For each monitor, we construct measures of pollution by taking the mean of the daily values either over the three trimesters before birth (for the birth outcomes models) or for each week after birth (for the infant mortality model).
For the pollutants of interest, the daily measures we use are the 8-h maximums of CO and O3 and the 24-h average of PM10, which correspond with national ambient air quality standards.
County level weather data come from the Surface Summary of the Day (TD3200) from the National Climatic Data Center.