That is to say, small quantities over a long period of time are harmless and, in some cases, are beneficial. Vitamin D and fluoride are two examples. We require small quantities of vitamin D daily for good health, and we know that fluoride is essential for good dental health. The same can be said of sodium chloride, common table salt. Not surprisingly, some chemicals are chronically toxic but acutely non-toxic.
Metallic mercury is one example.
A large ingestion of a single dose of metallic mercury will pass through the body without causing significant damage, but a buildup of mercury in small amounts over a lifetime can be lethal. Although there is little correlation between acute and chronic toxicity, each in its own way is dose-related. The greater the dose, either in small continuous quantities or in a single large quantity, the greater the effect will be. Caffeine in coffee, oxalic acid in spinach, ethanol in scotch, and acetylsalicylic acid in aspirin are just a few examples. Yet even in the face of evidence and common sense, the notion that exposure to trace quantities of foreign chemicals may actually produce beneficial effects is unacceptable to many people who have an anti-chemical bias.