TABLE ES-1 Types of Airborne Exposure Limits



Short-term exposure limit (STEL)a

The level at which an unprotected worker can operate safely for one or more 15-minute periods (depending on the agent) during an 8-hour workday. The STEL was introduced as part of the 2003/2004 AELs.

Worker population limit (WPL)b

The concentration at which an unprotected worker can operate safely 8 hours a day, 5 days a week, for a working lifetime, without adverse health effects.c

General population limit (GPL)

The concentration at which the unprotected general population can be exposed 24 hours a day, 7 days a week, without experiencing any adverse health effects.

Immediately dangerous to life or health (IDLH) limit

The level of exposure that an unprotected worker can tolerate for 30 minutes without experiencing escape-impairing or irreversible health effects.

aThe traditional definition of a STEL (paraphrased) is the concentration at which a worker may be exposed for 15 minutes up to four times a day with 1 hour between exposures. At the end of the work period, the established time-weighted average (TWA) must be satisfied (ACGIH, 2002).

bThe 1988 WPLs were issued as TWAs—8-hour time-weighted averages—but implemented as ceiling values.

cFor purposes of quantitative risk assessment, the Occupational Safety and Health Administration considers a working lifetime to be 45 years (Federal Register, 1989).

SOURCE: Adapted from Current and Revised Airborne Exposure Limits for Chemical Warfare Agents, a chart provided by the Chemical Materials Agency at the June 2, 2004, AEL videoconference.

the new AELs for nerve and mustard agents, the committee was briefed by CDC staff, who also provided written responses to questions posed by the committee. The committee was also briefed by the Army and contractor experts and received written responses from them to its questions as well. Numerous documents pertaining to the CDC AELs and the Army’s systems, technologies, processes, and procedures for ensuring worker and public safety were also reviewed.

The committee developed recommendations on analytical methods with improved selectivity and sensitivity; on various aspects of near-real-time (NRT) airborne contaminant monitoring; on operational procedures and airborne contaminant monitoring for NECD and the EDS; on the applicability of the Resource Conservation and Recovery Act (RCRA) to the non-stockpile program; and on involving workers and the public in the implementation of the new AELs. This executive summary discusses the committee’s primary recommendations only; additional recommendations are included in Chapters 2 through 6.


The 2003/2004 AELs were developed using generally accepted methods of setting regulatory limits. Although there are no new data on toxicity beyond those used to establish the 1988 values, the existing data were reevaluated using modified, more conservative methods that reflect present-day practices for establishing uncertainty factors (Federal Register, 2003a, 2004). Because the charge to the committee was narrowly defined, the committee accepted the new CDC-derived AELs as the starting point for its evaluation of the monitoring program. That is, the committee did not evaluate the process used or the end points selected by the CDC in revising the 1988 limits, nor did it take a position on the appropriateness of the 2003/2004 CDC-recommended AELs. Nevertheless, the committee does express, in Chapter 3, its opinion that the new AELs will not produce a demonstrable risk benefit. This report contains a substantive discussion of the 2003/2004 AELs and the distinctions between them and the 1988 limits, because it was necessary to understand the degree of uncertainty in the new limits in order to understand the role of monitoring in implementing them. Table ES-2 presents the 1988 AELs and the revised (2003/2004) AELs.

New AELs for Nerve Agent

The 2003 WPLs and GPLs for GB were lowered from the 1988 values by a factor of 3 (an “uncertainty factor”) to account for individual variability within the worker and general population (Federal Register, 2003a). Since the WPLs for GA and VX are derived from the WPL for GB, the 2003 WPLs for GA and VX were also automatically reduced by a factor of 3. The WPL for VX, however, was further reduced by an additional factor of 3 (a “modifying factor”) to account for a sparse database, resulting in a 10-fold total decrease in the WPL from 1988. The CDC adjusted the GPL for VX, 3 × 10−8 mg/m3, upward by a factor of 20, to 6 × 10−7, so as to obtain a value that was protective for humans and that could be reliably monitored by available monitoring methods (Federal Register, 2002). The CDC justified this

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement