Motor vehicle-related air pollution and adverse birth outcomes in Los Angeles County, California, 1994--2000

A number of studies reported positive relationships between maternal exposure to outdoor air pollution and adverse birth outcomes. However, outdoor concentrations measured at monitoring stations may not take into account differential exposure to pollutants found in elevated concentrations near heavy traffic roadways. Therefore, we used an epidemiologic case-control study design to examine whether residential proximity to heavy traffic influenced the occurrence of low birth weight (LBW) and/or preterm birth in Los Angeles County between 1994 and 2000. We mapped subject home locations at birth and estimated exposure to traffic-related air pollution using a distance-weighted traffic density (DWTD) measure. We calculated odds ratios (ORs) and risk ratios (RRs) for being LBW and/or preterm per quintile of DWTD. For the time period 1994--1996, we observed an approximately 10--20% increase in risk of preterm birth (both normal and low weight) and term LBW in infants born to women potentially exposed to high levels of motor vehicle exhaust, as represented by DWTD. These risks appeared to be strongest for women whose third trimesters fell during November--May, consistent with elevated pollution in proximity to sources during more stagnant air conditions present in cooler months.; Although residential proximity to traffic did not appear to be associated with higher risks of term LBW or preterm and LBW birth during 1997--2000, residential proximity to trucks on freeways did appear to be associated with greater risks of these outcomes. This suggests more heavily polluting vehicles within the overall cleaner motor vehicle fleet, such as trucks, may now be more important. Positive associations between background CO concentrations and LBW and preterm birth in 1997--2000 suggest, overall, air pollution may still be harmful.; Annual average, winter and spring concentrations of CO, NO2, PM2.5 measured at monitoring stations in LA County were well correlated with DWTD, but fall and summer averages were only weakly or negatively correlated with traffic density. Such differences may be of importance to studies focused on assessing health impacts of short-term air pollution exposures. Additional measurements at locations in closer proximity to traffic are needed to further evaluate the validity of the DWTD measure.