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Assessment of Exposure-Response Functions for Rocket-Emission Toxicants
situations of interest to the Air Force. However, when an ERF cannot be constructed, it is usually possible to estimate an exposure concentration and duration that is unlikely to result in health effects (i.e., a NOEL). Thus, it should be possible to use LATRA to estimate the number of people who are likely to be exposed at levels exceeding a no-observed-effect level (NOEL) and to indicate the magnitude of the excess (see Chapter 5). That estimate does not provide an estimate of the number of individuals affected or severity of effects, but it does provide an estimate of the number of people at risk from exposures in excess of the NOELs.
For the examples of ERFs constructed in this section, the uncertainty in the measurements of exposure is approximately a factor of two, higher or lower. Hence, estimates of the incidence of effects based on those ERFs would not be accurate. The magnitude of the uncertainty could be determined by calculating the expected number of individuals at risk with the ERFs shifted higher and lower; however, there are additional uncertainties if animal data are used to estimate effects for humans and if the slope of the dose-response relationship for one end point is used to estimate the slope of dose-response relations for another end point.
Given the limited dose-response data available for the rocket-emission toxicants, Chapter 5 recommended that either an expert elicitation process be used to estimate ERFs or the hazard-quotient-hazard-index approach be used to characterize risks. The subcommittee feels that it has exhausted the possibilities for constructing ERFs from incidence data for HCl and HNO3. The subcommittee did not examine all of the dose-response studies available for NO2 for their potential to develop ERFs, however. Thus, it is possible that additional health-end-point specific ERFs could be developed for NO2 using one of the approaches described in this chapter and Chapter 5. However, risk characterization will be more clear if the same risk-assessment model is applied to all three rocket-emission toxicants.