However, it recommended that the GPL (3 × 10−6 mg/m3) for VX be the same as that for GB based on the limited technical capabilities of the air monitoring equipment available in 1988. STELs and IDLHs were not established for VX in 1988.

In 2003, the CDC revised the AELs for GA, GB, and VX. The revised GB WPL (3 × 10−5 mg/m3) and the GPL (1 × 10−6 mg/m3) (Table 3-1) were one-third of the 1988 values (Federal Register, 2003a). These new limits were based not on new experimental data for humans or animals but on an additional uncertainty factor of 3 that the CDC wanted to account for individual variability. For VX, the CDC adjusted the relative potency factor from 10 to 12 to reflect increased toxicity compared with GB and applied a modifying factor of 3 to account for an incomplete data set, resulting in a total composite adjustment of 36 for VX. Applying these factors resulted in a VX WPL of 1 × 10−6 mg/m3 and a calculated GPL of 3 × 10−8 mg/m3. However, CDC then adjusted the calculated GPL for VX upward, by a factor of 20, to 6 × 10−7 mg/m3 based on the technical capabilities of latter-day air-monitoring methods (Federal Register, 2003a). The CDC justified this by saying it could be expected that any exposure would be identified and corrected within 3 days (72-hour TWA).

STELs and IDLH limits were derived in 2003 for GA, GB, and VX. A STEL of 1 × 10−4 mg/m3 was set for GA and GB while a STEL of 1 × 10−5 mg/m3 was set for VX (Table 3-1) (Federal Register, 2003a). A STEL is an acceptable exposure for 15 minutes for unprotected workers.3 For GA and GB, such exposures should not occur more than four times per day, and at least 60 minutes should elapse between successive exposures. For VX, STEL exposures should occur no more than once a day (Federal Register, 2003a).

Several issues surrounding the CDC’s 1988 and 2003 AELs for GA, GB, and VX deserve consideration. The CDC based its 2003 recommendations on several sources of information:

  • Comments from expert scientists,

  • Risk assessment approaches used by regulatory agencies and other organizations, and

  • Information provided in recent U.S. Army evaluations of AELs for chemical warfare agents.

The CDC used the U.S. Environmental Protection Agency (EPA) conventional reference dose concentration risk assessment methodology for developing the AELs (Federal Register, 2003a). The CDC says that this methodology is conservative and does not reflect a change in the understanding of demonstrated human toxicity by these agents nor does it redefine that understanding. The CDC also indicated that no overt adverse health effects had been noted in association with the 1988 recommended exposure limits.

The EPA’s risk assessment methodology is used to promulgate reference dose concentrations for airborne chemicals—that is, airborne exposure limits—for general (including sensitive) populations over a lifetime (70 years).4 The EPA has also developed and now manages a mechanism for establishing short-term emergency exposure limits for airborne chemicals. The process functions through the National Advisory Committee to Establish Acute Exposure Guideline Levels (AEGLs) for Hazardous Substances.5 AEGL values define exposures to airborne chemicals intended to protect the general public (including sensitive individuals) after single exposures ranging from 10 minutes to 8 hours. The proposed short-term AEGLs were reviewed by a National Research Council committee and ultimately issued as a National Academy of Sciences publication (NRC, 2003). EPA has not, however, developed long-term reference dose concentrations for nerve agents GA, GB, and VX. Since the CDC’s recommended STELs are for a 15-minute exposure, the WPLs for 8 hours per day, and the GPLs for a lifetime based on a 24-hour TWA (albeit not one-time exposures), some of the AEGL methodology could be directly applicable to the Army for emergency responses. For

3  

The STEL is defined as an exposure that is acceptable for a short period of time, i.e., averaged over 15 minutes, without a respirator. Thus, the STEL recognizes that one’s exposure may be higher. The STEL is set to minimize observed symptoms over a short exposure period. If there is a potential for brief airborne exposures in excess of the STEL, an industrial hygienist will assign a respirator. Emergency personnel typically select a self-contained breathing apparatus for protection until the area can be characterized correctly. Then the correct respirator, if any, can be selected.

4  

The general population is considered to be more sensitive to chemical agent exposure than the military population, and more casualties would be expected. The reason for this is that the general population includes children, the elderly, and unhealthy individuals, none of whom are represented in the military population.

5  

Acute exposure guideline levels (AEGLs) are a hazard communication measure developed by the National Advisory Committee to Establish Acute Exposure Guideline Levels for Hazardous Substances. The committee developed detailed guidelines for devising uniform, meaningful emergency response standards for the general public. The guidelines define three tiers of AEGLs as follows:

AEGL-1: The airborne concentration of a substance above which it is predicted that the general population, including susceptible individuals, could experience notable discomfort, irritation, or certain asymptomatic nonsensory effects. However, the effects are not disabling and are transient and reversible upon cessation of exposure.

AEGL-2: The airborne concentration of a substance above which it is predicted that the general population, including susceptible individuals, could experience irreversible or other serious, long-lasting adverse health effects or an impaired ability to escape.

AEGL-3: The airborne concentration of a substance above which it is predicted that the general population, including susceptible individuals, could experience life-threatening health effects or death.

The guidelines for each level consider five exposure periods: 10 minutes, 30 minutes, 1 hour, 4 hours, and 8 hours (NRC, 2001d).



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