Despite the controversy about whether or not there are safe levels of exposure to carcinogens, there is no argument with the fact that there are practical thresholds for carcinogens. The incidence of cancer caused by chemicals is dose-related. The greater the dose, the greater the incidence. The length of time it takes for a cancer to develop after exposure (the induction period) is inversely related to dose. The greater the dose, the shorter the time or the smaller the dose, the longer the time. If the exposure is so small as to increase the induction period to 200 years, of what practical significance would that be to people whose average life span is less than 100 years? If the dose is so small that the incidence is decreased to one in a trillion, it is of no practical significance to us (because there aren’t a trillion of us on earth.)
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https://en.wikipedia.org/wiki/Mutagen