So: "The findings of this review suggest that the etiology of ASD [autism spectrum disorder] may involve, at least in a subset of children, complex interactions between genetic factors and certain environmental toxicants that may act synergistically or in parallel during critical periods of neurodevelopment, in a manner that increases the likelihood of developing ASD".
That's the conclusion reached by a huge review paper by Dan Rossignol and colleagues* (open-access). I'm going to say no more aside from inviting readers to put half an hour aside, read and digest the significant work that has gone into this paper. Oh, and aside from including the autism research tag-team that is Rossignol and Frye (see here and see here and see here), a certain S Genuis also appears on the authorship list (see here and see here).
* Rossignol DA. et al. Environmental toxicants and autism spectrum disorders: a systematic review. Translational Psychiatry. 2014; 4: e360; doi:10.1038/tp.2014.4
D A Rossignol, S J Genuis, & R E Frye (2014). Environmental toxicants and autism spectrum disorders: a systematic review Translational Psychiatry DOI: 10.1038/tp.2014.4