I'm skeptical that we can point to any particular disease and say with certainty that it was caused by exposure to a specific chemical. There are thousands, perhaps millions, of different chemical compounds both natural and manmade that an average person will be exposed to over a lifetime. How can we be reasonably confident that a particular one of them caused a given sickness?

One of the reasons why the cancer rate increased so dramatically during the 20th century is that average life expectancy also increased dramatically. More people are living longer, and many forms of cancer like prostate are associated with age. I heard some numbers yesterday morning that help put this in perspective: before the 20th century, as many as 20% of American children died before their 2nd birthday. I forget the exact number but a shockingly high number (I think it was 13 out of every 1000) women died in childbirth. Today, only about 1 woman out of every 14,000 dies in childbirth and the infant mortality rate is a tiny fraction of what it was a little over 100 years ago. Likewise, in the days before antibiotics, many people died from simple infections that are easily treated today. The point is that a lot of people who might've died of cancer died from other causes first.

Researchers do cluster analysis to try and determine if a particular area has a higher than average cancer rate. It's a sad fact that a certain percentage of children are still born with birth defects or will become ill. The researcher has to determine if a higher than expected number of people are becoming sick in a given area. If that proves to be the case, then they try to look for common factors like chemical exposure that might explain it. It's difficult work and a rather inexact science.