"To conclude, the evidence from the studies allows us to conclude that there is an association between PM [particulate matter] exposure and ASD [autism spectrum disorder] whose strength varies according to the particle size studied with the association with PM2.5 and diesel PM being stronger."
Although probably not great sentence structure to begin a post with a conclusion, the 'bottom line' reported by María Morales-Suárez-Varela and colleagues  summarises the current research looking at particulate matter (a.k.a pollution) and risk of autism. Surveying the current research literature - well, a window "from November 2015 up to January 2016" - authors reported finding a majority of studies showing "positive associations restricted to specific exposure windows which however do not reach statistical significance at times." This adds to other reviews of the research in this area .
I don't want to dwell too much on this paper and topic because it's something that has already received quite a bit of attention on this blog (see here and see here for examples). As with most research areas focused on autism, there is evidence for and evidence against any association/correlation between air pollution exposure and risk of autism. Given the various factors included under the heading of pollution (type, particle size, how exposure is measured) it's perhaps not surprising that there is not yet any 'smoking gun' (pardon the pun) when it comes to any possible association.
I however, am taken by the ideas that (a) genes probably play a hand in translating air pollution exposure to a heightened risk of [offspring] autism and (b) other conditions linked to air pollution such as asthma might also feature in any connection (see here for example). Indeed, in these times of seemingly ever-increasing air pollution (see here) and lots of possible connections, further investigations are very much indicated.
 Morales-Suárez-Varela M. et al. Systematic review of the association between particulate matter exposure and autism spectrum disorders. Environ Res. 2016 Dec 13;153:150-160.
 Lam J. et al. A Systematic Review and Meta-Analysis of Multiple Airborne Pollutants and Autism Spectrum Disorder. PLoS One. 2016 Sep 21;11(9):e0161851.
Morales-Suárez-Varela M, Peraita-Costa I, & Llopis-González A (2016). Systematic review of the association between particulate matter exposure and autism spectrum disorders. Environmental research, 153, 150-160 PMID: 27984759