A value of zero indicates the absolute certainty of no risk. A value of one is the absolute certainty that the risk will occur. Lets say the lifetime cancer risk from the hypothetical carcinogen XTC is 0.0001 at an average daily dose of 200 mg. This predicts that one of every 10,000 people exposed to a daily dose of 200 mg will develop cancer over their lifetime. Risks associated with accidents or injuries are calculated using actuarial figures and are relatively reliable.
Risk associated with environmental chemical exposures are much less reliable.
Risk assessment is used to estimate the extra risk caused by chemical exposure over the risk already present in the absence of exposure. The answer is the risk assessor s final characterization of risk. Uncertainty and science policy Risk assessors deal with uncertainty by treating it the same way for all chemicals. Science policy choices must be made about the model to be used to extrapolate low dose exposure. All the principles tend to be health protective.