One definition of acceptable risk that has been widely accepted in environmental regulation is if lifetime exposure to a substance increases a person's chance of developing cancer by one chance in a million or less. This standard is used, for example, as a cleanup goal for hazardous waste sites. To put this standard in perspective with risks encountered in daily living:

• The chance that the average American will develop cancer is one in four, and the chance of death from cancer is one in five.

• Thus, while 250,000 of every 1 million Americans now develop cancer, the large amounts of money required by environmental regulations are spent to prevent the number from rising to 250,001 of every 1 million Americans.

Remarkably, the one-chance-in-a-million environmental criterion, which has cost society billions of dollars, has never received thorough regulatory or scientific review. It is an arbitrary level proposed more than 30 years ago for entirely different purposes.

• Two scientists, asked to develop guidelines for safety testing in animal studies, suggested in an article in 1961, strictly for purposes of discussion, that "safe" be defined as equal to one chance in 100 million of developing cancer.

• The scientists said of the one-in-100-million number, "We just pulled it out of a hat."

• The Food and Drug Administration adopted one in 100 million in a 1973 notice in the Federal Register, changing it to one in 1 million by the time the final rule was issued in 1977.

The action by the FDA established that, for guideline purposes, a risk of one in 1 million or less was so small it would be considered the same as no risk - "essentially zero" in the official terminology. The FDA specifically stated that a risk could be higher than "essentially zero" and still be "acceptable." However, many regulations and guidance documents have interpreted the terms as synonymous, although this was never a requirement in any legislation until the revision of the Superfund law in 1990.

The "one in 1 million" criterion - or considering "essentially zero" as the only acceptable risk of cancer - has been applied almost exclusively to hazardous waste sites, pesticides and selected carcinogens, apparently because of a general perception that they pose the greatest risks to society. Sources such as air and drinking water are regulated less stringently, apparently because they are perceived as posing less risk, whether or not the data support that perception.

Source: Kathryn A. Kelly and Nanette C. Cardon, "The Myth of 10-6 as a Definition of Acceptable Risk," EPA Watch, Vol. 3, No. 17, September 15, 1994.