39 Question 6 - Is it accurate to say that, ounce for ounce and gram for gram, radioactive substances are a million times more harmful to life than any other environ- v mental pollutants? If not, what is a reasonable comparison? Answer 6 Table I shows that for most radioisotopes the mass required to produce short term toxic effects may be greater than that required for some chemical toxins . On the other hand, Table II shows that, for severe long term effects which eventually result in death, the mass required for the most effective radiocarcinogens (radiation sources that produce tumors) is much less than that required for the more effective chemical carcinogens; the radiation sources would appear to be as much as 100,000 or more times more effective on e gram basis. These large ratios do not apply to the more common and important radioisotopes such as tritium, cesium-137, or strontium-90, which, as the following discussion points out, may not be more effective ona | gran basis than potent chemical agents. There.is very great interest in determining the body burden levels that “induce subtle long term effects, although at present there is little experi- mental data available in mammals. A simple proportional interpolation of high level burdens is probably not valid because it appears that many radiatic effects exhibit threshold properties; that is, radiation doses below a threshold level produce essentially no detectable effect, The present explanation for this response is that cells are capable of repairing many forms of radiation damage provided the exposure is delivered at a low enough rate. The existence of similar repair mechanisms which protect cells from chemical carcinogens or mutagens (mutation producing agents ) is not well