This thesis aimed to test the association between environmental exposures and neurodevelopmental diagnoses, trait, and cognitive scores in a sample of children and adolescents living in Toronto, Canada. The influence of the environment on the prevalence and severity of Attention-Deficit Hyperactivity Disorder (ADHD), Obsessive-Compulsive disorder (OCD), and Autism Spectrum Disorder (ASD) is not yet clear.Studying the effects of these exposures in an urban environment is critical, as they represent potentially important modifiable risk factors. This thesis found evidence of an association between environmental exposures and OCD traits within the community.Consistent results in the analysis are promising; however, the results are preliminary and require further study. The results add to the uncertainty surrounding the effect of the environment on ADHD. Analysis of the combined results support the use of quantitative measures for assessing neurodevelopmental disorders, and the idea that disorders like ADHD and OCD exist at extreme ends of a spectrum of traits within the community.Chapter 4 -Conclusion.