There’s two ways to answer this question. First, environmental can refer to the home environment of the child and second, environmental can also refer to the surrounding landscape and area where the child’s home exists. In both cases, the answer is slightly different.
When referring to a child’s home environment, only extreme abuse and exposure to high levels of a toxic substance while in utero or just after birth and leading up to the child’s second birth is thought to be a probable cause of autism. This is because blows to the child’s head or high lead levels can alter the brain’s growth and development. Because it isn’t certain that the child would not develop autism if not in these conditions, some pediatricians are willing to argue that it does contribute to the development of autism.
As for environmental, i.e., the earth, trees, plants, animals and landscapes a child’s home resides in, again there is no founded proof that it environment in this context is the cause of autism. However, children exposed to toxins in the lakes and rivers or the fish and game taken and eaten isn’t disproven and it is very difficult to make a clear claim in this regard. Because children do suffer from other diseases and disorders from toxic substances in soil, air and water, the potential for autism being related to these is slim, but a very minor possibility.