Public Health and Environmental Impacts of Monosodium Methanearsonate As Used in Bark Beetle Control in British Columbia
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Public Health and Environmental Impacts of Monosodium Methanearsonate as used in Bark Beetle Control in British Columbia Prepared for Ministry of Forests Silviculture Practices Branch by F.N. Dost, DVM, ATS February 1995 FS 48 HSI 95/2 Summary This report examines the potential health and environmental effects of monosodium methanearsonate (MSMA), as used in British Columbia to create lethal trap trees for control of bark beetle infestation. MSMA is applied as a frill injection to individual trees. The highest dosage of use is about 40 ml of MSMA product per tree, depending on tree size. Approximately 750 kg of MSMA product are applied in British Columbia each year on average. The area of use is widely dispersed covering most of the interior of the province where bark beetle infestations may occur. The average annual total area of treatment is 650 hectares. In any one landscape drainage infested with bark beetles, it is unlikely that, of the many thousands of trees that may exist there, more than 50 trees would be treated with MSMA. Existing evidence indicates that use of MSMA in bark beetle control does not impose measurable risk on applicators, the public or wildlife. Burning of slash or mill waste, or use of treated trees as firewood also does not represent a health threat, as judged through comparison of expected arsenic levels in smoke with other markers of air contamination. Arsenic concentration in various parts of trees treated with MSMA or the related cacodylic acid has been measured in several studies. A worst case situation where only treated trees enter a mill, with contaminated sawdust in air at the maximum allowable wood dust concentration, would lead to arsenic intake 2000-5000 fold lower than permitted by British Columbia industrial health and safety regulations. These conclusions have been reached after review and consideration of, in the context of bark beetle control, (a) the toxicology of MSMA and related compounds, (b) the disposition of these substances in the body, (c) the behavior of MSMA in the environment, and (d) exposure of workers, the public and lower species. All of the information has been integrated in an assessment of risk associated with this use of MSMA. The information reviewed includes both publications in the open literature and summaries of manufacturer's confidential registration data made available for the purpose of this report. The toxicity of MSMA is limited. It has some potential for mild skin and eye irritation, and when ingested in sufficient amounts causes gastrointestinal erosion. Studies of its cancer causing potential have been negative, and it has a limited and questionable ability to cause mutation. Reproductive effects appear only at dose levels that cause evident maternal injury. MSMA is excreted rapidly by mammals, including humans. Exposure of applicators has been determined to be low, and their intake of MSMA is far below thresholds of toxicity. However, it is evident that better use of basic protective garments, training and work discipline can lower exposures still further. In nature, inorganic arsenic salts are continuously converted to methylated acids and oxides, including MSMA, which are much less toxic than inorganic arsenic compounds. Formation of methylated derivatives is also a primary means of detoxication in mammals. Soil microorganisms slowly produce the corresponding arsines from the oxygen-bearing acids and oxides, but the reaction is rapidly reversed in air. While the arsines are relatively toxic, they disappear quickly enough that concentrations do not become significant. Reversion of MSMA to inorganic arsenic in plants or mammals, including humans, apparently does not occur to a measureable extent. Some microorganisms are able to accomplish this reaction. Because MSMA is either injected or applied directly to cuts in the bark at the base of selected trees in a stand, there is very little environmental contamination. Measurement of MSMA application to agricultural soil at several times normal application rates over multiyear periods shows that soil buildup and offsite movement do not occur, even under such extreme conditions. Measurement of arsenic in streams through treated forests has not detected increases over background levels. A spill of MSMA is unlikely to contaminate water sources unless lost directly into or immediately adjacent to a water body. Exposure of the public using the forest is extremely unlikely. ii Acknowledgements The initial effort in development of this report was a survey and review of available literature, commercial information and unpublished reports by Marnie Duthie, a student at the University of Victoria in a Cooperative Education Project with the Silviculture Branch, MOF. John Henigman of Silviculture Branch, MOF prepared the section on methodology and patterns of MSMA use and provided critical review from the perspective of forest insect control. Dr. John Manville, Pacific Forestry Centre, Canadian Forest Service and Dr. K.J. Reimer, Department of Chemistry, Royal Roads Military College reviewed the report and provided extremely valuable assistance and advice. A critique by Dr. E.M. Belletts, Chemical Consultants International, of toxicological and chemical considerations of the report was very helpful. This report could not have been completed adequately without access to recent data generated for re-registration of MSMA in the United States. The cooperation and generosity of the manufacturers of MSMA, collectively identified as MAA Task Force Three, in making the findings available, is acknowledged with thanks, as is the efforts of Dr. Belletts in expediting our access. iii Table of contents Summary....................................................................................................................................................................ii Acknowledgements .................................................................................................................................................... iii Table of contents........................................................................................................................................................ iv Introduction.................................................................................................................................................................5 Background ................................................................................................................................................................7 Chemistry................................................................................................................................................................7 Basis for control of bark beetle ................................................................................................................................10 Patterns of MSMA use in British Columbia forestry ...................................................................................................10 Other methods of bark beetle control........................................................................................................................11 Pile and burn......................................................................................................................................................11 Sanitation logging ...............................................................................................................................................11 Environmental behavior of MSMA ................................................................................................................................12 Metabolic fate of MSMA and other methyl arsenicals....................................................................................................16 Toxicology of MSMA..................................................................................................................................................18 Clinical observations in humans ...............................................................................................................................18 General toxicology: experimental observations ..........................................................................................................18 Effects observed after single doses and administration over periods up to a few weeks (acute and subacute toxicity)...19 Effects observed after long term administration (chronic toxicity).............................................................................21 Reproductive effects ...............................................................................................................................................22 Carcinogenicity and genetic effects..........................................................................................................................23 Genetic toxicity (mutations and DNA damage) ......................................................................................................23 Carcinogenicity...................................................................................................................................................24 Environmental toxicity................................................................................................................................................25 Terrestrial species ..................................................................................................................................................25 Exposure