- All of the commonly found real carcinogens have been found – like asbestos. They were easy to find.
The ones that are being discovered now are weak carcinogens, for example they may cause cancer in 1 out of 100,000 people who come in contact with it. Very minor risk (unless you are the one!).
Should you avoid such a small risk? How will it affect your lifestyle? Will slowing down 1 mpg on the highway protect you even more? Will taking one less car trip to the grocery store a year protect you more?
Every thing you do has risk. Low level carcinogens are in there, but low on the scale.
- How do you measure something that effects only 1/100,000 people? If you study 100,000 each in a control group and a test group, you might not even get a single case of cancer. And if you do, how do you extrapolate from a single case?
The way you do is to a) not use people and b) give megadoses.
So, will people get cancer because rats do? Maybe.
More importantly, will a megadose give proportional, linear results?
What I mean, is say 1000mg are given to rats, and the result is extrapolated to mean 1000 cancers per 100,000 people. So, the reasoning goes, because people are exposed to 1mg, you get 1 cancer per 100,000.
But this assumes linearity – if 1000mg causes 1000 cancers, then 500 causes 500, and 1 causes 1. But people may have tolerance for small amounts, but less tolerance for large amounts. So, if the 1000/1000 is correct, it may turn out that 500mg causes only 50 cancers, and 1mg causes 0.00001.
This is not unreasonable. Salt will kill anyone if eaten in large quantities. But at half that dose, hardly anyone will die. At normal rates of consumption, it kills no one (unless you consider the effects on people with hypertension).
But it’s very difficult to measure how carcinogenic a weak carcinogenic substance is at different (especially low) levels. So most of the time when you hear about numbers like 1/20,000 people will get cancer, it’s usually a worst case scenario and may well be inflated.