P_E ( S,C,T ) = P ( Y ≥ S/C,T ),

where P_E (S,C,T ) is the ERF for an exposure characterized by concentration, C, and time or duration of exposure, T , and is equal to the probability of the severity equaling or exceeding a given severity category, S, at a specified exposure concentration, C, and duration, T .

As indicated in Chapter 1, the task of the subcommittee was to review and provide recommendations on several issues surrounding the exposure-response components of the LATRA model. Further description of the model and issues surrounding those components is provided below.

The toxicity reference values originally used by the Air Force in the LATRA-ERF model to represent a 1%-effect level for HCl, NO₂, and HNO₃, had been established by other groups almost a decade earlier (e.g., NRC 1987,1991). The toxicity data for those substances needed to be reevaluated with respect to sensitive populations, severity of effect, and the availability of dose-response information. Those reevaluations are presented in Appendices D, E, and F, respectively. Use of that information to derive ERFs for LATRA is explored in Chapter 6.

The LATRA-ERF model divides potentially exposed populations into at least two population subgroups, sensitive and normal, and develops separate ERFs for each subgroup. Sensitive populations are defined as children (less than 15 years of age), the elderly (more than 64 years of age), and all persons with bronchitis, asthma, or other physiological stress, especially upper-respiratory ailments (Gene Killan, U.S. Air Force Space Command, personal commun., May 6,1996). Under LATRA, the remainder of the population is considered "normal" and is assumed to be composed of healthy adults. Census data are used to determine the