Like the assembling of a prehistoric animal’s skeleton, this careful piecing together of evidence can never furnish final or absolute answers. There will always be a few missing parts, first because experimenting on human beings is not, thankfully, considered ethically acceptable. Human carcinogens must, therefore, be identified through inference. One set of clues is provided by observations of people who have been inadvertently exposed to substances suspected of having cancer-causing tendencies. But often these people have been exposed to unknown quantities over unknown periods of time. […]
Another reason for scientific uncertainty [over what chemicals cause cancer, etc.] is that the widespread introduction of suspected chemical carcinogens into the human environment is itself a kind of uncontrolled experiment. There remains no unexposed control population to whom the cancer rates of exposed people can be compared. Moreover, the exposures themselves are uncontrolled and multiple. Each of us is exposed repeatedly to minute amounts of many different carcinogens and to any one carcinogen through many different routes. From a scientific point of view, such combinations are especially dangerous because they have the capacity to do great harm while yielding meaningless data. Science loves order, simplicity, the manipulation of a single variable against a background of consistency. The tools of science do not work well when everything is changing all at once.
Steingraber, Sandra. Living Downstream: An Ecologist’s Personal Investigation of Cancer and the Environment (p. 32-33). Da Capo Press. Kindle Edition.
This quote exhibits why absolute scientific “proof” of chemicals causing cancer and other disorders is hard to come by, and one reason why studies of suspected toxins usually turn up mixed results. Just because some studies fail to find a link between a particular chemical and cancer (or autism, Alzheimer’s, etc.) does not necessarily mean that the chemical does no harm. I maintain that we should err on the side of caution in all areas relating to human health and environmental health — both of which are, naturally, interconnected — if we wish to save ourselves from the explosion of disease that now envelops us.