It is also important to note that the means in Table 1 mask considerable variation in pollution levels both across monitors and over time.
In the most polluted areas, mean CO levels started at 4 ppm at the beginning of the sample period, but declined to roughly 1 ppm by 2005.
Figs. 2–4 plot pollution levels at one particular pollution monitor (the Camden Lab monitor in Camden) over time and residual pollution levels after controlling for the time and monitor effects and the weather variables included in our regression models.
The “a” series plot 3 month moving averages (corresponding to the measures of pollution we use in birth outcome models), while the “b” series plot 7 day moving averages (corresponding to the measures of pollution we use in the infant mortality models).
These plots show that although adjusting for these factors accounts for seasonal and annual trends, there is still considerable variation left to identify the effects of pollution.
Panel D of Table 1 shows means of the control variables available in the Vital Statistics data, the decennial census, and the weather data.