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155
Original article
The accumulation of dust mite allergens on mattresses
made of different kinds of materials
Nualanong Visitsunthorn
1
, Varakorn Chirdjirapong
1
, Visanu Pootong
1
, Orathai Jirapongsananuruk
1
,
Punchama Pacharn
1
, Sirirat Weeravejsukit
1
, Vanna Mahakittikun
2
, and Pakit Vichyanond
1
Sum m ar y
Background: Different mattress materials may
affect the accumulation of allergens.
Objective: To compare the amount of group 1
dust mite allergens (Der p1 + Der f1) on
mattresses made of different kinds of materials
before and after use.
Methods: Sixty new mattresses made of kapok,
synthetic fiber, coconut fiber and sponge-like
polyurethane, were placed in the house
officers’ dormitory at Siriraj hospital,
Thailand. The dust samples were collected
before (0), 1, 2, 3, 6, 9 and 12 months after the
mattresses were used. Group 1 dust mite
allergens were analyzed using two-site
monoclonal antibody ELISA.
Results: Der f1 made up 86.7 % of group 1
allergens found in the matress dust. After the
2
nd
month, only the mean level in sponge-like
polyurethane mattress was under 2 µg/g dust
(sensitized level). At the 6
th
month, the mean
levels were 13.1 in coconut, 21.7 in kapok and
17.3 µg/g dust in synthetic fiber, all of which
were more than 10 µg/g dust (symptomatic level).
At the 9
th
month, the level in sponge-like
polyurethane mattress was increased to 11.2
µg/g. At 12
th
month the level in coconut fiber,
sponge-like polyurethane synthetic fiber and
kapok mattresses were 20.2, 22.4, 28.9 and 32.2
µg/g dust respectively.
Conclusions: The accumulation rate in kapok
and synthetic mattresses was significantly
higher than coconut and sponge-like
polyurethane mattresses. The mean level of
group 1 mite allergens exceeded 10 µg/g dust
after the 6
th
month of use in coconut fiber,
kapok and synthetic fiber and at the 9
th
month
in sponge-like polyurethane mattress. (Asian
Pac J Allergy Immunol 2010;28:155-61)
Key words: mite allergen, mattress, Der p1, Der
f1, group 1 allergens
Abbreviations:
HDM = house dust mite
Introduction
The incidence of atopic diseases has been
shown to be increasing all over the world. In
Thailand, the incidence of asthma was 12.7% and
allergic rhinitis was 38.7%.
1
Domestic mites were
found to play a major role in the initiation and
development of allergic diseases, mainly asthma
and allergic rhinitis.
2
The two major dust mite
species that associated with respiratory allergic
diseases were Dermatophagoides pteronyssinus
and Dermatophagoides farinae.
2,3
House dust
mite (HDM) and their allergens were found in
mattresses, carpets, rugs, chairs, sofas, couches
and clothing.
4,5
Among multiple locations in the
house, the highest concentration of group 1 mite
allergens was found in mattresses.
5
Beds have
been suggested to be the most important source of
HDM allergen exposure for young children, who
spend more than half of their time in close contact
with bedding.
6
Airborne levels of mite allergens
were low compared to the high concentrations
found in mattresses and bedding.
From the
1
Division of Allergy and Immunology,
Department of Pediatrics, Faculty of Medicine, Siriraj
Hospital, Mahidol University, Bangkok, Thailand
2Department of Parasitology, Faculty of Medicine,
Siriraj Hospital, Mahidol University, Bangkok,
Thailand
Corresponding Author: Nualanong Visitsunthorn
Mailing address: Division of Allergy and
Immunology,Department of Pediatrics, Siriraj Hospital,
Mahidol University. 2 Prannok Rd, Bangkoknoi,
Bangkok 10700, Thailand
E-mail address: sinvs@mahidol.ac.th
Asian Pac J Allergy Immunol 2010;28:155-61
156
A previous study showed that the sensitizing
level of group 1 antigens was >2 µg/g dust and
the level that induced allergic symptoms was >10
µg/g dust.
7
Mite-sensitive patients with allergic
asthma, allergic rhinitis, and atopic dermatitis
should receive advice on how to avoid mite
allergens in their mattresses.
7-9
Several attempts
were made to reduce the amount of house dust
mites in mattresses.
3,10,11
Previous studies showed
a significant reduction in house dust mite allergen
levels in mattresses covered with polyurethane
encasing.
12-15
Vacuuming of mattresses had been
proven to be ineffective or to give only short-term
benefit.
16,17
A combination of acaricide and tannic
acid were not effective in creating a low-allergen
environment and their long-term toxicity was
uncertain.
18,19
In an environment with a high
content of house dust mite allergens, chemical
treatment of carpets and mattresses
20
or chemical
treatment of carpets and bedding encasements
21
is
insuffcient to produce a sustained beneficial
reduction in mite allergen levels. Mite-
impermeable mattress encasings have a significant
but modest effect on dust mite allergen levels of
mattresses with low initial mite allergen
levels.
13,22
In clinical practice, a combination of encasing
of mattresses and washing sheets, pillowcases,
blankets, and mattresses pad at least weekly in hot
water (>55
o
C) are often recommended.
10,11
Despite much researches aimed at developing
mite allergen avoidance strategies, conflicting
data about the mite allergen content of mattresses
made of different kinds of materials still exists.
11
Surprisingly, only a few studies about choices of
mattress materials have been adequately
completed.
23, 24
The most common kinds of mattress materials
in Thailand and many countries in Asia are kapok,
sponge-like polyurethane, synthetic fiber, and
coconut fiber. Allergic patients are adviced to
replace their kapok mattresses with synthetic fiber
mattresses, despite the lack of scientific evidence
to support this. The aim of this study is to
compare the accumulation rate of dust mite
allergens on new mattresses made of different
kinds of materials over a twelve-month period
after use.
Methods
The study was approved by Ethics Committee
and the written informed consent had to be signed
before the study began. Sixty new mattresses
made of 4 kinds of materials; kapok, synthetic
fiber, coconut fiber and sponge-like polyurethane
(15 of each), were obtained for regular use by
house officers living in the house officer’s
dormitory at Siriraj Hospital, Mahidol University,
Bangkok, Thailand. These mattresses were used
for a 12-month period without any being cleaned.
Each mattress was partially covered with a cotton
bed sheet that was cleaned by regular washing
method every 1-2 weeks. The mattresses were
used regularly at least 4 days per week. The floor
of all of the rooms was made of cement without
carpet. Air ventilation occurred via three windows
and two doors. The room temperature and
humidity levels on the upper surface of mattresses
were recorded each time dust samples were
collected. The house officers’ dormitory was
surrounded by a cluster of tall buildings and is
approximately 500 meters far from the Chao
Praya River. To control for the effect of different
seasons on HDM and their allergens, the starting
times of the study of each type of mattresses were
randomly distributed throughout the year.
Collection of dust samples and allergen
extraction
Dust samples from the mattresses were
collected before use (0), and after 1, 2, 3, 6, 9, and
12 months of use. Each sample was collected
from the upper surface of the mattress by two-
minute vacuuming with the same vacuum cleaner
(Kelvinator 1000 W) attached to a dust collector
(ALK laboratory, Denmark) as described in the
third international workshop of indoor allergens
and asthma.
25
The samples were sealed in plastic
bags and stored at 4
o
C until they were analysed.
For allergen extraction, 0.1 g of fine dust samples
was stored in 2 ml of phosphate buffered saline
overnight at 4
o
C. Two-fold dilution of dust
extracts from 1:20 to 1:80 in 1% bovine serum
albumin-phosphate buffer saline-tween was
conducted for measuring of group 1 allergens.
Allergen determination
Der p1 and Der f1 concentration in dust
samples were determined using two-sided
monoclonal antibody methods
26
using mAb 5H8
for Der p1, mAb 6A8 for Der f1 and the standards
from Biotechnologies, UK. Flat-bottom ELISA
plates (Nunc, Denmark) were coated with
monoclonal antibodies for determination of
specific group 1 allergens. One percent bovine
serum albumin was used as diluents for blocking a
Dust mite allergens on mattresses
157
non-specific reaction and 0.05% PBS-tween was
used as a washing solution throughout the
experiment. The allergen concentration in the dust
samples was expressed in micrograms of group 1
allergens (Der p1 + Der f1) per g of fine dust.
Statistical analysis:
In this study, dust samples were collected
from each mattress at variable intervals: 0, 1,
2, 3, 6, 9, and 12 months. Group 1 mite allergens in
different kinds of mattress material were compared
over a 12-month-period by using ANOVA to
calculate the p value.
A linear mixed model was used due to multiple
measurements, irregular (unequal) time intervals
and missing data.
27,28
In our linear mixed model,
the dependent variable was the log of dust mite
allergen and the independent variable was time in
months. Time was considered to be a fixed effect
in the model whereas each mattress was deemed
to be a random effect.
Each type of mattress (kapok, synthetic fiber,
coconut fiber and sponge-like polyurethane) had
its own population linear regression line. The
slope of the linear equation showed rate of
accumulation of log of mite allergens (per month).
Since the regression equation was linear, the slope
was constant over time. That is, one type of
mattress will have one constant rate of
accumulation of log of mite allergens. As it is
similar to an ordinary simple linear equation, it
can be used to predict the mean value of the log of
the dust mite allergens after any period of use.
The population linear regression was calculated
by the equation: pop = interceps + slope×time.
All statistical analyses were performed using
SAS 11.0. A 2-sided significant level of 0.05 was
employed.
Results
The vast majority of group 1 allergens from
this study were Der f1 (86.7% of total group 1
mite allergens). The average humidity was 60.6 %
and average ambient temperature was 29
o
C. There
was no significant difference in humidity and
ambient temperature between the rooms that
contained different mattress materials.
Group 1 mite allergens gradually increased
overtime as shown in Figure 1. The levels of
group 1 allergens in all kinds of mattress materials
showed no significant difference at the end of the
1
st
month. By the 2
nd
month, mean group 1
allergen levels were 13.7 in synthetic fiber, 7.4 in
coconut fiber and 5.9 µg/g dust in kapok
mattresses. Only the mean level in sponge-like
polyurethane mattress was 1.1 µg/g which was
under the allergy sensitized level (2 µg/g dust).
7
The level in sponge-like polyurethane increased to
3.2 µg/g at the 3
rd
month and 5.2 µg/g at the 6
th
month. At the 6
th
month, the mean levels were
13.1 in coconut fiber, 21.7 in kapok and 17.3 µg/g
in synthetic fiber mattresses which were more
than the level that induce allergic symptoms (>10
µg/g dust).
7
The mean level in sponge-like
polyurethane mattress was increased to 11.2 µg/g
by the 9
th
month. At the end of the study (12
th
month), the levels in all kinds of mattress
Figure 1.
Group I mite allergens in different kinds of mattress over time
Asian Pac J Allergy Immunol 2010;28:155-61
158
materials were more than 20 µg/g dust. The level
in coconut fiber, sponge-like polyurethane and
synthetic fiber were 20.2, 22.4 and 28.9 µg/g dust
respectively. The level in kapok mattresses was
32.2 µg/g dust which was the highest level of all.
Mean, median and interquartile range of group 1
mite allergens in different kinds of mattress at
different time points were shown in Table 1.
Comparison of Group 1 mite allergens in different
kinds of mattress materials over a 12-month-
period by using independent sample t-test to
calculate the p value showed significant
difference (p <0.05) between synthetic vs sponge-
like polyurethane from the 2
nd
to the 3
rd
month,
kapok vs sponge-like polyurethane at the 6
th
to 9
th
months and kapok vs coconut at the 9
th
as shown
in Table 2.
The predicted mean values (population
regression line; pop = intercepts + slope×time) of
natural log of mite allergens during the whole
study period for each type of mattress are
presented in Figure 2. The linear mixed models
showed that four types of mattress could be
classified into two groups according to the rate of
accumulation of log of mite allergen levels.
Mattresses produced from kapok and synthetic
fibers were classified in the same group while the
mattresses produced from sponge-like
polyurethane and coconut fibers were in the other
group. There were statistically significant
differences in allergen accumulation rate between
kapok vs. coconut, kapok vs. sponge-like
polyurethane, synthetic vs. coconut, and synthetic
vs. sponge-like polyurethane mattresses (p <0.05).
The statistical comparison of predicted means of
log of mite allergens between each type of
mattress at each time point is shown in Table 3.
There were statistically significant differences of
log of group 1 allergens between kapok vs.
coconut, synthetic vs. coconut, and synthetic vs.
polyurethane mattresses at and after six-month
period and those between kapok vs. polyurethane
mattresses at and after three-month period (p
<0.05).
Table 2. Comparison of (Der p1 + Der f1) on time
Month : p-value
Mattress 1 2 3 6 9 12
Kapok vs.
Synthetic
0.506 0.065 0.299 0.589 0.162 0.73
Kapok vs.
Coconut
0.59 0.701 0.591 0.225 0.005* 0.118
Kapok vs.
Sponge
0.247 0.079 0.259 0.009* 0.001* 0.396
Synthetic
vs. Coconut
0.25 0.13 0.125 0.581 0.336 0.35
Synthetic
vs. Sponge
0.575 0.002* 0.045* 0.075 0.166 0.604
Coconut
vs. Sponge
0.64 0.35 0.52 0.13 0.56 0.85
* p <0.05
Discussion
The distribution of group 1 allergens in this
study showed that Der f1 was the major species of
dust mites in this dormitory which was near the
Chao Praya River. A previous study carried out in
Thailand showed that Der p1 was the major house
dust mite allergen in the houses.
29
The difference
in the distribution of group 1 allergens
Table 1.
Mean, median and interquartile range of group 1 mite allergens in different kinds of mattress at
different time points
mattress Dust mite allergens Der pI + Der f I (microgram/gram dust)
month 0 1 2 3 6 9 12
Coconut Mean 0.1513 0.9707 7.3693 5.0893 13.1120 14.0640 20.2333
Median 0.14 0.96 1.36 1.05 4.61 13.30 16.60
Inter quartile range 0.05-0.22 0.39-1.45 0.64-12.83 0.75-1.92 0-23.2 0-27.4 0-33.7
Kapok Mean 0.2727 1.4180 5.9807 7.3747 21.7533 30.5667 32.2400
Median 0.25 1.44 2.24 1.47 25.40 28.10 31.30
Inter quartile range 0.21-0.35 1.06-1.83 0.57-6.00 0.53-2.44 0.00-31.7 26.10-43.50 17.40-42.80
Sponge Mean 0.3060 1.1060 1.2723 3.2080 5.1867 11.1840 22.3667
Median 0.10 0.96 1.04 1.18 .00 8.60 12.90
Inter quartile range 0.05-0.58 0.23-1.75 0.15-2.02 0.13-4.78 0.00-11.40 0.20-15.4 1.00-17.90
Synthetic Mean 0.0307 1.2667 13.7393 13.2880 17.3680 20.5733 28.9667
Median 0.00 1.30 13.73 3.41 3.10 10.70 25.30
Inter quartile range 0.00-0.04 0.80-1.89 1.46-22.40 1.63-21.90 0.00-29.29 0.80-43.80 12.10-32.50
Dust mite allergens on mattresses
159
in Thailand might be due to the different humidity
in different locations.
30-32
At the 2
nd
month of use,
mean group 1 allergen level in synthetic fiber
mattress was 13.7 µg/g, which is already more
than the level that induce allergic symptoms while
the mean levels in coconut fiber and kapok
mattresses were 7.4 and 5.9 µg/g. Only the mean
level in sponge-like polyurethane mattress was 1.1
µg/g which is lower than the allergic sensitization
level.
The study showed that the mattresses produced
from sponge-like polyurethane had the lowest rate
of mite allergen accumulation. Synthetic fibers
and kapok mattresses had the highest
accumulation rate. The accumulation rate of
coconut-fiber mattresses was slightly higher than
those from sponge-like polyurethane materials.
Over time, the mite allergen accumulation could
occur on sponge-like polyurethane mattresses, but
at a lower rate than for synthetic and kapo k
Figure 2.
Population regression line
of log of mite allergens
Der p
1
+
Der f1
(pop = intercepts + slope ×
time)
Table 3.
Results of linear mixed model of log (
Der p1 +Der f
1
) on time:Comparison of predicted log
(Der p1 +Der f1)
Month : p-value
Mattress 1 2 3 6 9 12
Kapok vs. Synthetic 0.8355 0.8285 0.8233 0.8237 0.8452 0.8721
Kapok vs. Coconut 0.5707 0.3697 0.2144 0.0276* 0.0052* 0.0025*
Kapok vs. Sponge 0.1517 0.0772 0.0355* 0.0032* 0.0008* 0.0007*
Synthetic vs. Coconut 0.7209 0.4984 0.3106 0.0489* 0.0100* 0.0046*
Synthetic vs. Sponge 0.2226 0.1227 0.0612 0.0067* 0.0018* 0.0013*
Coconut vs. Sponge 0.3875 0.3845 0.3877 0.4446 0.5605 0.6860
* p < 0.05
Asian Pac J Allergy Immunol 2010;28:155-61
160
mattresses. Statistically significant difference in
mite allergen accumulation existed when compare
sponge-like polyurethane or coconut-fiber
mattresses with kapok and synthetic mattresses.
The structure of sponge-like polyurethane
mattresses was spongy with numerous pores
inside while coconut mattresses had relatively
large natural fibers. The lower accumulation rate
is probably related to the structure and properties
of the mattress materials and the biology of house
dust mites. Further study in this area is needed to
explain the difference between mattress materials.
Although the rate of accumulation was lower
in sponge-like polyurethane, the level was more
than 2 and 10 µg/g dust at the 3
rd
and 9
th
month,
respectively. The antigen levels were higher than
10 µg/g dust after the 2
nd
month in synthetic fiber,
the 4
th
month in kapok, the 5
th
month in coconut
fiber and the 9
th
month in sponge-like
polyurethane mattresses. If we accept that the
allergy sensitizing level for group 1 antigen is >2
µg/g dust and the level that induces allergic
symptoms is >10 µg/g dust,
7
the residents in this
dormitory were exposed to very high levels of
mite antigen. The group 1 antigen levels in all
kinds of mattresses at the12
th
month of use were
much higher than the levels that induce allergic
symptoms. High HDM antigen levels were also
found in houses in UK and USA.
30,31
In
Manchester, UK, Two-thirds of homes contained
Der p1 levels > 2 µg/g dust and 40.3% contained
Der p1 greater than 10 µg/g dust.
30
House dust
mite allergen concentrations in US beds were 46.2
and 24.2% at or greater than 2.0 and 10.0 µg/g
dust respectively.
31
One of the explanation for this
is that mite allergens persisted in the households
for a long period of time. The median half life of
Der f1 in mattresses dust samples was 10 years in
exposed homes, 18 years in store rooms and 1
year in green houses.
33
The natural decay of Der
f1 was about 10 years in houses.
33
The results from the linear mixed model are in
line with the independent sample t-test. The
population linear regression line for each kind of
matress material showed that the accumulation of
group I mite allergens in kapok and synthetic fiber
were significant higher than in coconut and
sponge-like polyurethane.
Our study has showed that group 1 allergen
accumulation in all kinds of mattresses increased
significantly at three months after use (P value <
0.05). This is supported by the results of a
previous study which showed that Der p 1
increased
significantly at four months as compared with
baseline level.
34
This means that new mattresses
could become a significant source of exposure to
mite allergens after a short period of time (3-4
months after use) so there appears to be little
justification for advising mite sensitive patients to
replace their mattresses as a part of avoidance
regime.
34
Although the accumulation increased
with time, to start with the new mattress which
was produced from the materials that has the least
group 1 mite allergen accumulation rate, ie
spongy-like polyurethane or coconut mattresses,
should be better. Regarding a mite allergen
avoidance strategy, a combination of washing in
hot water and the use of mattress covers would be
preferable. A combination of the following:
choosing new sponge-like polyurethane or
coconut fiber mattresses with mite-impermeable
covers and washing sheets, pillowcases, blankets,
and mattresses pad at least weekly in hot water
should be the best method of prevention in HDM
allergen sensitive allergic patients.
In conclusion, this study showed that the
accumulation of mite allergen on mattresses made
of different kinds of materials was different. The
accumulation rate in kapok and synthetic
mattresses was significantly higher than for
coconut and sponge-like polyurethane mattresses
at and after the first 6 months of use. The mean
level of group I mite allergens exceeded the level
that induced allergic symptoms (10µg/g dust) at
the 6
th
month in coconut fiber, kapok and
synthetic fiber and at the 9
th
month in sponge-like
polyurethane mattress.
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