strophic abort, and (4) the probability of a catastrophic abort. Depending on the toxicity criteria used, as well as the quantity of propellants onboard, meteorological conditions, proximity of population centers, and so forth, far greater health risks generally are expected for catastrophic aborts than for normal launches. As a consequence, the total risk estimate produced by LATRA generally reflects the risks estimated for a catastrophic abort because the risks estimated for a normal launch usually are much lower.

The potential for combined effects of exposure to more than one compound at the same time is estimated after the risk profiles for individual compounds are estimated by "developing joint probabilities of effect from the individual toxics' probabilities of effect (assuming their independence)" (Philipson et al. 1996). This assumption was considered conservative because of the high level of correlation expected among exposures to the individual toxic emissions. The mode of action of the various toxicants needs to be considered. Such an investigation might provide support for response additivity or dose additivity of the constituents in a mixture.

The subcommittee considered the relative importance of the various uncertainties associated with the LATRA-ERF model, evaluating how well the important uncertainties are represented in the model and how they should be used to qualify the risk estimates. Those evaluations and the subcommittee's conclusions and recommendations also are provided in Chapter 5.

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