It's quite obvious that my uncle (now in his 60s) is on the spectrum. He's never been diagnosed though, as few people were in his day. He was just considered a bit odd and left to get on with it.
These days all schools have SEN coordinators and teachers receive training about conditions such as autism. That was certainly not the case in the old days.
I have a lot of time for the idea that chemicals in foods and household goods may be harmful to us in ways we don't yet understand. Bisphenol A for example was banned recently as you may well have heard (a widely-used chemical added to plastics such as baby bottles and tins of food, which is, it turns out a "hormone disruptor").
However there is a whole lot of quackery and bad science about such things, and I must admit I am particularly suspicious about claims to do with autism since the dangerous, media-hype and bad science circulating vaccines and autism.
Autism is a known condition, and there is research happening into it already. I'm fairly confident that if there was a link to something like mercury it would come out through the mainstream channels.
What I think is more of a risk to us, is chemicals used in industry, which maybe affect us in ways which are not so obvious, so which do not cause recognisiable conditions such as autism, but which could be responsible for causing bad health is less easily recognisable ways (perhaps contributing to heart disease or raising the risk of cancer or whatever). To find out about them, there would need to be large-scale research into the affects of such chemicals. No one is funding such research AFAIK. Who's interests would it be in? (apart from the public good of course!). If the effects of the chemicals are not immediately obvious, then they may go under the radar for generations to come.
Who knows, perhaps the risk from widespread use of chemicals is negligible (although personally I doubt it!). Or perhaps not.
We only starting using chemicals in such a big way relatively recently, so we are all effectively participating into a massive experiment as to what they do to us. But with incredibly unreliable results gathering, so it may take some time to come to any conclusions!