I'll start with the story of DDT and its effects on the top of the food chain in the avian world.
DDT was never sprayed on the eagle, nor was it sprayed on the fish that the eagles eat,
nevertheless, the eggs had trace amounts of DDT in them and it affected the thickness of the shell causing the eggs not to mature.
The source? Farmer spraying fields of plants, draining off into the streams, insects with DDT being consumed way down the food chain by small water dwelling animals, which were consumed by still larger fish until the size was large enough for an eagle to catch.
Think about it, that is a long way to travel.
We know, from other countries experience, that Arsenic can cause cancer, but at fairly high levels, but what about low levels?
We know from the non-smoking Uranium miners in Colorado and Utah that radon in very high levels can cause lung cancer. Reports and studies on low levels when i studied them were mixed as were the studies on radon in water (even more so because some studies showed an inverse relation between cancer and high levels of radon in the water).
Then there was the study that the US EPA did on tree milling areas where there would be a very fine dust produced. The study found that those workers who also worked producing methanol were not only NOT effected, but that the methanol vapors mitigated other factors such as smoking.
There is thee issue of asbestos, in the medium small particle range, causing a very bad disease, which is a direct cause and effect, or manganese which caused a mercury like brain problem when consumed in high quantities and of course a long history of problems with mercury itself.
Then we go to small levels of the next generation of pesticides after DDT, Aldrin (active component is dieldren) and chlordane. Dieldren has been described in literature as something that would assist cancer production if there is something else which would cause cancer. Chlordane is a concern for the endocrine system.
Now we get into the difficult stuff, pesticides have been found to effect immune systems at low levels and affect autoimmune diseases a generation down.
The issue with environmental factors is that they are complicated, small amounts of dieldren might not affect a person, but maybe their children or maybe a high dose of radon or a small dose of arsenic might be a trigger.
Eating process foods might be a factor. Other pesticides in foods might be a factor. Using chlorine bleach might be a factor.
Occupational exposure to ozone from a copier might be a factor or excess car exhaust from long commutes may play in to the equation.
One thing the US EPA did right is they talk about life time exposures to substances, but it can also be combination of things that could be factors.
This is why straight epidemiology does not work well in these situations, but it is the only tools we have.
This is complicated and direct connections are often hard to find.