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.

FINDING 5.3: Risk managers have often relied on clearly demarcated bright lines, defining boundaries between unacceptable and negligible exposures or risks, to guide their decisions. Congress has occasionally sought to include specified bright lines for risk in legislation. However, a strict bright-line approach to decision-making cannot explicitly reflect uncertainty about risks, population variation in susceptibility, community preferences and values, or economic considerations, all of which is required by the Commission's risk-management framework.

RECOMMENDATION: Bright lines or ranges of bright lines should be used as guideposts or goals for decision-making but should not be applied inflexibly. In addition to bright lines intended to protect the general population, bright lines to protect especially susceptible subpopulations--such as young children, pregnant women, or adults with lung disease--should be considered. Congress should leave the establishment of specific bright lines or ranges of bright lines to the regulatory agencies.

RATIONALE

Risk managers are accustomed to the clear guidance provided by bright lines for implementing and determining compliance with risk-based standards or guidelines. Measurable contaminant concentrations--such as permissible exposure limits (PELs) or threshold limit values (TLVs) in the workplace, action levels for food contaminants like aflatoxin on peanuts or mercury in swordfish, and national ambient air quality standards (NAAQS) for carbon monoxide or ozone in air--provide assurance that risks will be negligible as long as contaminant exposure concentrations are below the bright lines of those values. If risks or contaminant concentrations are found to exceed their bright lines, action is expected to be taken to protect workers, consumers, or the community. Small quantitative differences from those lines, whether above or below, can make a big difference in whether protective actions are taken. Nonetheless, bright lines provide a basis for consistent decision-making.

There are several potential problems in the use of specified bright lines. Bright lines are burdened by all the uncertainty, variability, and assumptions inherent in risk estimation; thus, the all-or-nothing nature of use of a bright line could be misunderstood and construed to imply that there is an exact boundary between safety and risk. Risk assessments themselves can be manipulated so that their results emerge above or below the bright line according to a risk manager's particular policy preferences. Bright lines have the potential to be applied inflexibly, leading to decisions that do not reflect the unique characteristics of particular populations. Implementing the Commission's risk-management framework will require the consideration of bright lines as a source of information about risk that is useful in the decision-making process, but they would not be the sole determinants of the outcome of that process. Roger Pryor, executive director of the Missouri Coalition for the Environment, testified before the Commission that although bright-line standards should be established on the basis of health considerations, other factors, such as cost and the role of cultural differences, should also play a role in risk-management decisions.

Congress has included bright-line risk provisions in several legislative bills in recent years. Not until the 1990 Clean Air Act Amendments, however, did Congress pass legislation specifying a quantitative risk, when it mandated the development of a strategy for evaluating residual risks after maximum available control technology (MACT) implementation based on an incremental lifetime cancer risk of 10^-6.

Bright lines have been well established by regulatory policy despite their absence in legislation. For example, the Food and Drug Administration regulates intentional and unintentional additives in food by calculating an "estimated daily intake" and comparing that value to a previously established "acceptable daily intake". When the ratio exceeds 1.0, the agency considers the exposure unacceptable (Flamm and Lorentzen 1988). Noncancer health effects are evaluated similarly under Superfund; contaminant doses are compared to bright-line values called reference doses. If the ratio is less than 1.0, adverse effects are considered unlikely, and no action is required.

Ranges of bright lines have also been adopted by regulatory policy. For example, under Superfund, a pair of bright lines has been used to define a potentially acceptable risk range for carcinogens. A contaminated site is considered to pose a negligible risk if a risk assessment of the site produces an upper-bound lifetime incremental cancer risk estimate not exceeding 10^-6. The site is considered to pose an unacceptable risk, requiring remediation, if the risk estimate is 10^-4 or higher. Between 10^-6 and 10^-4, remedial actions, if any, are determined case by case.

In addition to ranges of bright lines, multiple bright lines should be considered. For example, section 3.1.3 discusses the need to consider sensitive subpopulations in risk assessments. The results of such risk assessments might be expressed in terms of an estimated risk for the general population and a different estimated risk for a sensitive subpopulation. Those risk estimates could be used to establish a bright line for the general population and a different bright line for the sensitive subpopulation. Decisions about appropriate levels of risk reduction could then be made with the benefit of the knowledge of those differences. EPA's deputy administrator, Fred Hansen, noted in his testimony before the Commission that getting away from single bright lines would be consistent with incorporating environmental justice considerations into risk management.

Bright lines expressed as contaminant concentrations are easier to implement than bright lines expressed as risks.⁽¹⁾ Although concentration-based bright lines are derived from some judgment about what exposure constitutes negligible risk (or, in some cases, technologic feasibility), risk managers or compliance officers can easily determine whether they are being adhered to because concentrations can be measured. When bright lines are expressed as risks, uncertain and variable risk estimates must be compared to determine compliance. Comparing risks will become even more difficult as distributional approaches to risk estimation are implemented.

¹ Examples of bright lines based on contaminant concentrations are maximum contaminant levels (MCLs) for drinking water, which, although derived from some estimate of risk, can be easily measured and therefore enforced. Expressing MCLs in terms of risk would be more difficult to enforce because risks would have to be estimated from contaminant concentrations and other variables at each drinking-water source; this would be a cumbersome and uncertain way to determine compliance.