Air pollution and birth outcomes: a systematic review.
BACKGROUND
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.


OBJECTIVE
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.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Department of Pediatrics, Mount Sinai Hospital, Toronto, Ontario, Canada. pshah@mtsinai.on.ca
Prakesh S
PS
Shah
Taiba
T
Balkhair
D000393:Air Pollutants / Q000032:analysis; D000397:Air Pollution / Q000706:statistics & numerical data*; D001724:Birth Weight; D005260:Female; D005865:Gestational Age; D006801:Humans; D007230:Infant, Low Birth Weight; D007231:Infant, Newborn; D019570:Inhalation Exposure / Q000032:analysis / Q000706:statistics & numerical data; D008297:Male; D018811:Maternal Exposure / Q000706:statistics & numerical data; D052638:Particulate Matter / Q000032:analysis; D011247:Pregnancy; D011248:Pregnancy Complications / Q000453:epidemiology*; D011256:Pregnancy Outcome / Q000453:epidemiology*; D047928:Premature Birth / Q000453:epidemiology; D013458:Sulfur Dioxide / Q000032:analysis
D000393:Air Pollutants; D052638:Particulate Matter; D013458:Sulfur Dioxide