Association of ambient air pollution and meteorological factors with primary care visits at night due to asthma attack
Environ Health Prev Med
Association of ambient air pollution and meteorological factors with primary care visits at night due to asthma attack
Shin Yamazaki 0 1 2
Masayuki Shima 0 1 2
Yoshiko Yoda 0 1 2
Katsumi Oka 0 1 2
Fumitake Kurosaka 0 1 2
Shigeta Shimizu 0 1 2
Hironobu Takahashi 0 1 2
Yuji Nakatani 0 1 2
Jittoku Nishikawa 0 1 2
Katsuhiko Fujiwara 0 1 2
Yasuyuki Mizumori 0 1 2
Akira Mogami 0 1 2
Taku Yamada 0 1 2
Nobuharu Yamamoto 0 1 2
0 K. Oka F. Kurosaka S. Shimizu H. Takahashi Y. Nakatani J. Nishikawa K. Fujiwara Y. Mizumori A. Mogami T. Yamada N. Yamamoto Himeji Medical Association , 3-7-21 Nishiimajuku, Himeji 670-0061 , Japan
1 M. Shima Y. Yoda Department of Public Health, Hyogo College of Medicine , 1-1 Mukogawacho, Nishinomiya 663-8501 , Japan
2 S. Yamazaki (&) Department of Healthcare Epidemiology, Graduate School of Medicine and Public Health, Kyoto University , Yoshidakonoecho, Sakyo-ku, Kyoto 606-8501 , Japan
Aim The association of outdoor air pollution and meteorological elements with primary care visits at night due to asthma attack was studied. Methods A case-crossover study was conducted in a primary care clinic in Himeji City, Japan. The subjects were 956 children aged 0-14 years who visited the clinic with an asthma attack between the hours of 9 p.m. and 6 a.m. Daily concentrations of particulate matter, ozone, nitrogen dioxide, and a number of meteorological elements were measured, and a conditional logistic regression model was used to estimate odds ratios (ORs) of primary care visits per unit increment of air pollutants or meteorological elements. The analyses took into consideration the effects of seasonality. Results Of the 956 children, 73 (7.6 %) were aged \2 years and 417 (43.6 %) were aged 2-5 years. No association between daily ozone levels and primary care visits due to asthma attack at night in the spring or summer was found. An inverse relation between suspended particulate matter and primary care visits due to asthma attack was detected in the winter. ORs in the summer per degree increment in daily mean temperature was 1.31 [95 % confidential interval (CI) 1.09-1.56], and ORs in the autumn per hourly increment in daily hours of sunshine was 0.94 (95 % CI 0.90-0.99). Conclusion The findings of our study fail to support any association between daily mean concentration of air pollutant and primary care visits at night. However, we did find evidence indicating that certain meteorological elements may be associated with primary care visits
Air pollution; Asthma; Ozone; Meteorological elements; Particulate matter
Children’s exposure to air pollution is of special concern
because their immune system and lungs are not fully
developed, thereby potentially putting them at risk of
developing asthma or worsened pulmonary function.
Further, children spend a significant amount of time outdoors,
where concerns of exposure to pollution from traffic, power
plants, and other sources are generally higher than indoors.
Exposure to ambient air pollutants, such as particulate
matter (PM), ozone, and nitrogen dioxide (NO2), is
associated with many adverse health outcomes ranging from
increased symptoms of allergic airway disease to increased
]. Children are considered to be more
sensitive to air pollution than adults [
], and asthmatic
children are particularly vulnerable to the adverse health
effects of air pollution. Studies of asthmatic children have
concluded that exposure to high concentrations of ozone or
PM significantly enhances the risk of respiratory
symptoms, asthma medication use, and reduced lung function,
such as decline in peak expiratory flow or forced expiratory
flow in a short time [
]. However, while a number of
studies have been conducted in the field of air pollution
epidemiology, few have examined the association between
meteorological elements and asthma attack, and the
adverse health effects of low-level exposure to air
pollutants remain unclear.
Previous studies have shown that the meteorological risk
factors of care visits due to asthma attack are high
], large changes in temperature [
high atmospheric pressure [
], low relative humidity
, and large changes in humidity [
]. However, these
data may suffer from publication bias.
In this study, we collected and analyzed data on the
mean levels of air pollutants and meteorological elements
to determine whether there was an association between any
of these environmental factors and primary care visits of
children at night due to asthma attack.
The setting of this study was Himeji City Emergency
Clinic, Himeji, Japan, which had been established for the
purpose of treating emergency cases between 9 p.m. and
6 a.m. on weekdays. Himeji City is located in the western
part of Japan, within 100 km of central Osaka, and faces
the Setonaikai Sea. The city is 534 km2 in area, and its
population is approximately 540,000. The subjects of our
study were city residents aged 0–14 years with a past
history of asthma attack who had visited the municipal
emergency primary care clinic between 9 p.m. and 6 a.m.
at some point between 1 April 2010 and 31 March 2012.
We excluded patients who visited the clinic on national
holidays for reasons discussed in the Statistical methods
section. The medical records of all patients were provided
retrospectively, and subjects’ age, gender, diagnosis, and
date of visit were recorded. Eligible subjects were patients
diagnosed with asthma by their primary care physician and
for whom bronchodilators were prescribed. The study was
approved by the Himeji local government with respect to
using personal identification information obtained from the
municipal clinic (Himeji City Emergency Clinic, Himeji,
Air pollutants and meteorological elements
Data on daily concentrations of suspended PM (SPM),
NO2, and ozone from 1 April 2010 through to 31 March
2012 were obtained from the Himeji local government.
SPM is defined in the Japanese Air Quality Standard as any
particle collected with an upper 100 % cut-off point of an
aerodynamic diameter of 10 lm. The 50 % cut-off
diameter for SPM is assumed to be approximately 7 lm; we
therefore refer to this variable as PM7. The monitoring
station where these air pollutants were measured was
located on a residential street in the city. We also measured
hourly concentrations of PM with a 50 % cut-off
aerodynamic diameter of B2.5 lm (PM2.5), PM with a 50 %
cutoff aerodynamic diameter of B10 lm (PM10), and organic
black carbon (OBC) using the R&P TEOM-1400 ambient
particulate monitor (Rupprecht & Patashnick Co., Albany,
NY) at a point nearby the monitoring station from April
2010 through March 2012. Meteorological elements such
as daily mean values for atmospheric pressure, relative
humidity, temperature, wind speed, as well as total hours of
daylight were also assessed. These data were obtained from
the Japan Meteorological Agency.
The study design was that of a time-stratified
case–crossover analysis, which is an accepted technique for assessing
brief changes in risk associated with transient exposures
]. Case–crossover analyses require exposure data for
cases only and can be regarded as a special type of case–
control study in which each case serves as its own control.
This design has the advantage of inherently controlling for
potential confounding caused by fixed individual
characteristics, such as sex, race, diet, and age. ‘‘Time-stratified’’
indicates the method by which the control periods were
chosen. Specifically, we stratified time into months to
select days for control periods that fell on the same day of
the week within the same month as the date of the primary
care visit (day of the index period). For example, if the
primary care visit at night due to asthma attack occurred on
September 18, the three control days were September 4, 11,
and 25. Therefore, this approach also controls for
longterm trends, seasonality, and day of the week. The merits of
case–crossover designs in studies of health effects of air
pollution have been discussed in detail by Schwartz .
We excluded patients who visited the clinic on national
holidays because of bias in control selection. That is, if
patients whose visits occurred on holidays were included as
subjects, the estimated relative risks were lower than
expected because the concentration of air pollutants on
holidays (days for the index periods) was
usually/systematically lower than that on non-holidays (days for control
We examined associations between daily mean
concentrations of each air pollutant and the risk of primary
care visits at night due to asthma attack. These
concentrations were subject-specific values averaged over the day
of the indexing. We estimated odds ratios (ORs) of primary
care visits at night due to asthma attack per 10 lg/m3
difference in SPM in a single-pollutant model adjusted for
1-day mean atmospheric pressure (hPa), relative humidity
(%), temperature ( C), wind speed (m/s), hours of daylight
(h). Similarly, we also estimated ORs of primary care visits
per 10-ppb difference in NO2 and in ozone. In addition, we
estimated ORs of primary care visits at night due to asthma
attack per 10 lg/m3 difference in SPM, per 10 ppb
difference in NO2, and per 10 ppb difference in ozone in a
multi-pollutant model adjusted for the same variables as
the single-pollutant model. We also assessed the
association between primary care visits due to asthma attack and
the concentrations of air pollutants or meteorological
elements 1 day before the visits.
All models took into consideration the effects of season
and unusually high and low temperatures; modified effects
were examined using a four-level indicator variable for the
spring (April through June), summer (July and August),
autumn (September through November), and winter
months (December through March).
The PHREG procedures of SAS release 9.2 (SAS
Institute, Cary, NC) were used to perform the conditional
logistic regression. All tests were two-tailed, and alpha was
set at 0.05. We computed ORs and their 95 % confidence
intervals (CIs). Given that several test procedures were used
in our study, multiple testing issues arose. However, we did
not devise any countermeasures for these issues as we
believed that elevated risks of air pollutants in this study
should be demonstrated by the precautionary principle.
Characteristics of the subjects enrolled in our study are
shown in Table 1. Of the 956 children evaluated, 73 were
aged \2 years, 417 were preschool children aged
2–5 years, and 466 were school children aged 6–14 years.
Figure 1 shows the number of cases in each month of the
The daily mean concentrations of air pollutants and
other meteorological data are shown in Table 2. The mean
concentration of ozone was higher in the spring months
than in the other months.
Table 3 shows the association between air pollutants
and primary care visits at night due to asthma using the
single-pollutant model. We noted no association between
air pollutants and primary care visits, other than SPM in
winter. When we analyzed these data according to three
age groups, namely, children aged 0–1 year, 2–5 years, and
6–14 years, the results were almost the same as those
obtained for the whole study cohort (data not shown).
Table 4 shows the results for the multi-pollutant model.
We noted statistical significance in the inverse relation
between ozone levels and primary care visits in winter, as
well as between SPM and primary care visits in winter.
With respect to meteorological elements, we observed an
association between temperature increase and primary care
visits in the summer, and between decline in hours of
daylight and primary care visits in he autumn. No
6-14 years old
2-5 years old
0-1 years old
Associations are shown as ORs and their 95 % CIs per unit increment of each indices
NO2 Nitrogen dioxide, SPM suspended particulate matter, PM2.5 Particulate matter (PM) with a 50 % cut-off aerodynamic diameter of B2.5 lm
PM10 PM with a 50 % cut-off aerodynamic diameter of B10 lm, OBC organic black carbon, SD standard deviation
statistically significant associations were noted between
primary care visits due to asthma attack and the
concentrations of air pollutants or meteorological elements 1 day
before the visits except for the association between
temperature and primary care visits in the summer (OR 1.36,
95 % CI 1.08–1.70).
To ascertain whether there is an association between
ambient air pollution/meteorological factors and primary
care visits at night due to asthma, we examined these factors
among children with a past history of asthma attack who
had visited the municipal emergency primary care clinic in
Himeji, an industrial city in the western part of Japan,
between 9 p.m. and 6 a.m. at some point between 1 April
2010 and 31 March 2012. We noted no association between
1-day ozone levels and primary care visits due to asthma
attack at night in the summer, nor did we note any
association between other air pollutants and primary care visits,
except for an inverse relation between ozone and SPM and
primary care visit due to asthma attack in the winter season.
However, we did find evidence indicating that several
meteorological elements, such as temperature and hours of
daylight, may be associated with primary care visits due to
asthma attack. The OR in the summer months per degree
increment of daily mean temperature was 1.31 (95 %CI
1.09–1.56), and that in the autumn per hourly increment of
hours of daylight was 0.94 (95 % CI 0.90–0.99).
Ozone and primary care visits due to asthma attack
Our findings regarding a potential association between
ozone and physician visits due to asthma attack were not
consistent with those of previous studies. A recent U.S.
Environmental Protection Agency analysis of ambient
ozone health effects concluded that children with asthma
suffer acute adverse health consequences at current
ambient levels of ozone [
], and Babin et al. [
] and Moore
et al. [
] also observed an association between pediatric
emergency room visits for asthma exacerbation and
outdoor ozone levels. In a previous Japanese study as well,
Yamazaki et al. [
] noted an association between ozone
and physician visits due to asthma attack in the summer.
However, the findings in our study show no association
between ozone levels and primary care visits in the warmer
months. We propose that a number of factors could explain
these results. First, compared to the previous Japanese study
by Yamazaki et al. [
], in which investigators analyzed the
association between hourly concentrations of air pollutants
and hourly records of primary care visits due to asthma
attacks, we analyzed daily concentrations and primary care
visit data. As such, we were unable to detect any effects of
acute hourly high-level exposure of air pollutants. For
example, in our study, the maximum concentration of ozone
in the warmer months (April through September) was a daily
high of 62 ppb (Table 2), compared with a value of 224 ppb/
h in Yamazaki et al.’s study [
]. Second, outcomes such as
‘‘emergency room visit’’ or ‘‘primary care visit’’ might not be
appropriate in studies on adverse health effects of air
pollution conducted in developed nations, as attentive asthma
medication for the prevention of these attacks is relatively
widely available in these countries. With respect to our
finding of an inverse association between ozone and primary
care visits in the winter, we speculate that results from the
multi-pollutant model may suffer from the effect of
multilinearity among air pollutants. Pearson’s correlation
coefficients between ozone and NO2, ozone and SPM, and NO2
and SPM were -0.69, -0.12, and 0.44, respectively. Finally,
the detection of this inverse association may be the result of
an alpha error of the test.
PM and NO2 and primary care visits due to asthma
Our findings failed to support any association between
daily mean concentration of PM/NO2 and primary care
visits at night. Moreover, the inverse relation of SPM on
primary care visits with asthma attack was detected in the
winter. We speculate that this was an alpha error detection
because we did not find an association of PM2.5 and PM10
with primary care visits in the winter in our study.
Meteorological data and primary care visits due
to asthma attack
With respect to the relationship between primary care visits
due to asthma attack and temperature, several studies have
examined respiratory morbidity effects of heat waves with
consistent findings [
]. Our results support a positive
association between high temperature and trigger of
primary care visits for asthma attack. To our knowledge, ours
is the first study to examine the relationship between
primary care visits for asthma attack and hours of daylight.
Further studies will be needed to examine the association
between change in meteorological risk factors and
A number of limitations to our study warrant mention.
First, the significance of the association between air
pollution and primary care visits at night due to asthma attack
is diminished because primary care visits due to asthma
attack are a surrogate measurement for asthma
exacerbation. The time lag between exposure to elevated
concentrations of air pollutants and asthma exacerbation
can vary, with an equal variation in time lag between
asthma exacerbation and time of primary care visit. These
variations in time lag affect the statistical association
between air pollutants and primary care visits at night due
to asthma attack. Second, we were unable to evaluate the
effect of indoor air pollution on primary care visits. In
Japan, indoor space heaters are usually used in the winter,
particularly December through February, and as such the
concentrations of indoor air pollutants are reported to be
considerably high in the winter [
]. The observed inverse
association between outdoor air pollutants and primary
care visits in winter may therefore have been confounded
by these space heater-generated indoor air pollutants.
Third, selection of subjects for this study may have been
subject to issues with external validity, as we restricted our
population to nighttime patients. Fourth, the estimated ORs
in this study may suffer from non-differential
misclassification, causing our results to be biased toward null.
In conclusion, while we noted no association between
daily concentration of ozone and primary care visits with
asthma attack at night in the summer, we did find evidence
suggesting that meteorological elements such as
temperature and hours of daylight might be associated with primary
care visits due to asthma attack.
Acknowledgments This research was supported by the
Environmental Research and Technology Development Fund C-1005) of the
Ministry of the Environment, Japan.
Conflict of interest All authors declare that they have no competing
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