Whether there are environmental triggers, or not, is not known at this time, in regards to ASD, but research is being conducted.
There is currently no evidence showing an actual increase in the incidence of ASD, that cannot be explained by the broadening of the diagnostic criteria, and by more awareness of ASD, which leads to more diagnosed cases.
ASD is not something that there is a reliable diagnostic test for, such as there is with viral or bacterial diseases. ASD is diagnosed using the observation of various behavioural traits. This makes it very difficult to conclude that there is an increase in the incidence of ASD, as using the diagnostic method of observation of behavioural traits, leaves a wide margin of error in diagnosis.