I personally think it was due to genetic alterations and bioengineering experiments. The dog race had dogs racing unearthly fast, most likely due to bio-engineering. There was biohazard signs throughout the movie, and multiple talks and insinuations about biogenetical experiments. Much like people are saying engineered food is disrupting hormones causing girls to mature too early, I think the screenwriter is insinuating that this is the cause of the infertility. Of course I could be dead wrong because they never went out and said it.
