Bright Lines

A "bright line" is a single numerical value between unacceptable and negligible magnitudes of risk or exposure concentrations of concern. Bright lines are chosen to provide pragmatic definitions of "safe" and "unsafe" for those making risk-management decisions and for those implementing or enforcing decisions. An example of a bright line is an excess-cancer risk of about 10-5: if a risk assessment predicts that more than one case of cancer is likely to occur as a result of exposure to a substance in a population of 100,000 people exposed to it, that risk is judged unacceptable and protective action is required; a predicted risk of less than 10-5 is considered negligible and requires no protective action. Risk-based decisions are generally converted to measurable exposure or emission limits for implementation and compliance. Regulated parties are expected to demonstrate that estimated exposures or risks are below the bright line to operate a manufacturing facility, introduce a new product to the market, or sell foods with low concentrations of contaminants.
Bright lines are generally used with single point estimates of risk to judge safety; Science and Judgment in Risk Assessment characterizes bright lines and point estimates of risk as "magic numbers" whose use is inconsistent with knowledge about the distributions of risk and their inherent uncertainty (NRC 1994a). Strict use of bright lines is also inconsistent with the risk-management framework and with the inclusion of cost and other considerations in decision-making. Bright lines that are health-based standards provide useful goals, however, to guide a decision-making process.

http://www.riskworld.com/Nreports/1996/risk_rpt/html/nr6aa016.htm

I didn't know.