Daily levels of ambient air pollution over the study period are presented in Table 2. Gaseous pollutant data were available for the entire study period (1992 to 2002), while daily particulate matter data were only available from 1998 onwards.
Studies with purely ecologic exposure assessments (e.g.,maternal residence in a polluted vs. unpolluted area) or studies with quantitative traffic density data but without pollutant data (e.g., Cordier et al. 2004) were not included in the review.
Plant data were for both the years, while climate and pollutant data were for year 2004 only, in order to remove the variability due to changes in the atmospheric variables.
Most studies use air pollutant data from large networks of fixed site monitors, combined with large retrospective birth cohorts obtained from government birth registries.
Atmospheric pollutant data were obtained from a fixed-site monitoring station.
Pollutant data were from the US Environmental Protection Agency Air Quality System.
* N = number of sampling days with valid pollutant data (varied due to equipment failures).
Approximately 20% of all cases were missing pollutant data for lags of interest.
We stratified the pollutant data by high-wind and low-wind days.
Pollutant data were non-normally distributed, so we used Spearman's correlation coefficients.
Table 2 displays the meteorologic and pollutant data stratified by season.
