ticular ideas further, some of the broader assumptions made here to understand risk should be mentioned.

Risk is interpreted primarily as a combination of the probability of occurrence of some hazard and the harm corresponding to that hazard. This is both a highly technical and somewhat vague interpretation, involving the related ideas of hazard, occurrence, and some combining process. A hazard has the potential to produce harm, injury, or some other undesirable consequence. In saying a slippery road is hazardous, it is not meant that any harm or a car accident has occurred. It simply means that the road conditions could potentially cause an accident. In this case the hazard is a car accident, which may or may not happen. Likewise, if a transgenic crop has a hazard, it does not mean that any harm has or will occur. Hazard identification is one of the most subjective and potentially contentious elements of risk analysis. While this report is limited to a consideration of environmental risks, there is some ambiguity in deciding what is and is not an environmental hazard. Does this include or exclude the potential for adverse impacts on human health that are mediated by the environment (not directly by food consumption)? Does it include or exclude the potential for adverse impacts on farming practices and profitability? Is a nonspecific effect on habitats or ecosystems an identifiable hazard? Is an effect on an ecological process an environmental hazard? The characterization of hazards in this chapter reflects an answer to each of these questions.

The occurrence of a hazard is a probability that the hazard would occur. This typically depends on many factors. The probability that an accident will occur on a slippery road will depend on how many cars travel the road, how fast they are going, how much they accelerate and decelerate, the skill of the drivers, and other factors. Clearly, these probabilities will be highly conditional on the environment and other details about the situation, and therefore they will be variable both spatially and temporally. Likewise, the probability that any hazard associated with a transgenic crop would occur is likely to vary spatially and temporally. In much of the literature on risk, the probability of occurrence is called an exposure probability, referring to the probability that people are exposed to the hazard. In this report the term exposure is frequently used as a shorthand notation for the probability that a hazard would occur.

The combining of hazard and occurrence probabilities to characterize risk can be contentious, ranging from simple mathematical formulations to complex deliberative processes involving many people. The committee leaves this process deliberately unspecified, so that the range of formulations can be used as needed in the report. In its simplest form, risk can be understood as a weighted probability in which the probability of occur-



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