Several mechanisms are suspected to underlie adverse birth outcomes among mothers
exposed to air pollutants, including inflammation, direct toxic effects on fetuses
and the placenta, displacement of the oxygen-hemoglobin dissociation curve, and formation
of DNA adducts.
To systematically review the association between air pollutants and birth outcomes
of low birth weight (LBW), preterm (PTB) and small for gestational age (SGA) births.
Electronic databases and bibliographies of identified articles were searched for English
language studies reporting on birth outcomes. Included studies were assessed for risks
of bias in the selection, exposure assessment, confounder adjustment, analyses, outcomes
assessment, and attrition. Unadjusted and adjusted estimates from included studies
were extracted. Methodological differences between the studies were evaluated.
A total of 41 studies, mostly with a moderate risk of biases due to indirect assessment
methods employed, met the eligibility criteria. Exposure to sulphur dioxide was associated
with PTB, exposure to fine particulate matter (PM) of ≤2.5 μM was associated with
LBW, PTB and SGA births, and exposure to coarse PM of ≤10 μM was associated with SGA
births. The evidence for nitrous oxide, nitrogen dioxide, ozone and carbon monoxide
was inconclusive.
Reported associations, and lack thereof, between individual air pollutants and birth
outcomes have differed across published studies. This heterogeneity and/or absence
of association may be due to difficulty in quantifying exposure, method of ascertainment,
time of measurement and collinearity between pollutants. Important future research
directions include developing improved methods to detect the duration and intensity
of exposure, including entire populations, as well as performing well-designed nested
studies that ascertain complete outcomes, avoiding residual confounding, and adjusting
for residential mobility.
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