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Children's Health
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Airborne Endotoxin Is Associated with Respiratory Illness in the First 2 Years of Life Robert Dales,1 David Miller,1 Ken Ruest,2 Mireille
Guay,1 and Stan Judek1 1Air Health Effects Division, Health Canada, Ottawa, Ontario, Canada; 2Research
Division, Canada Mortgage and Housing Corporation, Ottawa, Ontario, Canada Abstract To determine the influence of endotoxin on the incidence of acute respiratory illness during the first 2 years of life, we carried out a longitudinal follow-up study, beginning at birth, of 332 children born in Prince Edward Island, Canada. We measured 5-day averaged air endotoxin in the homes of children, whose parents provided information by daily symptom diaries and twice-monthly telephone contact for up to 2 years. Endotoxin concentration was 0.49 ± 3.49 EU/m3 (geometric mean ± geometric SD) , and number of annualized illness episodes was 6.83 ± 2.80 (mean ± SD) . A doubling of the air endotoxin concentration was associated with an increase of 0.32 illness episodes per year (p = 0.0003) , adjusted for age, year of study, breast-feeding, environmental tobacco smoke, child care attendance, indoor temperature, and income. Indoor mold surface area and fungal ergosterol were not significantly associated with endotoxin. Airborne endotoxin appears to be a risk factor for clinically symptomatic respiratory illnesses during the first 2 years of life independent of indoor fungus. Key words: bacteria, children, endotoxin, housing, respiratory illness. Environ Health Perspect 114:610-614 (2006) . doi:10.1289/ehp.8142 available via http://dx.doi.org/ [Online 3 November 2005]
Address correspondence to R. Dales, Ottawa Hospital (General Campus) , 501 Smyth Rd., Box 211, Ottawa, Ontario K1H 8L6, Canada. Telephone: (613) 737-8198. Fax: (613) 739-6266. E-mail: rdales@ottawahospital.on.ca This work is supported by Health Canada, Canada Mortgage and Housing Corporation, Prince Edward Island Reproductive Care Program, Prince Edward Island Medical Society, Carleton University, and Prince Edward Island Department of Health and Social Services. The authors declare they have no competing financial interests. Received 23 March 2005 ; accepted 3 November 2005. |
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Endotoxins are lipopolysaccharide
components of the outer membranes
of gram-negative bacteria. Endotoxin
has been implicated in bysinnosis,
organic dust toxic syndrome,
and illness in swine confined
animal feeding operations workers
(Douwes et al. 2002). Endotoxin
in settled dust in residential
environments has been associated
with an increase in asthma symptoms,
asthma medications, and reductions
in lung function in those with
atopy or asthma (Douwes et al.
2000; Gehring et al. 2001a; Michel
et al. 1991, 1996; Park et al.
2001a; Rizzo et al. 1997). Despite
these adverse effects, early
exposure may reduce future allergies
and asthma (Lapa e Silva et al.
2000; Litonjua et al. 2002; Reed
and Milton 2001; Von Ehrenstion
et al. 2000; Von Mutius et al.
2000). Most studies were of adults
or school-age children, with
two focusing on infants. In the
present study we examined the
association between airborne
endotoxin and the incidence of
respiratory illnesses in children
during the first 2 years of life.
We accounted for exposure to
a potential confounder, indoor
fungus, which has been associated
with respiratory symptoms and
may be associated with the presence
of indoor endotoxin (Gehring
et al. 2001b; Verhoeff and Burge
2004).
Study design. Data
for the present study were abstracted
from an ongoing study of the
influence of indoor environmental
factors on respiratory illness
during the first 2 years of life.
The study began in 1997 in the
province of Prince Edward Island,
Canada, which has a population
of approximately 150,000. The
study was approved by the ethics
review boards of the Ottawa Hospital
and the Health Protection Branch
of the Canadian government. Recruitment
occurred during the late autumn
and winter (cold season) of each
year when the ground was frozen.
Because of resource constraints,
we recruited approximately 60
consecutive newborns each year.
All physicians who practice obstetrics
in the province participated
in recruitment. Women in the
third trimester of pregnancy
received letters from the physicians’ offices
describing the study and requesting
participation. Interested women
were contacted by telephone to
obtain informed consent. Excluded
from the study were babies born > 4
weeks premature, those with neonatal
respiratory difficulties requiring
prolonged hospitalization at
birth, and those whose families
expected to change residence
within 2 years of birth. Only
one child per household was studied.
Baseline information was obtained
on sociodemographics and family
history. The participating parents
maintained a daily symptom diary
from birth until 2 years or until
the study ended, on large multipurpose
calendars. Each study family
was phoned twice monthly to document
information from the diary. If
parents had omitted recording
symptoms on a daily basis, they
provided information for the
previous 2 weeks based on recall.
Parental reporting of child care
attendance was also recorded
every 2 weeks.
Definition of respiratory
illness. We adapted
the method of Samet et al.
(1992) to define a respiratory
illness episode, the purpose
being to identify discrete
acute illnesses as opposed
to persistent ongoing symptoms,
such as a chronically runny
nose. We defined the beginning
of an illness episode as
2 consecutive days with any
one of the four following
symptoms: stuffy nose, cough,
wheeze, and shortness of
breath. The illness episode
starts on the first of these
2 consecutive days and ends
when there are 2 consecutive
days with none of these symptoms,
the last day of the illness
episode being the last day
with a symptom.
The primary outcome of interest
was the number of illness episodes
prorated on an annual basis (number
multiplied by 365/days of observation).
A secondary outcome measure,
illness days, was defined as
the sum of all days occurring
within illness episodes, also
prorated on an annual basis.
For example, if a child had two
illness episodes each lasting
3 days, six illness days would
be assigned. If a child had two
illness episodes each lasting
5 days, 10 illness days would
be assigned.
Air sampling for endotoxin
and ergosterol. Sampling
was done within the first
year of birth, and for 81%,
within the first 4 months.
Air from the child’s
bedroom was sampled for both
endotoxin and ergosterol
through a three-piece cartridge
equipped with a polycarbonate
filter for approximately
5-7 days with a Buck model
SS sampling pump (AP Buck,
Orlando, FL, USA) calibrated
at 2 L/min at the beginning
and end of the sampling.
Forty-eight hours after the
pump was installed, the flow
rate was checked to ensure
it was within 5% of the initial
reading. Very high dust concentrations
can clog the filter and reduce
the pump flow. If this happened,
the pump was stopped and
air endotoxin concentration
was calculated based on the
reduced sampling time. The
total volume of air sampled
ranged from 6.0 to 23.9 m3,
limited by the need to maintain
an acceptable flow rate.
Once collected, the cartridges
and filters were sealed in
new plastic bags and stored
at room temperature under
dry conditions.
For endotoxin analysis, the
filters were extracted with depyrogenated
water (LRW, Associates of Cape
Cod Ltd., East Falmouth, MA,
USA) assisted by sonication.
Samples taken before April 2001
were analyzed by the Limulus amoebocyte
lysate (LAL) assay gel clot method
from Associates of Cape Cod.
The detection limit was 0.25
EU/filter. All subsequent analyses
were done by the LAL chromogenic
method, also from Associates
of Cape Cod. The detection limit
was 0.1 EU/filter. All analyses
were performed by the same analyst
in the same laboratory according
to the manufacturer’s instructions.
Apart from differences in the
lower limit of detection, the
two methods gave similar results.
We analyzed ergosterol, an
indicator of fungal biomass,
by gas chromatography and mass
spectroscopy as described by
Foto et al. (2005). The volume
of air sampled ranged from 10.7
to 25.4 m3. Ergosterol
was determined using an Agilent
model 5973 quadrupole mass spectrometer
(Agilent Technologies, Inc.,
Palo Alto, CA, USA) operating
in the electron ionization mode
at 70 eV. Compounds were separated
on an Agilent 6890 series gas
chromatograph equipped with a
30 m 0.25
mm inner diameter 0.25 µm
ZB-5 capillary column. The concentration
of ergosterol was determined
against an authentic external
standard in the selective ion
mode using m/z 363
and 396. The detection limit
in selective ion mode was 4.5 ± 0.6
ng/mL. Ergosterol standard (Sigma
Chemical Company, St. Louis,
MO, USA) was recrystalized, freeze-dried,
and stored at 4°C.
Definition of covariates
other than ergosterol. During
the postnatal interviews,
several characteristics were
recorded every 2 weeks for
a period of 2 years: the
presence of furry or feathered
pets in the house, the presence
of smokers inside the house,
whether the baby was breast-fed,
and the number of hours per
week the child was cared
for outside the home. Based
on this information, we created
a summary variable for each
of the characteristics. For
the first three characteristics,
we calculated the percentage
of postnatal interviews where
the characteristic was declared.
For example, if 20 of 50
postnatal interviews mentioned
the presence of pets inside
the house for a particular
child, the value of the pet
variable would be 0.4. The
last characteristic, the
number of hours per week
that the baby was cared for
outside the home, was averaged
over the entire 2-year period
to create the child-care
variable. For presentation,
we then categorized some
summary variables. The pet
variable was categorized
into “never declared
pets,” “sometimes
declared pets,” and “always
declared pets,” which
divided responses almost
equally into thirds. The
exposure-to-smoke variable
was categorized into terciles.
The breast-feeding and the
child-care variables were
kept as continuous data.
The age variable was defined
as the age of the child in
the middle of the span of
the follow-up period. “Mold
surface area” refers
to the total surface area
of the home with mold visible
to trained home inspectors.
Statistical analysis. We
tested the association between
the number of illness episodes
per year and airborne bedroom
endotoxin concentration using
multiple linear regression analysis.
Valid endotoxin results were
obtained for 351 houses. A total
of 19 homes were excluded from
the analysis--15 because of missing
temperature data and four because
of missing income data--leaving
332 homes for analysis. Eleven
babies of 332 (3.3%) exited the
study before turning 2 years
of age (mean age, 1.1 years),
and 56 babies had not reached
2 years of age (mean 1.7 years)
by the last day of data collection
used for this analysis. This
is an ongoing study with children
entering and exiting at different
times. The illness episodes and
covariates were annualized and
thus adjusted for duration of
follow-up. To test the effect
of the 11 babies who exited early,
we repeated the analysis with
and without them, and no differences
were found in the illness-endotoxin
association.
Mold surface area was expressed
as ranks from highest to lowest.
Endotoxin values followed a log-normal
distribution, so they were log-transformed.
A multiple linear regression
model with the number of respiratory
illness episodes per year as
the dependent variable and the
natural logarithm of endotoxin
concentration as the primary
independent variable of interest
was built with the stepwise method.
A categorical variable--the year
of sample collection--was added
to the model to account for any
seasonal variations in illness
from year to year and also the
change in the lower limit of
detection of the endotoxin analytic
technique after 2001. Endotoxin
and year of home sampling were
held in the model along with
any variables with p-values < 0.10,
resulting in the final model,
with variables endotoxin, year
of home sampling, temperature,
age, mean hour per week that
the baby was cared for more than
1 day a week outside the home,
percentage of postnatal interviews
in which the baby was breast-fed,
income, and categorized percentage
of postnatal interviews where
smokers were declared in home.
A potential confounder was
defined as a variable (Tables
1 and 2) that, if added to the
model, would change the parameter
(β)
of the natural logarithm of endotoxin
by > 10%. No confounders were
found for the illness episodes
model. The residuals from the
regression equation were normally
distributed (Shapiro-Wilk statistic
= 0.9932, p = 0.1362).
We also examined the homogeneity
of variance assumption, and the
chart of residuals against predicted
values showed no particular pattern.
Interactions biologically plausible
were also tested, and none were
found to be statistically significant
at the 5% level. We found no
evidence of interaction between
allergies or asthma in parents
and endotoxin. Careful examination
of each of the partial residual
plots (i.e., the component-plus-residual
plot) did not reveal any sign
of nonlinearity in the relationship
between illnesses and air endotoxin.
Further, adding a square term
for endotoxin did not improve
significantly the R2 of
the model.
The endotoxin measurement was
made only at the beginning of
the 2-year follow-up. To determine
the robustness of the endotoxin-illness
association, we measured it at
several time points between the
initial endotoxin measurement
and symptom assessment. We would
assume that a true causal association
would remain stable or weaken
over time. If the association
increased or fluctuated randomly
with time of follow-up, this
would reduce the probability
of a causal association. We measured
the results from 90-, 180-, 270-,
360-, 450-, 540-, 630-, and 720-day
windows around the time of endotoxin
sampling. The regression model
obtained previously for a 2-year
period was applied to each of
these windows. The β-coefficient
for the effect of the natural
logarithm of endotoxin on illness
episodes and total illness days
along with its 95% confidence
interval were graphed against
the size of the window.
The characteristics of the
332 children, overall and stratified
by bedroom airborne endotoxin
level, are presented in Tables
1 and 2. Of the categorical variables
(Table 1), only year of testing
was significantly associated
with endotoxin, with no secular
trends ( p < 0.0001).
The pets variable was not associated
with endotoxin concentrations.
For dogs, the geometric mean
and geometric standard deviation
(GSD) were 0.46 ± 3.82
if dogs were never reported present
in the home and 0.54 ± 3.15
if ever reported to be in the
home ( p = 0.22). There
was also no significant difference
in endotoxin values between homes
where dogs were reported in < 50%
of interviews compared with at
least 50% of interviews ( p =
0.12). Similarly, there was no
significant association between
cats and endotoxin. Of the continuous
variables (Table 2), only indoor
relative humidity was positively
related to endotoxin ( p =
0.01).
Illness episodes correlated
best with the individual symptoms
of cough and stuffy nose; Pearson
correlation coefficients were
0.69 and 0.76, respectively,
both at p < 0.01. For
illness days, respective values
were 0.83 and 0.95. Wheeze was
also significant for both illness
episodes and illness days at
0.37 and 0.41, respectively (p < 0.01).
The annualized number of respiratory
illness episodes and total days
of illness episodes were positively
related to endotoxin at p =
0.13 and p = 0.07, respectively
(Table 3). All of the individual
respiratory symptoms were greater
in the higher compared with the
lower endotoxin group, but only
the incidence of wheeze reached
statistical significance, being
a relative 248% greater in the
higher compared with the lower
endotoxin group (p =
0.01).
The unadjusted Pearson correlation
coefficients between log-transformed
endotoxin and illness episodes
and illness days were 0.105 (p =
0.056) and 0.106 (p =
0.053), respectively. The association
for number of days with wheeze
was 0.271 (p < 0.0001),
but other individual variables
were not significant at p =
0.05. The adjusted associations
for illness episodes and total
illness days were highly significant
(Tables 4 and 5). The multiple
linear regression model for illness
episodes resulted in a β-coefficient
of 0.46 (SE 0.13) for the natural
logarithm of endotoxin (p =
0.0003), which means that each
1.0 unit increase in the natural
logarithm of airborne endotoxin
concentration was associated
with 0.46 more illness episodes
per year. An alternative expression
of the relation would be that
a doubling of air endotoxin concentration
was associated with an increase
of 0.32 illness episodes per
year (p = 0.0003), adjusted
for age, year of study, breast-feeding,
environmental tobacco smoke,
child care attendance, indoor
temperature, and income. Also,
starting from the geometric mean
(0.49) and increasing endotoxin
by its geometric mean resulted
in 4.7% excess illnesses per
year. Similarly, doubling air
endotoxin was associated with
an increase of 3.25 illness days
per year (p = 0.005),
adjusted for age, year of study,
breast-feeding, child care attendance,
indoor temperature, and sex.
Starting from the geometric mean
(0.49) and increasing endotoxin
by its geometric mean resulted
in 5.5% excess illness days per
year.
Significant covariates in the
regression of illness episodes
were year of testing, indoor
temperature, age, child care,
environmental tobacco smoke,
and income (all p < 0.05).
Similar results were found with
illness days with a β-coefficient
of 4.68 (SE 1.66, p =
0.005).
In Figure 1, the β-coefficient
for the effect of the natural
logarithm of endotoxin on illness
episodes and total illness days
is graphed against the size of
the window. The magnitude of
the association between illness
episodes and endotoxin levels
was almost linearly decreasing
with the use of longer observation
periods extending further from
the original sampling. Because
the effect of endotoxin levels
on illness episodes was highly
significant for a 2-year period,
it would be even more significant
for shorter observation periods.
Air endotoxin was positively
associated with an increase in
episodes of respiratory illness
among children during their first
2 years of life despite adjustment
for many host and environmental
factors, including indicators
of fungal exposure. The method
of endotoxin collection is unique
compared with previous studies
of indoor air, most of which
sampled floor dust rather than
airborne dust, which may be better
correlated with inhalation exposure.
Air sampling, done infrequently
in previous studies, usually
consisted of a grab sample (up
to 30 min), whereas our 5-day
collection period would be expected
to provide a more stable average
estimate of exposure. Another
unique feature of exposure is
that endotoxin sampling was done
during the cold season when the
ground is frozen and usually
snow covered. This makes it less
likely that the measured endotoxin
in air is simply a reflection
of what was present outdoors
at the time of the sampling.
Rylander (2002) suggested that
fungal products, and specifically
(1-3)-β-d-glucan,
may coexist with endotoxins and
thus may confound the association.
In the present study, ergosterol,
a marker of fungal growth in
Prince Edward Island homes, did
not influence the illness-endotoxin
association.
Michel et al. (1991) measured
endotoxin concentrations in the
house dust of 28 adult subjects
with chronic stable asthma. Exposure
to higher levels (> 5.6 ng
lipopolysaccharide/mL) was associated
with poorer asthma control measured
by symptoms, medication use,
and lung function. In a subsequent
study, Michel et al. (1996) refined
their previous findings in a
group of 69 adults with asthma
who were sensitized to house
dust mites. Asthma control was
related to house dust endotoxin,
averaging 2 ng/mg dust in those
exposed to Der p 1 levels > 10 µg/g
dust, but not in those exposed
to lower levels of the major
dust mite antigen. Rizzo et al.
(1997) and Douwes et al. (2000)
reported adverse effects of settled
dust endotoxin in school-age
children with asthma, atopy,
or asthma symptoms but not in
those without an atopic history.
Park et al. (2001a) and Gehring
et al. (2001a) reported increased
wheezing in infants living in
homes with increased levels of
settled dust endotoxin, consistent
with the findings of the present
study in which airborne endotoxin
was measured and fungal burden
was accounted for using airborne
ergosterol. Indoor fungus therefore
is not likely to have confounded
the observed relation, although
fungus has been associated with
respiratory symptoms and may
be associated with indoor endotoxin
(Gehring et al. 2001b; Verhoeff
and Burge 2004). Compared with
occupational settings, the indoor
air endotoxin concentrations
in our study were low (on average, < 2
EU/m3), yet associations
with adverse health effects were
observed. The relatively large
sample size with daily symptom
monitoring over 2 years in each
subject appeared to give us the
power to detect these effects.
Further support from health effects
at low levels comes from Park
et al. (2000), who reported airborne
endotoxin in Boston homes during
the warm season to be generally < 1
EU/m3, and corresponding
floor dust was < 100 EU/mg
(estimated values taken from
a log-scaled graph). Before our
study, Park et al. (2001a) also
found associations between settled
dust endotoxin and wheeze.
Endotoxins are also postulated
to confer health benefits. Lipopolysaccharide,
the main component of endotoxin,
may shift the cytokine response
toward a Th1 response
and away from a Th2
response, thereby reducing the
chance of developing atopy (Lapa
e Silva et al. 2000). Consistent
with this theory is the observation
that children growing up on farms,
where endotoxin exposure is higher
than in urban areas, have less
atopy (Von Ehrenstion et al.
2000; Von Mutius et al. 2000).
Evidence thus far suggests that
early childhood exposure to endotoxin
may protect against future asthma,
but later in life endotoxin appears
to exacerbate asthma (Reed and
Milton 2001). The present study
indicates that very early exposure
is not benign but associated
with increased illness episodes.
Litonjua et al. (2002) reported
on 226 children between the ages
of 1 and 5 years with a parental
history of atopy who were followed
for 4 years. House dust endotoxin
was associated with reported
wheezing that decreased with
increasing duration of follow-up.
This observation suggests that
early exposure offers future
protection, or that the initial
endotoxin measure became less
representative of ongoing endotoxin
exposure over time. The present
study was somewhat different:
beginning at birth, including
all children irrespective of
parental history of atopy, using
airborne rather than dust endotoxin,
including indicators of respiratory
illness in addition to wheeze,
and considering confounding by
indoor mold exposure. Nevertheless,
even with children not selected
based on atopic parents, we also
found that wheeze was the symptom
with the strongest association
with endotoxin, and the effect
size became smaller with increased
duration of follow-up.
Sources of endotoxins. Gram-negative
bacteria are found in water,
soil, and outdoor air. Reported
indoor sources of gram-negative
include contaminated humidifiers,
pets, storage of food waste,
and increased amounts of settled
dust (Park et al. 2001b). The
need for water availability is
consistent with our finding that
relative humidity was positively
associated with air endotoxin,
not previously described. Gehring
et al. (2001a) found that dust
concentrations were higher with
cats and dogs present. These
studies found that endotoxin
was higher in old buildings,
with longer duration of occupancy,
low ventilation rate, and poor
housekeeping. Indoor pets were
not associated with air endotoxin
in the present study, which was
carried out during the cold season
with frozen ground and often
snow cover. Perhaps pets would
be less likely to go outside
and subsequently bring in soil
on their paws.
In summary, the present study
supports a positive association
between airborne endotoxins and
the incidence of acute respiratory
illnesses during the first 2
years of life, independent of
allergic history and exposure
to indoor mold that may coexist
with contamination by bacterial
endotoxin. |
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Last Updated: April 20, 2006
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