<p class="first" id="d1594249e582">This national cohort study of more than 1.3 million
births in China from December
1, 2013, to November 30, 2014, assesses the association of concentration of airborne
particulate matter of 1 μm or less with the risk of preterm birth and identifies subgroups
of pregnant women who may be vulnerable.
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<h5 class="section-title" id="d1594249e588">Question</h5>
<p id="d1594249e590">Is the concentration of airborne particulate matter with a median
diameter of 1 μm
or less (PM
<sub>1</sub>) associated with the risk of preterm birth in China?
</p>
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<h5 class="section-title" id="d1594249e596">Findings</h5>
<p id="d1594249e598">This national cohort study of more than 1.3 million births found
that increases in
PM
<sub>1</sub> concentration of 10 μg/m
<sup>3</sup> during the entire pregnancy as well as at each trimester were significantly
associated
with an increased risk of preterm birth.
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<h5 class="section-title" id="d1594249e607">Meaning</h5>
<p id="d1594249e609">Exposure to PM
<sub>1</sub> air pollution is associated with an increased risk of preterm birth,
and control
measures to reduce PM
<sub>1</sub> air pollution may lower the future incidence of preterm birth.
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<h5 class="section-title" id="d1594249e620">Importance</h5>
<p id="d1594249e622">Airborne particulate matter pollution has been associated with
preterm birth (PTB)
in some studies. However, most of these studies assessed only populations living near
monitoring stations, and the association of airborne particulate matter having a median
diameter of 1 μm or less (PM
<sub>1</sub>) with PTB has not been studied.
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<h5 class="section-title" id="d1594249e628">Objective</h5>
<p id="d1594249e630">To evaluate whether PM
<sub>1</sub> concentrations are associated with the risk of PTB.
</p>
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<h5 class="section-title" id="d1594249e636">Design, Setting, and Participants</h5>
<p id="d1594249e638">This national cohort study used National Free Preconception Health
Examination Project
data collected in 324 of 344 prefecture-level cities from 30 provinces of mainland
China. In total, 1 300 342 healthy singleton pregnancies were included from women
who were in labor from December 1, 2013, through November 30, 2014. Data analysis
was conducted between December 1, 2016, and April 1, 2017.
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<h5 class="section-title" id="d1594249e641">Exposures</h5>
<p id="d1594249e643">Predicted weekly PM
<sub>1</sub> concentration data collected using satellite remote sensing, meteorologic,
and land
use information matched with the home addresses of pregnant women.
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<h5 class="section-title" id="d1594249e649">Main Outcomes and Measures</h5>
<p id="d1594249e651">Preterm birth (<37 gestational weeks). Gestational age was
assessed using the time
since the first day of the last menstrual period. Cox proportional hazards regression
analysis was used to examine the associations between trimester-specific PM
<sub>1</sub> concentrations and PTB after controlling for temperature, seasonality,
spatial variation,
and individual covariates.
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<h5 class="section-title" id="d1594249e657">Results</h5>
<p id="d1594249e659">Of the 1 300 342 singleton live births at the gestational age
of 20 to 45 weeks included
in this study, 104 585 (8.0%) were preterm. In fully adjusted models, a PM
<sub>1</sub> concentration increase of 10 μg/m
<sup>3</sup> over the entire pregnancy was significantly associated with increased
risk of PTB
(hazard ratio [HR], 1.09; 95% CI, 1.09-1.10), very PTB as defined as gestational age
from 28 through 31 weeks (HR, 1.20; 95% CI, 1.18-1.23), and extremely PTB as defined
as 20 through 27 weeks’ gestation (HR, 1.29; 95% CI, 1.25-1.34). Pregnant women who
were older (30-50 years) at conception (HR, 1.13; 95% CI, 1.11-1.14), were overweight
before pregnancy (HR, 1.13; 95% CI, 1.11-1.15), had a rural household registration
(HR, 1.09; 95% CI, 1.09-1.10), worked as farmers (HR, 1.10; 95% CI, 1.09-1.11), and
conceived in autumn (HR, 1.48; 95% CI, 1.46-1.50) appeared to be more sensitive to
PM
<sub>1</sub> exposure than their counterparts.
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<h5 class="section-title" id="d1594249e671">Conclusions and Relevance</h5>
<p id="d1594249e673">Results from this national cohort study examining more than 1.3
million births indicated
that exposure to PM
<sub>1</sub> air pollution was associated with an increased risk of PTB in China.
These findings
will provide evidence to inform future research studies, public health interventions,
and environmental policies.
</p>
</div>