After exclusions, 76,788 births were included: 22,911 (30%) in New Haven County, CT; 21,947 (29%) in Hartford County, CT; 25,450 (33%) in Fairfield County, CT; and 6480 (8%) in Hampden County, MA. summarizes the study population overall and by county. Mothers were primarily white and married, with a mean age of 29.3 years and 13.8 years of education. Fairfield County had the lowest risk of term LBW, highest fraction of births as a first child, lowest tobacco or alcohol use during pregnancy, highest maternal education, and oldest mothers.
Summary of Study Population Characteristicsa
) provides the largest and second largest constituent contributions to each source from positive-matrix-factorization analysis, for each county. Elemental carbon and sulfur are the main contributors to almost all source factors for almost all counties, comprising an average of 86%–90% of contributions to the motor-vehicle, oil-combustion, and regional-source factors. These constituents comprise an average 56% of the road-dust factor. Chloride and sodium comprise most (61%) of the salt-source factor.
Because of the dominance of a small number of constituents across most sources, we did not identify indicator constituents based on the largest contributor to each source, but rather on constituents that were more closely linked to each source. In other words, for each source, we identified source tracer constituents that most likely originated from that particular source. Whereas eTable 2
shows the main constituents for each source factor, eTable 3
) shows the main sources resulting from each constituent. Zinc, copper, lead, elemental carbon, bromide, and potassium tended to be associated with the motor-vehicle source more than with any other source. Crustal elements (silicon, aluminum, titanium, manganese, calcium, and iron) and barium mostly originated from the road-dust-source factor. Vanadium and nickel were most associated with oil combustion, chloride with salt, and sulfur and sodium with the regional-source category. These results were used to identify constituents to function as approximate indicators of source categories, so that associations of PM2.5
source exposures and birth weight could be examined.
For each constituent, we calculated the ratio of the highest percentage contribution to a source to the second highest percentage, to identify constituents that are more closely linked to a particular source. For example, the ratio for bromide was 1.6 (= 41.6/26.6) indicating that bromide is about 1.6 times more likely to be in the motor-vehicle-source than the regional-source factor (the sources with the highest and second highest percentage contributions of bromide). Indicator constituents were selected as zinc and elemental carbon for motor vehicles as zinc is about 6 times and elemental carbon 3 times more likely to be in motor vehicle PM2.5 than the source with the next highest percentage. Silicon and aluminum were selected for road dust; vanadium and nickel for oil combustion; chloride for salt; and sulfur for the regional-source factor. The selected indicator constituents (ie, those with the highest ratios) were chloride, vanadium, and silicon.
summarizes study subject exposures for these selected constituents and source factors. Exposures were highest for the motor-vehicle and regional sources and lowest for salt. Based on the constituents’ relative mean standard deviation (standard deviation/mean × 100%), the largest variations in exposures were for chloride and vanadium and the smallest for sulfur and zinc. Based on relative mean standard deviation, the salt factor had the largest variation in exposures of the 5 identified sources, and the regional source factor the lowest. eTable 4
) shows correlations among gestational exposures to the constituents, with the highest correlation between aluminum and silicon (0.98).
Summary of Study Population Exposures (µg/m3) During Pregnancy to PM2.5 Mass, Constituents, and Sources (n = 76,788)
shows the association between IQR increase in gestational exposure to PM2.5
mass, constituents and sources, and birth weight. For each exposure, separate analyses were performed for a linear model using birth weight as a continuous variable (, column 3) and a logistic model comparing risk of small-at-term to not small-at-term births (, column 4). Central estimates indicated a decrease in birth weight and higher risk of small-at-term birth for all sources and constituents except for oil combustion and risk of small-at-term birth. Higher levels of all road-dust indicators (silicon and aluminum) and oil-combustion indicators (vanadium and nickel) were associated with lower birth weight for both models. Higher levels of zinc were associated with lower birth weight and increased risk of small-at-term birth. Elemental carbon was associated with increased risk of small-at-term birth. The linear model results for elemental carbon showed an association with decreased birth weight for the central estimate, although results were imprecise. Higher PM2.5
levels from all sources were associated with lower birth weight based on central estimates. All source factors other than oil combustion were associated with higher risk of small-at-term birth in terms of central estimates, although the only strong association was for road dust. The central estimates for PM2.5
total mass indicate that higher levels are associated with lower birth weight, although findings are marginal for the logistic model. Previous work did find such an association,6
and the current study’s findings may result from lower statistical power.
Change in Birthweight (g) per Interquartile Range (IQR) Increase in Gestational Exposure to PM2.5 Mass, Constituents, and Sources
shows associations between trimester exposures to PM2.5 sources and constituents and birth weight. Associations were observed in the third trimester for oil combustion, elemental carbon, zinc, nickel, and vanadium. For most exposures, trimester values were not highly correlated (average correlation 0.29), although there were exceptions (elemental carbon correlations among trimesters 0.82–0.90). Using models adjusted for correlation among trimesters’ exposures, consistent effects were observed across models in the third trimester for oil combustion, elemental carbon, nickel, and vanadium ().
Change in Birthweight (g) per IQR Increase in Trimester Exposure to PM2.5 Mass, Constituents, and Sources
Effect modification by race was investigated for pollutants and sources exhibiting associations in the main model (road dust source, zinc, elemental carbon, silicon, aluminum, nickel, and vanadium). shows associations between these pollutants or sources and birth weight for infants of African-American mothers and infants of white mothers. For all pollutants, a given increment in exposure had larger associations among infants of African-American mothers than those of white mothers (approximately, 35%–100% higher). However, these differences by race did not reach statistical significance for any given pollutant.
Change in Birthweight (g) per IQR Increase in Gestational Exposure to Specific PM2.5 Constituents and Sources, by Race of Infant’s Mother