If dioxin and ionizing radiation cause cancer, then it stands to reason that less exposure to them should improve public health. If mercury, lead and PCBs impair intellectual development, then less should be more. But a growing body of data suggests that environmental contaminants may not always be poisonous--they may actually be good for you at low levels.
Called hormesis, this phenomenon appears to be primarily an adaptive response to stress, says toxicologist Edward J. Calabrese of the University of Massachusetts at Amherst. The stress triggers cellular repair and maintenance systems. A modest amount of overcompensation then produces the low-dose effect, which is often beneficial.