A gem from Andrew Weil

...hormesis may help explain why people who lead strenuous lives with plenty of moderate physical challenges may be healthier and live longer than those in more comfortable circumstances. A 2008 paper titled "Hormesis in Aging" by researchers from the Laboratory of Cellular Aging, Department of Molecular Biology, University of Aarhus in Denmark concluded that "single or multiple exposure to low doses of otherwise harmful agents, such as irradiation, food limitation, heat stress, hypergravity, reactive oxygen species and other free radicals have a variety of anti-aging and longevity-extending hormetic effects."

All of which suggests that one of the best routes to health is to make yourself a little uncomfortable now and then. The most profitable discomforts are likely those with which human beings have a long evolutionary history such as physical exertion, getting hungry, regularly tipping back a modest measure of alcohol, short-term exposure to cold or heat, and so on. Conversely, novel stressors -- such as the stew of noxious synthetic chemicals in the modern environment with which we have no evolutionary history -- are best regarded as guilty until proven innocent.

Which brings up a word of caution: Throughout history, irresponsible politicians and commentators have cited the hormetic effect to justify reducing restrictions on pollution -- claiming that a little poison or radiation in the water, air or food supply is good for us. This is dangerous nonsense. Hormesis appears to be of value only when dosages are very carefully controlled, which does not describe releasing random mixtures of toxins, especially synthetic ones, into general circulation. There's still a great deal we don't understand about hormesis. Until we do, the smartest policy for governments and industry is to keep the public's exposure to environmental toxins as low as possible.