Part One

Introduction

In this review we address health issues related to chemical warfare (CW) agent disposal and stockpile destruction. Munro et al. (1) detailed the acute and chronic toxicity of the nerve agents tabun (ethyl N,N-dimethylphosphoroamidocyanidate; GA), sarin (isopropyl methylphosphonofluoridate; GB), and O-ethyl S-[2-(diisopropylamino)ethyl]methylphosphonothioate (VX). Earlier, Watson and Griffin (2) reviewed the acute and chronic toxicity of the vesicant agents with special emphasis on mustard carcinogenicity, and Munro et al. (3) evaluated nerve and blister agent antidote use, toxicity, and decontamination procedures in the context of civilian application.

During this era of CW agent demilitarization and cleanup of sites and facilities associated with chemical agent production, testing, and storage, information on the properties and toxicity of CW agent degradation products is important for risk management in site operations and restorations. Although a variety of breakdown products and impurities have previously been documented, the significance to environmental and/or occupational health has not been established.

In this review we assemble the scattered and often fragmentary literature on environmental fate of the CW agents as well as what is presently known about the potential health effects and ecotoxicity of each agent's degradation products and contaminants. We have eliminated certain compounds from potential concern and focused attention on those with known significant environmental persistence and toxicity. With the exception of the sulfur mustards, most of the CW agents are not persistent in the environment because they are subject to a variety of abiotic and biotic degradation mechanisms. It is important to identify persistent and/or toxic chemical agent breakdown products to assist in cleanup processes and to ensure worker and public safety. We generally use "environmental persistence" to refer to the presence of compounds in soil; environmental persistence is moderate for compounds that may be stable for weeks to months and high for compounds that are stable for months to years. "Persistent compounds" are characterized by low vapor pressure, low water solubility, and low rates of natural abiotic and biologic degradation. Examples include polychlorinated biphenyls and dioxins; these compounds are not related to CW agents. However, compounds with moderate-to-high water solubility that are not readily degradable or subject to hydrolysis may persist in dry soil and/or leach into groundwater, where they persist for long periods.

Toxicity depends on the route of exposure. Relevant routes of exposure for CW agent breakdown products are oral, inhalation, and dermal. Compounds that are lethal to 50% of tested animals [median lethal dose (LD₅₀) or median lethal concentration (LC₅₀)] at < 50 mg/kg, < 50 mg/m³, and < 200 mg/kg after single exposures are considered highly acutely toxic by the oral, inhalation, and dermal routes, respectively (4). Compounds with LD₅₀ or LC₅₀ values of 50-500 mg/kg, 50-500 mg/m³, and 200-500 mg/kg for the respective routes are considered moderately toxic, and compounds with values higher than these ranges are considered to be of a low order of toxicity. Toxic values for chronic exposures by the respective categories and routes of exposure are generally an order of magnitude lower. For aquatic organisms, LC₅₀ values of < 1 mg/L and < 0.1 mg/L are considered highly acutely and chronically toxic, respectively.

The primary warfare agents of concern within the U.S. CW agent inventory include several vesicant or blister agents: sulfur mustards {undistilled sulfur mustard (H), sulfur mustard (HD), and an HD/ agent T mixture (HT)}; nitrogen mustards [ethylbis(2-chloroethyl)amine (HN1), methylbis(2-chloroethyl)amine (HN2), tris(2-chloroethyl)amine (HN3)], and the organic arsenical Lewisite]; four nerve agents [VX, GA, GB, and soman (pinacolyl methylphosphonofluoridate; GD)], and the blood agent cyanogen chloride (CK) [Table 1 (2,5-20)]. The nitrogen mustards, GD nerve agent, and CK were not stockpiled as part of the U.S. chemical weapons inventory, and Lewisite and GA were produced in limited quantities in the United States. The emphasis in this review is on those potential degradation products resulting from agent contact with soil or water, especially from buried chemical weapons and wastes. The principal degradation processes include photolysis, hydrolysis, oxidation, and microbial degradation. Volatilization is an important mechanism for the transfer of some CW agents from soil and water to air. Decontamination procedures may incorporate some or all of these processes. We do not discuss combustion.

We assessed each of these processes/sources of degradation products to assist those responsible for disposal, cleanup, and destruction operations to anticipate possible hazards. For example, knowledge of the environmental fate of CW agents would aid in choices of measures needed to ensure the safety of workers involved in hazardous waste cleanup and to ensure adequate remediation. It would also assist in determining the extent of possible contamination where agent wastes or munitions have been buried.

Different operations and conditions involve these various processes and therefore potentially have different breakdown products associated with a given CW agent. In general, however, photolysis is relevant in the case of spills, particularly to soil surfaces and surface waters, as well as in the event of airborne release of an agent. Hydrolysis is pertinent to warfare agents buried in moist soil, to disposal in bodies of water, or to inadvertent releases or spills into surface water bodies. Compared to many other environmental contaminants, these agents and some of their degradation products are susceptible to hydrolysis, which minimizes their transport to groundwater. Hydrolysis is also a relevant disposal option for VX stored in ton containers (alkaline hydrolysis or neutralization followed by supercritical water oxidation) (21,22) and is an alternate disposal option for HD stored in ton containers (hot water neutralization followed by biodegradation of the hydrolysate) (21,23,24). Oxidation is relevant to compounds in contact with air or natural oxidants in soil or water, and also for decontamination systems (oxidative detoxification). Microbial degradation is of interest in cases of burial or spills on soil and the hot water neutralization/biodegradation of hydrolysate option for HD disposal. Decontamination with any of the various chemical solutions currently in use by the army may result in the production of intermediates of varying toxicities. Experimental or proposed novel decontamination methods are not considered here.

We identified products associated with each of the degradation processes from field and laboratory studies as well as from analyses of stored containers. We then assessed the persistence and toxicities, both mammalian and environmental, of the degradation products.

We obtained the information in this review through an extensive literature search. Searches of the following computerized databases were updated as of July/August 1997: Medline [National Library of Medicine (NLM), National Institutes of Health, Bethesda, MD], Toxline (NLM), Defense Technical Information Center (DTIC; Washington, DC), and Registry of Toxic Effects of Chemical Substances (RTECS; National Institute of Occupational Safety and Health/NLM). Searches of Current Contents (Institute for Scientific Information, Philadelphia, PA) continued through May 1999. Although we sought information on all identified degradation products, we placed emphasis on those products or contaminants identified as present in storage containers at concentrations of > 0.1%. The chemical nomenclature in this paper is based on nomenclature in ChemID (NLM). We do not present toxicity data for inorganic degradation products or for well-characterized organic entities such as ethanol or isopropyl alcohol.

Data on environmental fate, mammalian toxicity, and ecotoxicity are often lacking or minimal for many of the minor degradation products and contaminants identified in this paper. In addition, much of the data are historic or incompletely reported by contemporary standards. Evaluation of those degradation products and contaminants for which both environmental fate and toxicity information are available indicates that relatively few degradation products are persistent over long periods in the environment, and most of those that are persistent exhibit moderate-to-low levels of mammalian toxicity.

Last Updated: November 3, 1999