[Federal Register Volume 68, Number 196 (Thursday, October 9, 2003)]
[Pages 58348-58351]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 03-25583]



Centers for Disease Control and Prevention

Final Recommendations for Protecting Human Health From Potential 
Adverse Effects of Exposure to Agents GA (Tabun), GB (Sarin), and VX

    The National Center for Environmental Health published a document 
in the September 17, 2003, edition (Volume 68, Number 180, Pages 54460-
54462) of the Federal Register entitled ``Final Recommendations for 
Protecting Human Health from Potential Adverse Effects of Exposure to 
Agents GA (Tabun), GB (Sarin), and VX.'' A printing error altered a 
value in Table 1. The error has since been corrected. The document is 
being republished in its entirety for the convenience of the reader.
AGENCY: Centers for Disease Control and Prevention (CDC), Public Health 
Service, Department of Health and Human Services.

ACTION: Notice of final recommendations for protecting human health 
from potential adverse effects of exposure to agents GA, GB, and VX.


SUMMARY: Agents GA, GB, and VX are stored and are in the process of 
being destroyed by the Department of Defense (DoD). Public Law 99-145 
(50 U.S.C. 1521) mandates that all unitary (self-contained) lethal 
chemical munitions be destroyed. Public Law 91-121 and Public Law 91-
441 (50 U.S.C 1512) mandate that the Department of Health and Human 
Services (DHHS) review DoD plans for disposing of these munitions and 
make recommendations to protect public health.

EFFECTIVE DATE: January 1, 2005. An implementation period is necessary 
to allow the DoD to make program adjustments and allow time for changes 
to environmental permits as required.

FOR FURTHER INFORMATION CONTACT: Dr. Paul Joe, Acting Chief, Chemical 
Demilitarization Branch, National Center for Environmental Health, CDC, 
4770 Buford Highway, M/S F-16, Atlanta, Georgia 30341.

SUPPLEMENTARY INFORMATION: On January 8, 2002, DHHS, CDC published

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proposed ``Airborne Exposure Limits for Chemical Warfare Agents GA 
(tabun), GB (sarin) and VX'' in the Federal Register (Vol. 67, No. 5, 
Pages 894-901, Tuesday, January 8, 2002), seeking public comment. This 
notice discusses major comments received, describes decisions regarding 
the public comments, and states the final recommendations. CDC received 
comments from the U.S. Army, the Agency for Toxic Substances and 
Disease Registry (ATSDR), the CDC's National Institute for Occupational 
Safety and Health (NIOSH), state of Utah, U.S. Army contractors, and 
two individuals.
    The comments fell into the following general categories: 
assumptions used in the risk assessment, selection of uncertainty 
factors, determination of the relative potency factor for the VX 
exposure limits, and technical feasibility of air monitoring at the 
lower exposure limits. The key comments potentially impacting CDC's 
recommendations are discussed below.
    The U.S. Army recommended that adjustment in the risk assessment 
algorithm for breathing rate be eliminated because the critical 
endpoint in deriving the exposure limits is miosis, a clinical sign 
that is recognized as a local effect on the muscles of the iris of the 
eye. This biologic endpoint is widely considered to be a direct effect 
of the nerve agent vapor on the surface of the eye (not related to 
breathing rate). Scientists from CDC/NIOSH however, indicated that the 
data do not completely rule out the potential contribution of inhaled 
agent to the miosis effect. The weight of the scientific data appears 
to support the Army's recommendation on this matter, and CDC has 
decided to eliminate the breathing rate adjustment. Eliminating the 
breathing rate adjustment increases the worker population limit (WPL) 
by a factor of slightly more than two. No significant change in the 
general population limit (GPL) would occur by eliminating the breathing 
rate adjustment.
    In the derivation of the WPL for GB, CDC/NIOSH experts recommended 
that an additional uncertainty factor of three be added to account for 
individual worker variability. Although workers are medically screened, 
the recommendation is a reasonable public health decision. CDC 
therefore has incorporated the additional uncertainty factor of three 
into the risk assessment algorithm. Making this adjustment lowers the 
exposure limits by a factor of three. This adjustment and elimination 
of the breathing rate factor suggested above essentially cancel each 
    In the derivation of the VX exposure limits by using relative 
potency, the Army questioned the use of a relative potency of 12 with 
the application of a modification factor of three for the incomplete VX 
data set. The application of a relative potency of 12 with a modifying 
factor of three effectively resulted in a relative potency of 36 
between the calculated exposure limits for GB and VX. As discussed in 
the January 8, 2002, Federal Register proposal, the relative potency 
factor of 12 was based on a 1971 British study that measured the 
ability of VX to cause 90 percent pupil constriction in rabbits. 
Because the critical effect in the study used to derive the GB exposure 
limit was miosis, CDC believes that miosis was appropriate to use as 
the health effect in determining the relative potency of VX. CDC/NIOSH 
experts and the state of Utah supported the proposed relative potency 
of 12 with a modifying factor of three. Therefore, CDC is retaining its 
relative potency assumptions for deriving the VX exposure limits.
    As discussed in the January 8, 2002, Federal Register proposal, CDC 
adjusted the VX GPL because available air-monitoring methods do not 
reliably detect VX at the calculated value of 3 x 10-\8\ mg/
m3. In the adjustment, CDC assumed that potential exposure 
would be identified and corrected within three days, precluding chronic 
exposure. Several people who provided comments pointed out that a 
similar adjustment also could have been made for the GB GPL. CDC 
recognizes that the assumptions used to derive the GPLs for GB and VX 
differ. Indeed, this adjustment could be applied to the GB exposure 
limits; however, the air-monitoring technology is currently functioning 
near the recommended level. CDC recommends no upward adjustment of the 
GB exposure limits; this recommendation is consistent with the accepted 
industrial hygiene practice of keeping exposure to the minimum 
practicable level.
    The derivation of the VX exposure limits may be biased low because 
of the inadequate VX toxicity database. CDC believes that reliable air 
monitoring is a crucial aspect for implementing the exposure limits. 
Although CDC would have preferred a better toxicity database for VX, as 
well as improved air-monitoring methods for VX, these items are not 
currently available. Consequently, CDC is not further adjusting the 
final recommendation to the GPL for VX. However, CDC will reevaluate 
the VX exposure limits in the future if significant new VX toxicity 
data are available for setting exposure limits, new risk assessment 
evaluation methods are demonstrated superior to methods used herein, or 
substantive technological advances in air monitoring methods are made.
    Army contractors and CDC/NIOSH experts expressed concerns about the 
technical feasibility of meeting the new exposure limits. On the basis 
of these comments, CDC has adjusted the VX short-term exposure limit 
(STEL) to 1 x 10-\5\ mg/m3 but added the 
provision that excursions to this special VX STEL should not occur more 
than once per day (in the typical STEL, four excursions per day are 
allowed). A lower STEL value would have required a longer response time 
for near real-time instruments; the recommended STEL is a result of 
balancing the detection capabilities and response time. A shorter 
instrument response time associated with the recommended STEL will 
minimize exposures. This adjustment to the VX STEL should not affect 
worker health.
    To account for other technical feasibility concerns, CDC recommends 
that the GB and VX STEL be evaluated with near-real-time 
instrumentation, whereas the GB and VX WPLs and GPLs may be evaluated 
with longer-term historical air monitoring methods. CDC further 
recommends that, in implementing the WPLs, STELs and GPLs, specific 
reduction factors for statistical assurance of action at the exposure 
limits are not needed because of safety factors already built into the 
derivation of the exposure limit. This recommendation assumes that the 
sampling and analytical methods are measuring within +/-25% of the true 
concentration 95% of the time. If this criterion is not met, an alarm 
level or action level below the exposure limit may be required.
    The Army recently indicated to CDC that the exposure limits as 
listed and implemented in this announcement are technically feasible to 
detect with the instrumentation and methods currently in use. However, 
whether the agent destruction sites can monitor at these exposure 
limits and still meet current quality control standards has not been 
determined. To allow the Army to implement program changes, regulatory 
adjustments, and to evaluate quality control issues, the final 
recommended exposure limits will become effective January 1, 2005.
    Final Recommendations: CDC presents final recommendations for 
airborne exposure limits (AELs) for the chemical warfare agents GA 
(tabun or ethyl N,N-dimethyl-phosphoramidocyanidate, CAS 77-81-6); GB 
(sarin or O-isopropyl-

[[Page 58350]]

methylphosphonofluoridate, CAS 107-44-8); and VX (O-ethyl-S-(2-
diisopropylaminoethyl)-methylphosphonothiolate, CAS 50782-69-9). CDC 
based its recommendations on comments by scientific experts at a public 
meeting convened by CDC on August 23-24, 2000, in Atlanta, Georgia; the 
latest available technical reviews; and the risk assessment approach 
frequently used by regulatory agencies and other organizations. 
Additionally, CDC reviewed the substantial background information 
provided in the recent U.S. Army evaluations of the airborne exposure 
criteria for chemical warfare agents. AELs for chemical warfare agents 
GA, GB, and VX were reevaluated by using the conventional reference 
concentration risk assessment methodology for developing AELs described 
by the U.S. Environmental Protection Agency. This methodology is 
considered conservative; however, the calculated exposure limits are 
neither numerically precise values that differentiate between 
nonharmful and dangerous conditions, nor are they precise thresholds of 
potential human toxicity. The recommended changes to the AELs do not 
reflect change in, nor a refined understanding of, demonstrated human 
toxicity of these substances but rather the changes resulted from 
updated and minimally modified risk assessment assumptions. Overt 
adverse health effects have not been noted in association with the 
previously recommended exposure limits. This may be due to rigorous 
exposure prevention efforts in recent years as well as the conservative 
implementation of the existing limits (i.e., 8-hour time-weighted 
average exposure limits have been implemented as short-duration ceiling 
    Recommended AELs for GB: CDC recommends a WPL value of 3 x 
10-5 mg/m\3\, expressed as an 8-hour time-weighted average 
(TWA). Additionally, CDC recommends a STEL of 1 x 10-4 mg/
m\3\ to be used in conjunction with the WPL. Exposures at the STEL 
should not be longer than 15 minutes and should not occur more than 
four times per day, and at least 60 minutes should elapse between 
successive exposures in this range. The STEL should not be exceeded 
during the work day, even if the cumulative exposure over the 8-hour 
TWA is not exceeded. CDC recommends a decrease in the GPL to 1 x 
10-6 mg/m\3\. The WPLs and GPLs values are approximately 
threefold lower than levels previously recommended by CDC in 1988. An 
immediately-dangerous-to-life-or-health (IDLH) value of 0.1 mg/m\3\ is 
recommended for GB.
    Recommended AELs for GA: Although not as well-studied as GB, GA is 
believed to be approximately equal in potency to GB. Therefore, CDC 
recommends the same exposure limits for GA as for GB.
    Recommended AELs for VX: CDC recommends that the VX WPL, expressed 
as an 8-hour TWA, be decreased to 1 x 10-6 mg/m\3\. 
Additionally, CDC recommends a VX STEL of 1 x 10-5 mg/m\3\. 
An excursion to the STEL should not occur more than one time per day 
(compared to four times per day for a typical STEL). The recommended 
WPL is a factor of 10 lower than the CDC's 1988 recommendation. CDC 
recommends that the GPL for VX be decreased to 6 x 10-7 mg/
m\3\ (a factor of five lower than CDC's 1988 recommendation). An IDLH 
value of 0.003 mg/m\3\ is recommended for VX. CDC's final 
recommendations are summarized in Table 1 below.


[[Page 58351]]


    CDC does not specifically recommend the use of these AELs for uses 
other than transportation, worker protection during the destruction 
process, or general population protection. For example, the 8-hour WPL 
historically has been used for the Army-designated 3X decontamination, 
surveillance activities of leaking containers in storage, and charcoal 
unit mid-beds. CDC did not evaluate the applicability of the WPLs for 
these activities; the specific technical and safety requirements for 
each activity need to be considered individually.
    This announcement does not address the allowable stack 
concentration (ASC). The ASC is a ceiling value that serves as a 
destruction process source emission limit and not as a health standard. 
It typically is used for monitoring the furnace ducts and final exhaust 
stack, providing an early indication of an upset condition. Modeling of 
worst-case credible events and conditions at each installation should 
confirm that the WPL is not exceeded on-site or that the GPL is not 
exceeded at the installation boundary as a consequence of a release at 
or below the ASC.
    The Director, Management Analysis and Services Office, has been 
delegated the authority to sign Federal Register notices pertaining to 
announcements of meetings and other committee management activities for 
both CDC and ATSDR.

    Dated: October 3, 2003.
Alvin Hall,
Director, Management Analysis and Services Office, Centers for Disease 
Control and Prevention.
[FR Doc. 03-25583 Filed 10-8-03; 8:45 am]