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Effects of ambient air pollution on monthly wheeze and asthma exacerbation in southern California school children
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Effects of ambient air pollution on monthly wheeze and asthma exacerbation in southern California school children
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INFORMATION TO USERS
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EFFECTS OF A M BIENT A IR POLLUTION ON M O NTHLY WHEEZE AND
ASTHMA EXACERBATION IN SOUTHERN CALIFORNIA SCHOOL
CHILDREN
by
Joshua Millstein
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment o f the
Requirements for the Degree
MASTER OF SCIENCE
(BIOM ETRY)
December 2001
Copyright 2001 Joshua Millstein
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UMI Number 1411033
UMI’
UMI Microform 1411033
Copyright 2002 by ProQuest Information and Learning Company.
All rights reserved. This microform edition is protected against
unauthorized copying under Title 17, United States Code.
ProQuest Information and Learning Company
300 North Zeeb Road
P.O. Box 1346
Ann Arbor, Ml 48106-1346
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UNIVERSITY OF SOUTHERN CALIFORNIA
T H E GRADUATE SCH O O L
UNIVERSITY PARK
L O S ANG ELES. CA LIFO RN IA 9 0 0 0 7
This thesis, written by
A i/a
under the direction of h Thesis Committee,
and approved by all its members, has been pre
sented to and accepted by the Dean of The
Graduate School, in partial fulfillm ent of the
requirements fo r the degree of
M a ste r o f S c ie n c e
D m *
r in t * December 17 , 2001
ITTEE IIS CO
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DEDICATION
For Edan
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iii
ACKNOWLEDGEMENTS
My greatest thanks are directed to my committee chair and mentor, Dr. Frank
Gilliland, for his guidance, support and generous commitment o f time. His wisdom
and dedication made this project a rewarding experience. I would also like to extend
my gratitude to Dr. Kiros Berhane for his substantial contribution o f time and his
deep understanding o f generalized linear models. Dr. James W. Gauderman also
provided essential and much appreciated guidance in statistical concepts. It has been
a pleasure and an honor to work with this outstanding group o f researchers.
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iv
TABLE OF CONTENTS
Dedication ii
Acknowledgements iii
List of Tables V
List o f Figures
vi
Abstract
vii
Introduction I
Methods
2
Results 6
Discussion 16
References 21
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LIST OF TABLES
Table I.
Table 2.
Table 3.
Table 4.
Correlations o f mean monthly air pollution levels in
twelve southern California communities in November
1994 through Octobler 1995
Demographics for children in group D o f the Children’s
health Study
Odds ratios for pollutants associated with monthly
incidence o f wheeze
Odds ratios for pollutants associated with monthly
asthma medication use
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Figure 1.
Figure 2.
Figure 3.
LIST OF FIGURES
Boxplots o f mean monthly pollution levels for PM, O3
and NO2 in each community
Boxplots o f mean monthly pollution levels for HNO3,
formic acid and acetic acid in each community
Trends in monthly pollution levels in 12 southern California
communities
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vii
ABSTRACT
We investigated the impact o f ambient air pollution on the respiratory health o f2034
forth-grade children in twelve southern California communities. During the spring
and summer o f 1995, the monthly prevalence o f wheeze was associated with
monthly levels o f particulate matter less than 1 0 microns (PMio), PMio-PMt.5 and
acetic acid within communities (p < .01; p < .01; and p < .05, respectively), but not
with ozone, NO2, PM2.5, nitric acid, or formic acid. Only PM to and PMto-PMis
maintained statistical significance in two-pollutant models that included other
pollutants. In children with physician diagnosed asthma, monthly prevalence of
asthma medication use was associated with ozone, nitric acid, and acetic acid
(p < .01; p < .05; and p < .05, respectively) within communities. However, only the
association with ozone maintained strength in two-pollutant models. We conclude
that ambient air pollution in southern California is associated with respiratory
morbidity in school children.
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I
INTRODUCTION
Over the past several decades, many epidemiological studies have
demonstrated a relationship between levels of ambient air pollution and the
prevalence of acute respiratory symptoms (1,2). Respirable particles less than 10 pm
in diameter (PMio) have been shown to be associated with acute upper and lower
respiratory illness (3-6). Ozone (O3) has been associated with asthma exacerbation
(7-9) and acute respiratory illness in non-asthmatics (10,11). PM2. 5, nitrogen dioxide
(N O i), sulfur dioxide (SO2), and acid aerosols have also been implicated in the
etiology o f various acute respiratory illnesses (1,2,4,12,13). However, inadequate
control o f confounding factors as well as interdependencies between pollutants and
climatic characteristics has made it difficult to clearly isolate specific environmental
factors that affect specific respiratory symptoms.
Southern California provides a useful setting for epidemiologic studies
because air pollution varies substantially in level, composition and concentration
across localities with different geographical characteristics, e.g., coastal, urban basin,
agricultural and mountainous. Studies in Southern California also provide the
opportunity to distinguish effects o f PM from SO2, because suspended PM in the
western U.S. does not contain appreciable amounts o f SO2 . In much o f the eastern
U.S., SO2, resulting mostly from coal burning plants, is a major component o f
suspended PM as sulfate and is thought to be an important contributor to respiratory
morbidity.
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2
We report here a longitudinal analysis o f the association o f monthly air
pollution levels with monthly prevalence o f asthma medication use and wheezing
episodes in children. Children are a vulnerable segment o f the population because
their lungs are still growing, and because they may receive a higher exposure to
pollutants due to more time spent outdoors, more vigorous activity while outdoors
and higher ventilation rates than adults (14). The Children’s Health Study
incorporates data from 1 2 communities, each with one monitoring station, allowing
us to assess the effects o f monthly average levels o f O3, NO2, PMto, PM1.5, PM 10 -
PM2.5 (coarse-fraction particulate matter - PM 10-2.5) and acid vapors on respiratory
illness prevalence.
METHODS
The Children’s Health Study includes school-aged children from 12
communities within a 200-mile radius o f Los Angeles. These 12 communities were
selected with the objective o f maximizing variability in exposure to air pollution
while minimizing correlations between pollutants. Comprehensive descriptions of
the study design have been reported elsewhere (13). The present analysis uses
baseline questionnaire data from a cohort o f fourth grade students (mostly 9 year-
olds) who entered the study in 1995. The written questionnaire, completed by a
parent or guardian upon entrance into the study, characterized the respiratory history
and associated risk factors o f each child. In this report, we focus on month-by-month
accounts o f wheezing and asthma medication use. For the 12 month period previous
to the questionnaire date, parents were asked to check boxes next to months in which
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3
their child had wheezed or used asthma medication. Only children with physician
diagnosed asthma were included in the asthma medication analyses, but excluded if
parents selected “don’t know” to the asthma medication use item. Children were
excluded from the analysis o f wheeze if all data for wheezing were missing.
Information on age, gender, ethnicity, and tobacco smoke exposure was also
obtained from the entrance questionnaire responses.
Monitoring instruments were set up in each participating community to
measure several airborne pollutants (13). These data were used to estimate monthly
means o f 24 hour mean NOi, 10AM-6PM O3,24hr PM 10, and 2-week PM2. 5, nitric
acid (HNO 3), acetic acid and formic acid. Data from the National Climatic Data
Center were used for all monthly average temperature estimates
(http://www.ncdc.noaa.gov/). Most o f the questionnaires were completed in late
October and early November of 1995, so pollution measurements were taken for
November o f 1994 through October o f 1995 to optimally match time o f exposure to
the outcome. The mix o f air pollutants and other environmental factors, e.g.
temperature, airborne pollen, and contagious respiratory illnesses, differ substantially
between the winter and summer months. The atmospheric processes that drive air
pollution levels are different enough between these two periods to warrant seasonal
models for explaining the influence o f measured factors on respiratory illness. We
addressed this issue by including analyses o f the data stratified into two six-month
periods, March through August and September through February.
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4
A supplemental questionnaire that determined time spent outdoors could have
provided an additional basis for stratification into two groups. The prior notion is
that the group with more time spent outdoors should have a stronger dose-response
for pollutants
with higher outdoor concentrations. However, to maintain a more adequate sample
size, we looked at the effect o f eliminating the lower 30% o f children ranked by time
spent outdoors.
Statistical Analyses
The following multilevel model was used to examine the relationship
between community-specific monthly ambient air pollution levels and the monthly
prevalence of wheeze and asthma medication use. The first level,
l°git(Yljc) = Ac / +<& ,
is a logistic model. Here, Aq represents community and month-specific intercepts
adjusted for personal covariates x, which include age, gender, tobacco smoke
exposure, and ethnicity, and doctor diagnosed asthma if the outcome is wheeze. The
model assumes a binary outcome YjjC , which indicates the presence or absence of a
positive response for each month, where i, j and c denote the individual, month and
town, respectively. The second level,
A,, = Ac +yMJ + /?,[BS(TEMPjc-TE M P c)}+/3z (Z J e ~ Z c) + £cf,
is a linear regression model. Intercepts from the first level are regressed on 144
community and month-specific air pollution levels, and 1 2 community-specific
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5
intercepts Ac are estimated. Because time o f year is associated with possible
confounding factors, e.g. contagious respiratory illnesses and ambient airborne
pollen or other allergens, the term yMj is included to adjust for the effect y o f month
M j on the adjusted logit o f Yijc. To account for a possible nonlinear association
between temperature and the outcome, a spline term, with 2 knots at the I s t and 3r d
quartiles, was fitted to the deviations of community-specific monthly temperatures.
The basis matrix o f a spline on the temperature deviations was included in the model
as the term, BS(TEMPjc - TEMPe) , where BS indicates a B-spline basis
representation o f the flexible regression spline on temperature (15) and
TEMP - TEMPc is the deviation o f the community-specific monthly temperature
from the community-specific annual mean. The deviation o f the community-specific
monthly average pollution level Z jc - Z c has coefficient /? ,, the primary parameter
o f interest, which indicates the within-community effect o f monthly average
pollution level on the monthly logit (prevalence) o f wheeze or asthma illness. The
third level model has the form,
Ac —cc + fti TEMP c + ■ p x Z c + s c.
Community-specific intercepts from the second level are regressed on community-
specific average pollution levels. The effect across communities o f average annual
and seasonal air pollution Z c is indicated by fix and adjusted for average annual and
seasonal temperature TEMPc. The equations are combined by substituting A^ and
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6
Ae in the first and second levels with the expressions on the right o f the equality
signs in the second and third levels, respectively. The result is a mixed-effects model
(16) of the form,
logit(YIIC ) = a + yM l + & c + p x[BS(TEMPje -TEMP<)] + 0 ,( Z Je- Z c ) +
P^TEM Pc + P KZc
that simultaneously estimates the effects o f monthly and yearly or seasonal average
pollution levels on the monthly prevalence o f asthma or wheeze illness within and
between communities after adjusting for personal and community-level factors.
Town-by-month and town o f residence membership are modeled as random effects
with variance components Scj and Sc , each assessed to follow a normal
distribution, respectively. An independent correlation structure was assumed
conditional on the random effects. The models were fitted using the Glimmix SAS
macro for generalized linear mixed models (included in SAS/STAT software,
version 8 .0 0 ).
RESULTS
Pollution levels were relatively low for Atascadero, Santa Maria, and
Lompoc in the northern less populated region, were somewhat elevated at Lancaster
but were highest for San Dimas, Upland, Mira Loma and Riverside, all in the greater
L.A. area (Figures I and 2). In general, pollution levels peaked during the summer
months (Figure 3). Ozone was most strongly correlated with HNO3 and acetic acid,
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7
BOXPLOTS OF MEAN MONTHLY POLLUTION LEVELS FOR PM. 0 3 ,
AND NO> IN EACH COMMUNITY
1 0 0 Annual
0, »-
iPMI
no, a > - - _
( P O O )
uwuat _ — _ Sept-Fad. - _ M ar.-A uj. l a ■ a
■ ■ • i.H i'Iii,. ...................
.■ |I. ii... . ‘ i 1 . • . ..
3 0 —
« « > -
( ( i / n A
H 1 f s 1 1 1 1 i } I S 1 1 1 1 11 > I ? IS 3 1 ! ? j S i ? i 11
Figure I. The left panels shows boxplots (n = 12 for each) o f mean monthly pollution
levels, November, 1994 through October 1995, for PM, O 3, and NO 2 in each
community. The line within the box represents the median, and the whiskers
represent the value closest to but not exceeding a standard span from the quartiles.
The middle and right panels (n = 6 for each boxplot) show the same mean monthly
pollution levels as the left panel but divided into two six-month periods.
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8
BOXPLOTS OF MEAN MONTHLY POLLUTION LEVELS FOR HN03 , FORMIC ACID AND
ACETIC ACID IN EACH COMMUNITY
« - Annual — Sept - Feb. M ar.-Aug.
HNO, ,
( p o o l
formic
( p p b )
acaoc * - —
( P O O )
Figure 2. The left panel shows boxplots (n = 12 for each) o f mean monthly pollution
levels, November, 1994 through October 1995, for HNO 3, formic acid and acetic
acid in each community. The line within the box represents the median, and the
whiskers represent the value closest to but not exceeding a standard span from the
quartiles. The middle and right panels (n = 6 for each boxplot) show the same mean
monthly pollution levels as the left panel but divided into two six-month periods.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
HNOj (ppb) O j (ppt>)
9
TRENDS IN M O NTHLY POLLUTION LEVELS IN 12 SOUTHERN
CALIFORNIA COM M UNITIES
o
s
o
n
o
6 8 12 10
S
s
s
o
▼
8 10 12 6
A t ------- LN
---
ML AT
IE LO -------
R J SM
LA
---
LB
---
SO UP
rt
2 4 6 8 1 0 1 2 2 « 6 8 1 0 1 2
m onth m onth
Figure 3. Lines represent mean monthly pollution levels for the 12 Children’s Health
Study communities from November 1994 to October 1995, for O 3, PM io.^, HNO?
and acitic acid. Month number one represents January; two is February, etc.
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10
and PMio and PM 10-2, 5 were most strongly correlated with each other (Table I). The
correlation between PMio and PM 2 . 5 was high across but not within communities.
The lifetime prevalence o f wheeze was usually more than twice that o f
asthma (Table 2). O f the 2081 fourth grade children who entered this study in 1995,
2034 completed the asthma and wheeze questions, of whom 14.5 percent reported
doctor-diagnosed asthma and 32.3 percent reported some lifetime history o f
wheezing. O f the 293 children with doctor-diagnosed asthma, 265 reported some
lifetime history o f wheezing. Thus, a third o f the children with some lifetime history
o f wheeze also had
doctor-diagnosed asthma. The total number of months during which wheezing
occurred was 967 (436 summer months and 531 winter months) and was 546 for
asthma illness (247 summer months and 299 winter months).
In the summer period, respirable particulate matter (PMio or PM 1 0 -2 .5 ) was an
important predictor o f wheeze prevalence (Table 3). Within-community monthly
prevalence o f wheeze was significantly associated with monthly average PMio,
PM,o.2 5 , and acetic acid (OR = 1.50, p = 0.007; OR = 1.35 p = 0.005; and OR = 1.52,
p = 0.020; respectively; Table 3). When we looked at all combinations o f two
pollutants in models that include PMio-2^ or PMio for within-community effects, we
found that the strength o f PMto-2-5 and PMio associations for the summer months
were not significantly affected by the inclusion of other pollutants. However, the
within-community association o f wheeze and acetic acid diminished considerably
when PMio (or PM10.25) was added to the model (acetic acid OR = 1.21, p = 0.352).
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TABLE I. CORRELATIONS OF MEAN MONTHLY AIR POLLUTION LEVELS IN TWELVE SOUTHERN
CALIFORNIA COMMUNITIES DURING NOVEMBER 1994- OCTOBER 1 995
below diagionai: between communities above diagionai: within communities
0 , NO, PM,„ PM(W, PM, j UNO, formic acetic temperature
0 3
-
0.23 0.76 0.77 0.09 0.81 0.70 0.83 0.84
NO, 0.03 - 0.39 0.29 0.28 0.55 0.59 0.45 0.16
m u > 0.13 0.59 - 0.91 0.33 0.75 0.74 0.78 0.71
PM, 0.2 , 0,02 0.50 0.97 - -0.08 0.66 0.60 0.72 0.68
PM,., 0.24 0.65 0.96 0.86 - 0.31 0.42 0.25 0.16
IIN O j 0.44 0.89 0.63 0.50 0.72 - 0.89 0.91 0.69
formic 0,25 0.95 0.59 0.46 0.69 0.95
- 0.86 0.62
acetic 0.41 0.81 0.78 0.70 0.81 0.91 0.87 - 0.77
temperature -0.01 0.58 0.49 0.45 0.50 0.56 0.54 0.63 -
TABLE 2 . DEMOGRAPHICS F O R CHILDREN I N GROUP P O F T H E CHILDREN’S HEALTH STUDY
Percent o f Total b y Town
12
3
o
Upland 3
Santa Maria ^
Atascadero
San Dimas
Riverside 2
Mira Loma ^
Long Beach *
Lompoc 2
Lancaster 2
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
question (yes/no); "H as your child's chest ever sounded wheezy o r whistling including tim es w h en h e o r s h e h a d a cold?'
47
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TABLE 3. ODDS RATIOS FOR POLLUTANTS ASSOCIATED WITH
________________MONTHLY INCIDENCE OF WHEEZE______________________
November - October March - August September - February
Interquartile
pollutant OR Cl OR Cl OR Cl range
0 , 0.96 (0.78. 1.18) 1.14 (0.79. 1.64) 0.99 (0.72, 1.37) 27.83 (ppb)
NOj 1.05 (0.96. 1.14) 1.06 (0.87, 1.29) 1.07 (0.95, 1.20) 5.74 (ppb)
PM,„ 1.10 (0.95, 1.27) 1.50* (1.13,
1.99) 0.94 (0.78, 1.13) I3.39(pg/mj )
I'M II).J s 1.09 (0.96. 1.23) 1.35' (1.09, 1.67) 0.95 (0.80, 1.13) 11.44 (pg/ni3)
I’Mjs 1.00 (0.90.
111)
0.95 (0,75, 1.20) 0.98 (0.86, 1.12) 5.24 (pg/mJ)
IINOj 1.05 (0.91. 1.21) 1.12 (0.90, 1.40) 1.06 (0.87, 1.29) 1,64 (ppb)
acetic acid 1.17 (0.95, 1.45) 1.52* (1.07, 2.16) 1.09 (0.81, 1.47) 2.66 (ppb)
formic acid
L°J
(0.88, 1.16]
L!5
(0J4, 1.43) 1.05 (0.89, 1.24) 0.98 (ppb)
♦p < 0 . 0 5 / p <0.01
14
The annual prevalence o f wheeze was also positively and significantly associated
with average annual PM 10-2.5 across communities (OR = 1.15, p = 0.039). The six-
month prevalence o f wheeze remained significantly associated with average six-
month PMio-zs across communities in both summer (marginally) and winter (OR =
1. 11, Cl = (1.00, 1.24), p = 0.055 and OR = 1.26, Cl = ( 1.02, 1.54), p = 0.028,
respectively; OR’s scaled to an interquartile range for PM 1 0 -2 .5 o f 7.6 pg/m3 across
communities).
The strongest air pollution predictor for asthma medication use was O3
(Table 4). In the single pollutant models, we found significant associations between
monthly prevalence o f asthma medication use and O3, HNO3 and acetic acid
(OR = 1.49, p = 0.007; OR = 1.24, p = 0.027; OR = 1.42, p = 0.017, respectively)
within communities (Table 4). After stratifying into seasonal periods, no effect for
any pollutant attained the 0.05 significance level. All associations between asthma
medication use and annual or summer period PMio or PM 10-2.5 levels were positive
(OR >1), but none reached the 0.05 significance level. In 2-poIIutant models that
included O3 and a second pollutant, the strength of the O3 association was
maintained in all models, but the significance level decreased in models that included
HNO3 or acetic acid (OR = 1.51, p = 0.116 and OR = 1.41, p = 0.116, respectively).
However, the correlations between O3 and these pollutants were rather high (0.81
and 0.83, respectively; Table I). In contrast, the strengths and significance levels for
the effects o f HNO3 and acetic acid were substantially reduced with O3 included in
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TABLE 4. ODDS RATIOS FOR POLLUTANTS ASSOCIATED WITH
_______________________MONTHLY ASTHMA MEDICATION USE_____________________
November - October March - August September - February
Interquartile
pollutant OR Cl_____________OR Cl_____________OR Cl______________range
Oj 1,49* (1,11, 2,00) 1.25 (0.73,
NO, 0.94 (0.83, 1.06) 0.96 (0.74,
FMiu
1.09 (0.89, 1.34) 1.20 (0.80,
1.06 (0.89, 1.26) 1.13 (0.84,
p m 2J 1.02 (0.88, 1.18) 0.99 (0.72,
IIN O j 1.24* 0.02, 1.51) 1.15 (0.85,
acetic acid 1.42* 0.07, 1.90) 1.38 (0.87,
formic acid 1.16 (0.97, 1.40) 1,12 (0.78.
♦ p < 0,05, * p < 0.01
2.13) 1.17 (0.75, 1.81) 27.83 (ppb)
1.24) 1.02 (0.87, 1.21) 5.74 (ppb)
1.80) 0.87 (0.67, 1.15) I3,39(pg/in3)
1.53) 0.78 (0.61, 1,00) ll.44(pg/m 3)
1.36) 1.15 (0.95, 1.38) 5.24 (pg/m3)
1.55) 1.14 (0.87, 1.50) 1,64 (ppb)
2.20) 1.19 (0.79, 1.79) 2.66 (ppb)
1.60) I.J3 (0.90, L42) 0,98 (ppb)
16
the model (OR= 1.04, p = 0.84 and OR = 1.15, p = 0.43, respectively). No significant
effects were found across communities.
For the upper 70% o f children ranked by time spent outdoors, the
within-community association between monthly asthma medication use and average
monthly O3 (OR = 2.02, C l = (1.40, 2.91), p = 0.0002) was stronger than the
association for entire group o f children with physician diagnosed asthma.
DISCUSSION
For 2034 southern California school children, parental reported wheezing
was associated with monthly levels o f PM 10 and PM 10-2.5 within communities over
time, especially in the spring and summer months. Monthly levels o f O3 were
significantly associated with asthma medication use in children with doctor
diagnosed asthma within communities, and this association was stronger for children
who spent more time outdoors.
Our results are consistent with the findings o f the summer air pollution study
(17) that found an association in summer between PM 10 and lower respiratory
symptoms (OR = 1.53, C l = (1.20, 1.95), for a change from 20 to 50 pg/m3 ). This
association existed for PM2.5 as well, but a high correlation between particulate
measures (r = .92) precluded distinguishing between the two effects. In contrast, we
observed a lower correlation (r = -.08 within communities) between levels o f PM10
and PM2.5 and found no association between PM2.5 and incidence o f wheeze.
However, there was a high correlation between PM10 and PMto-zs (r = .91 within
communities) in our study, thus we were unable to distinguish between the effects o f
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17
these pollutants. The lack o f association with PMt_5, in contrast with Dockery et al.
(18), suggests that the effect o f PMio on the incidence o f wheeze may be attributable
to the PMio.2.5 fraction. Sulfur oxides form an important component o f PM2.5 in the
eastern U.S. (1) but not in Southern California. Thus, the sulfur oxide component of
ambient air pollution may have contributed to the significant findings for PM2.5 in
studies conducted in the eastern U.S. Associations have been found in other studies
between PM 10 levels and lower respiratory symptoms (19-21) within communities.
Strong associations are usually absent between wheeze status and measures o f PM 10
(13,18,22) across communities. In this study, associations across communities
between wheeze and measures of PMio did not reach the significance level, but those
between PM 10-2.5 and wheeze did. Thus, PM 10-2.5 may be the active factor
contributing to the development o f lower respiratory symptoms.
No associations were found between PMio and wheeze in the winter months,
but lower levels o f PMkkls (Figure I) in this part o f the year together with an
increase in non-pollution related respiratory illnesses may have decreased our ability
to detect effects. Significant associations were found between wheeze and NOo in
males (OR = 1.54, Cl = (1.04, 2.29)) by Peters et al. (13) for annual differences
across communities, but this relationship did not maintain significance in all subjects
(OR = 1.12, C l = (0.86, 1.45)). We found no significant effects o f NO2 but did not
have a large enough sample size to enable an additional stratification by gender.
Associations between the prevalence o f wheeze and acetic acid lost significance in
the 2-poIIutant models that included PMto or PMkkls- However, other studies have
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18
found chronic effects o f acid vapors on peak expiratory flow rates or other measures
o f lung (23,24,25) and respiratory symptoms (4,13). Hence, there may still be an
important contribution from acetic acid that is masked by PMio or PM 10-2.5 in the 2-
pollutant models due to high correlations.
Associations between O3 levels and hospital admissions or acute respiratory
symptoms for asthmatics are well documented (1,7-9,26) and are supported by
findings here, which further confirm the association between O3 levels and asthma
medication use. Asthma medication use was not associated with levels of O3 across
communities, but this result is not surprising considering evidence that there is some
degree o f adaptation to repeated O3 exposure (1,26.27). I f children with asthma tend
to adapt to normal community levels of O3, then one would expect an increase in
illness as a response to a large deviation from normal O3 levels but not high average
O3 levels. The loss o f significance for acetic acid and HNO3 in the 2-pollutant
models, like the loss o f significance for acetic acid with the wheeze outcome, may
only partially reflect high correlations. No statistically significant associations were
found between PMio (or PM10-15) and asthma medication in the summer months
when PM 10-2^ levels were high, but this finding contrasts with previous studies
(1,20,28-30) and may have resulted from weak statistical power after excessive
stratification. McConnell et al. (4) in a cross-sectional study observed that PMio was
related to the prevalence o f bronchitis and phlegm in asthmatics. This observation
highlights the question o f why no PMto effects for asthma medication use were seen
here, which may be explained by the limited sensitivity o f asthma medication use as
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19
a binary outcome. Also, the effects o f O3 and PMio (and PM 10-2. 5) were positive
during the summer period and the sample size was small. The increase in the strength
o f association between O3 and asthma medication use, after eliminating the lowest
30% o f subjects ranked by time spent outdoors, supports the idea that children who
spend more time outdoors are exposed to higher levels o f O3 and thus experience
higher rates of asthma illness.
An assumption o f this study was that the single monitoring station in each
community was adequate to characterize exposure levels for the study participants. If
pollutant levels vary greatly within communities, the monitoring station may give a
biased characterization o f the general exposure level, but if pollutant levels vary
gradually over a broad area, this assumption is probably appropriate (3 1). The
within-community estimates are more robust than those across communities, because
they depend on average changes over time, which are likely to be similar across
microenvironments. Care must be taken in interpreting cross-community effects
since such ecologic inferences are vulnerable to confounding (32), e.g., income level
may affect a subject’s personal risk by influencing lifestyle as well as by affecting
the likelihood o f living in a more vs. less polluted area. Concentrations o f O3 are
generally lower indoors than outdoors (3 1), due to its high reactivity, so there is
potential for over estimating exposure in subjects that spend a large amount of time
indoors.
This analysis provides evidence that fluctuations in ambient monthly PMio
levels, especially the coarse fraction, are associated with the prevalence o f monthly
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20
wheezing in children. In southern California, this association was strongest during
the spring and summer. We also provide evidence o f a strong association between
fluctuations in ambient monthly O3 levels and the prevalence o f monthly asthma
medication use in children with physician diagnosed asthma. The strength o f this
association was greater for children who spent more time outdoors. Community-
specific annual O3 levels were not associated with the annual prevalence o f asthma
medication use, but community-specific annual levels o f PM 10.2J were associated
with the annual prevalence o f wheeze. We conclude that fluctuation in ambient air
pollution in southern California is associated with the prevalence o f respiratory
morbidity in children.
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21
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Asset Metadata
Creator
Millstein, Joshua
(author)
Core Title
Effects of ambient air pollution on monthly wheeze and asthma exacerbation in southern California school children
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Graduate School
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Master of Science
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Biometry
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University of Southern California
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environmental sciences,health sciences, public health,OAI-PMH Harvest
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Gilliland, Frank (
committee chair
), Berhane, Kiros (
committee member
), Gauderman, William James (
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