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Assessment of Exposure-Response Functions for Rocket-Emission Toxicants
involving laboratory animals and humans can provide valuable data on the variability in uptake according to species and the specific region within the respiratory tract where the chemical might target. New experimental dosimetric approaches, such as those involving isotope ratio mass spectroscopy and cyclotron generation of gases, offer promise for improving the ability to make scientifically defensible predictions. The subcommittee recommends that the Air Force consider including interspecies dosimetric correction factors when applicable.
Instead of presenting one risk estimate for a launch that combines the risks of a normal launch and a catastrophic abort, the subcommittee believes that it would be more appropriate for the Air Force to present separate risks for normal and aborted launches or to provide separate conditional risks and combined risks.
The Air Force should ensure that any time-weighted-average exposure estimate used to determine risk is the maximum value possible. For example, the maximum 30-min time-weighted-average concentration passing over an exposure location should be compared with a 30min ERF or a reference exposure unlikely to cause an adverse health effect.
The subcommittee also recommends that the Air Force evaluate the relative accuracies of the exposure estimates from the rocket-exhaust dispersion model and the estimates of incidence of effects from the ERFs (or reference exposures as suggested here). If the Air Force can determine whether the exposure component or the effects component of the LATRA model is the more serious limit to the model's accuracy in predicting risk, it can invest effort in improving the less accurate component.
In summary, the LATRA-ERF model is a valid concept, but the current lack of toxicological data makes its implementation problematic. Some specific deficiencies have been noted above by the subcommittee, and some improvements in the LATRA-ERF model might be possible. In the interim, the subcommittee suggests that a hazard-quotient hazard-index approach be considered as a possible alternative. This approach would allow an estimate of the number of people exceeding a reference exposure level below which health effects are unlikely to occur. This approach would not attempt to estimate the incidence of health effects in an exposed population.