Cases were excluded for the following overlapping
reasons: women reported on the followup
questionnaire that the child did not have
ASD (n = 32); the child was adopted (n = 9);
they did not want to participate (n = 20);
or they did not report the child’s birth year
(n = 71). Children reported to have trisomy 18,
fragile X, an XXY genotype, or Down’s,
Angelman’s, Jacobsen’s, or Rett’s syndrome
also were excluded (n = 11). Of the remaining
children, 329 were born after 1987, when
air pollution data were available, but 4 had
insufficient address information for geocoding,
yielding 325 cases. In this study we refer to
children with autism, Asperger’s syndrome, or
other autism spectrum disorder as cases and use
“ASD” to refer to this case definition.
We validated the ASD diagnosis by
telephone administration of the Autism
Diagnostic Interview–Revised (ADIR) (Lord
et al. 1994) to 50 randomly selected case
mothers who indicated willingness to complete
the interview (81% of the 636 mothers who
responded to the follow up questionnaire were
willing to be interviewed). Diagnoses reported
by women who were willing versus unwilling
to participate in the substudy were similar
(25% autism, 51% Asperger’s, and 25%
PDD-NOS compared with 25% autism, 49%
Asperger’s, and 23% PDDNOS, respectively).
For the subsample of mothers who completed
the ADIR, 43 children (86%) met full ADIR
criteria for an autism diagnosis (based on minimum
scores in all three domains and onset by
3 years of age). The remaining children met the
onset criterion and communication domain
score, but missed full diagnosis by one point in
one domain (n = 5) or had qualifying scores in
one or two domains only (n = 2). Thus, all of
the children in the subsample exhibited some
autistic behaviors and may have been on the
autism spectrum.