But she was really feeding off of a concept developed a few years earlier. The “precautionary principle” was conceptualized when the National Academy of Sciences proposed a radical change in the risk assessment of exposure to radiation and carcinogens. It recommended changing the regulatory paradigm from a “threshold dose” model to a linear one.
The threshold paradigm was what one might call common sense. It held that humans could tolerate small doses of things that, in larger doses, could be harmful.
Sunlight is a perfect example. Low doses are actually required for survival, as ultraviolet radiation — the same general type that causes sunburn — catalyzes the formation of Vitamin D. But, as is obvious to anyone who lives in a sun‐drenched area, excessive exposure can lead to death in the near term (from dehydration) or the longer term (from skin cancer).
The “linear model” assumes that just a single molecule of a carcinogen or a single ionization from an X‐ray can induce cancer. The enthusiasm spawned by Earth Day soon gave us brand new regulatory agencies such as the Environmental Protection Agency and the Occupational Safety and Health Administration. The EPA routinely applies the linear model to carcinogens.