A risk is a likelihood of injury, damage, or loss. The concept has two elements: a normative judgment of a possible event or condition as adverse plus the chance or probability that it will come about. The magnitude of a particular risk is often thought of in terms of the severity of the potential harm weighted by the probability of its occurrence; a low probability of a serious harm may thus be seen as equivalent, in magnitude of risk, to a higher probability of a less serious harm. It may then be suggested that the acceptability of a given risk is determined by its magnitude, and hence that risks of equal magnitude are, or ought to be, equally acceptable (or unacceptable).

This way of thinking may be misleading, however, in at least three ways: (1) It suggests an often unattainable degree of precision in estimating the probabilities of the events that concern us. (2) It suggests gests commensurability of harms of very different sorts (loss of home, sight, life, loved one) that may occur in very different ways (during recreation, due to storm or earthquake, intentional or negligent actions of others, etc.). These and many other factors are morally and psychologically relevant to individual and social judgments of acceptability. (3) It suggests that it makes sense to think in terms of a general level of acceptable risk for a person or for a society. But to look for a general level of ac ceptable risk is like looking for a general fair price. Price for what? No risk is acceptable if it does not bring with it some benefit and it matters both whether those who bear the risk consent to bear it and whether they themselves reap the benefit.

Consent. Scarcely any activity can be carried on that does not impose some risk on someone? Often not (only) upon the agent, but upon others. We are not always morally required to obtain the explicit consent of others. We are not morally required to obtain the explicit consent of everyone potentially at some risk from an act such as driving to work or turning on the furnace before we may perform it. Some risks, though, such as those involved in medical research, may not be imposed without the explicit informed consent of those potentially at risk. There may be risks it would be morally impermissible to impose even with informed consent. How should we distinguish cases where consent is required from cases where it is not (and from cases, if any, where it may not suffice)? What, if anything, should count as consent in situations where direct consent is unfeasible or unreasonable? Is there a role for such a notion as implicit or hypothetical consent to risk? There are serious limits to the justificatory force of implicit or hypothetical consent. Inferring implicit consent by workers to workplace risks, for example, presupposes that workers are fully informed about the hazards they face, that they have reasonable alternatives available if they find the risks unacceptable, and thus that they are satisfied with the wage risk packages they currently get. Each of these assumptions is subject to serious question. To extrapolate from labor market behavior to other areas of life and infer implicit consent to risks of "equal magnitude" compounds the problem. Similarly, insofar as the moral significance of consent derives from respect for the autonomy of those whose consent is sought, appeals to hypothetical consent may be poor substitutes for the real thing. Providing for meaningful participation in the decision by those potentially at risk may more fully respect their autonomy (though, admittedly, even purely self-imposed risks raise issues of whether there are duties to the self that may be violated and of the justifiability of paternalism).

Justice. Risks resulting from an activity or policy may be borne wholly or largely by some members of a community, while the benefits are shared equally, or enjoyed entirely by persons other than those at risk. In such cases, it may be possible (and perhaps even obligatory) to compensate in some way those who bear the risks (and perhaps to compensate further those for whom the risk turns out badly) so that the interests of those at risk are not sacrificed either for the common good or for the benefit of others. (Questions would remain as to whether prior consent to such a risk-compensation package would be required.) The requirements of justice may differ depending on whether those who would be at risk from an activity already bear a disproportionately larger or smaller share of risks or are antecedently worse or better off in other ways than those who would benefit. How much weight one believes such considerations ought to have will depend in part on one's general moral views-the relative importance one attaches to individual rights, the general welfare, and social equality, for example. Equity considerations arise between local communities, regions, and nations as well as between individuals or groups within a community. What are our moral obligations in cases where those potentially at risk do not yet exist? We cannot ask their consent, and it does not seem reasonable to ascribe rights to persons who do not exist-and who may or may not exist, depending in part on our decisions. Yet, it does not seem morally acceptable to ignore the potential effects of our policies on future generations. How, then, should these considerations enter into our decisions?

Facts and Values. Because the concept of risk involves two elements, chance and adversity, some propose that decisions concerning risk involve two fundamentally different sorts of activity:(1) measurement or estimation of risk and (2) evaluation of its acceptability. The first is thought to be factual, objective, and scientific, while the second is normative, subjective, and personal or political. Many practitioners and theorists of risk assessment insist that their function is restricted to the first, while the second is the job of individuals or the policy makers who represent them. Others argue that there are normative and subjective elements present throughout the process-beginning with the judgment that a possible outcome is adverse and hence its possibility constitutes a risk, and continuing with judgments as to how conservatively to estimate probabilities, how to frame risks (whether in terms of probability of adverse or favorable outcome), what comparisons and alternatives to represent in characterizing the nature and magnitude of risk, and so on. They maintain that the factual and evaluative components of risk decisions are inextricably intertwined, and hence that the widely accepted division of labor in the decision process is ill-conceived.