The prevalence of Autism Spectrum Disorder (ASD) diagnoses in the United States has increased dramatically over the past 20 years, fueling investigations into possible environmental triggers for the disorder. Exposures to pesticides, persistent pollutants, prescription medications, and heavy metals through various routes have been examined, but very few studies have examined the potential role of chronic inhalation of hazardous air pollutants (HAPs) in the etiology of ASD. This thesis was designed to examine possible relationships between HAPs and ASD prevalence on a statewide level for the U.S., with sub-analyses on a finer, countywide level within the state of Maryland. Findings suggest consistent, positive associations between ASD prevalence and HAPs at the statewide level for the U.S. The findings do not persist at the county level in the Maryland sub-analyses. These results reinforce the concept of ASD as a spectrum of phenotypes best explained through multifactorial etiological models.