=0.02), the presence of a clothes drier inside (p=0.01) and an
electric heater being used as the primary form of heating (p=0.006).
When the type of accommodation was adjusted for by
multivariate regression analysis, higher bed levels of
Der p1 were associated with a longer
duration of residence in Dunedin (p=0.0005), a longer duration (>2 years) of
occupancy of the current accommodation (p=0.004), a mattress greater than one
year of age (p=0.0005) and an electric heater being used as the primary form of
heating (p=0.03).
No association was demonstrated between
Der p1 levels and the sex of the
student, use of a mattress cover, type of mattress on the bed, the washing of
bedding, age of carpet in the bedroom, the presence of carpet or rugs on the
bedroom floor, a lack of heating in the accommodation or the frequency of
clothes drier and washing machine usage.
Detailed tables of results are available from the authors on
request.
DiscussionDunedin is a city where, during the
university term, approximately 10% of the population is composed of University
of Otago students. This study was designed to quantify levels of
Der p1 in university student
accommodation and determine factors that might influence the levels.
We found that Der
p1 levels varied widely between the three different forms of student
accommodation. Students were exposed to higher levels of the allergen living in
family homes, boarding environments and in student flats when compared with
halls of residence. Similar low levels of Der
p 1 have been demonstrated in University Colleges compared with nearby
homes in Sydney, Australia.15
A number of possible explanations exist for this difference.
Rooms in halls of residences are cleaned and sheets laundered on a regular
basis. These rooms tended to have low pile, commercial grade carpet and be well
heated and ventilated. Halls of residences are typically vacant for a three
month period over summer, possibly resulting in a fall in house dust mite levels
from the ensuing reduction in food sources and humidity.
Other studies have shown that significant differences exist
between Der p1 levels in public
buildings when compared to domestic
dwellings.16,17 Our results suggest that the
level of Der p1 students are exposed to
in halls of residence is more in keeping with that of a public building. The
geometric mean floor level of Der p1 in
halls of residence was 0.26μg/g, compared with 0.58μg/g measured in
public buildings in two New Zealand cities.16
The geometric mean bed level of Der p1
in the halls of residence was 3.25μg/g, very similar to the 3.57μg/g
measured in hotel beds in the same
study.16
In all forms of accommodation the
Der p1 concentrations were
significantly higher in dust collected from beds than from bedroom floors,
consistent with results from both earlier New Zealand studies as well as
overseas research. Indeed the Der p1
levels measured in mattress dust in student flats and family homes were above
the recognised threshold for the exacerbation of allergy symptoms, and bedroom
floor levels exceeded the sensitisation
threshold.2
Our results suggest that bed and floor
Der p1 levels may be influenced by
different environmental factors. Higher bed levels were associated with factors
relating to a longer duration of residence in the studied dwelling. Our finding
of higher levels of Der p1 allergen in
mattresses greater than one year of age is in keeping with an earlier study by
Custovic et al,18 where it was demonstrated
that mattresses can become a significant source of exposure to mite allergens
after as little as four months, with allergen levels generally stabilising after
one year.
We found that higher floor levels of the allergen were
associated with the presence of a washing machine and clothes drier inside.
Although both factors may contribute to increased indoor relative humidity,
there is uncertainity as to the importance of this in terms of
Der p1 levels. The design of our study
did not allow for the actual measurement of relative humidity in the dwellings.
Earlier studies16,19,20 however have found that
the measurements of relative humidity did not consistently correlate with the
Der p1 levels. Possibly the measure is
an inadequate indicator of the more humid mite microclimate in beds and carpets
or alternatively mite growth occurs independently of humidity once minimum
conditions for mite breeding are
satisfied.19
We were unable to demonstrate in our study a relationship
between the Der p1 levels and the use
of a mattress cover or vacuuming of the mattress. This was most likely to be due
to the low number of students practicing these measures, which are generally
perceived as being for sufferers of asthma and allergy. Barrier covers have been
advocated to trap house dust mites and their allergen inside the mattress.
Although some studies have demonstrated a significant reduction in allergen
levels with their use,21,22 other studies have
failed to show benefit.23
It is difficult to explain why, even after adjusting for the
type of accommodation, higher floor levels of Der p1 were associated with fewer
people sharing that accommodation. Although this negative correlation was also
reported by Kuehr et al,24 most studies have
shown an association between increased occupancy and higher mites
levels.5,9,16 Our finding would suggest that in
our accommodation environments, food supply may not be the most important factor
in limiting mite population growth.
In conclusion, this study has demonstrated that significant
variations in the levels of Der p1
allergen can be found in different forms of university student
accommodation. Students are exposed to lower levels of allergen living in halls
of residence than in other types of accommodation. Further research needs to be
carried out in order to determine the relative impact of differing allergen
levels results on asthma and allergy symptoms in this population.
Author Information:
WM McRae, Senior Registrar in Respiratory Medicine, Dunedin Hospital; EM
Flannery, Technician; JO Cowan, Technician; CR McLachlan, Technician,
Respiratory Research Unit; GP Herbison, Biostatistician, Department of
Preventive and Social Medicine, University of Otago, Dunedin; RW Siebers, Senior
Technical Officer; J Crane, Associate Professor, Wellington Asthma Research
Group, School of Medicine, Wellington; CS Wong, Respiratory Physician, Dunedin
Hospital, Dunedin.
Correspondence. WM
McRae, Department of Medicine, Dunedin Hospital, Dunedin. Fax (03) 4747655;
email: wendy_mcrae@hotmail.com.
References:
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