Covariates. We examined family and community
socioeconomic indicators that may
be associated with ASD ascertainment. To
characterize community circumstances around
the time by which ASD was likely to have
been diagnosed, we used two U.S. Census
tract variables (linked by mother’s mailing
address) measured 6 years after the birth
of the child: median income and percent of
residents with a college education, which we
divided into quartiles. We used the maximum
of the mother’s parents’ education during her
infancy as a proxy measure of maternal childhood
socioeconomic status. The index child’s
current family income was based on the family
income reported by the mother in 2001. The
educational attainment of the mother’s partner
or spouse was reported in 1999. We also
examined factors that may be associated with
both ASD and air pollutant exposure: smoking,
year of birth, maternal age at birth, and
air pollution prediction model year. Smoking
during the index pregnancy was assessed in
2001. Year of birth was by mother’s report.
Maternal age at birth was calculated by subtracting
the child’s birth year from the mother’s
birth year. The air pollution prediction
model year (HAP year), 1990, 1996, 1999, or
2002, was modeled as a categorical variable.