We expect that potential errors in PM10 exposure estimates are more or less the same for different levels of PM10 (non-differential misclassification), resulting in an underestimation of effect esti- mates. In a study of Ostro et al. (1991) adjustment of exposure estimates for time-activity pattern information, such as time spent outdoors, led to a 43% increase in the estimated effect of air pol- lution, suggesting non-differential misclassification if exposure estimates were based on fixed-site monitoring stations. Moreover, Wilhelm and Ritz (2005) found stronger effects of air pollution (CO, PM10, and PM2.5) exposure if they limited their analysis to women living within 1 mile of a monitoring station. This indicates that our estimated effects of prenatal PM10 exposure are likely to be underestimated.