Allow me to quote at length from ET Jaynes' book, "Probability: The Logic of Science."
> A common error, when judging the effects of radioactivity or the toxicity of some substance, is
to assume a linear response model without threshold (that is, without a dose rate below which there
is no ill e ect). Presumably there is no threshold effect for cumulative poisons like heavy metal
ions (mercury, lead), which are eliminated only very slowly if at all.
> But for virtually every organic
substance (such as saccharin or cyclamates), the existence of a finite metabolic rate means that
there must exist a finite threshold dose rate, below which the substance is decomposed, eliminated,
or chemically altered so rapidly that it has no ill effects. If this were not true, the human race
could never have survived to the present time, in view of all the things we have been eating.
> Indeed, every mouthful of food you and I have ever taken contained many billions of kinds
of complex molecules whose structure and physiological effects have never been determined, and
many millions of which would be toxic or fatal in large doses. We cannot doubt that we are daily
ingesting thousands of substances that are far more dangerous than saccharin, but in amounts
that are safe, because they are far below the various thresholds of toxicity. But at present there is
hardly any substance except some common drugs, for which we actually know the threshold.
> A common error, when judging the effects of radioactivity or the toxicity of some substance, is to assume a linear response model without threshold (that is, without a dose rate below which there is no ill e ect). Presumably there is no threshold effect for cumulative poisons like heavy metal ions (mercury, lead), which are eliminated only very slowly if at all.
> But for virtually every organic substance (such as saccharin or cyclamates), the existence of a finite metabolic rate means that there must exist a finite threshold dose rate, below which the substance is decomposed, eliminated, or chemically altered so rapidly that it has no ill effects. If this were not true, the human race could never have survived to the present time, in view of all the things we have been eating.
> Indeed, every mouthful of food you and I have ever taken contained many billions of kinds of complex molecules whose structure and physiological effects have never been determined, and many millions of which would be toxic or fatal in large doses. We cannot doubt that we are daily ingesting thousands of substances that are far more dangerous than saccharin, but in amounts that are safe, because they are far below the various thresholds of toxicity. But at present there is hardly any substance except some common drugs, for which we actually know the threshold.