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to cause similar signs of poisoning. Biomethylation is the preferred <br />detoxification mechanism for absorbed inorganic arsenicals; methylated <br />arsenicals usually clear from tissues within a few days. <br />Episodes of arsenic poisoning are either acute or subacute; chronic cases <br />of arsenosis are seldom encountered in any species except man. Single oral <br />doses of arsenicals fatal to 50% of sensitive species tested ranged from 17 to <br />48 mg/kg body weight (BW) in birds and from 2.5 to 33 mg/kg BW in mammals. <br />Susceptible species of mammals were adversely affected at chronic doses of 1 <br />to 10 mg As/kg BW, or 50 mg As/kg diet. Sensitive aquatic species were <br />damaged at water concentrations of 19 to 48 ug As/1 (the U.S. Environmental <br />Protection Agency drinking water criterion for human health protection is 50 <br />ug/1), 120 mg As/kg diet, or (in the case of freshwater fish) tissue residues <br />>1.3 mg/kg fresh weight. Adverse effects to crops and vegetation were <br />recorded at 3 to 28 mg of water soluble As/1 (equivalent to abouj 25 to 85 mg <br />total As/kg soil) and at atmospheric concentrations >3.9 ug As/m . <br />Numerous and disparate arsenic criteria have been proposed for the <br />protection of sensitive natural resources; however, the general consensus is <br />that many of these criteria are inadequate and that additional information is <br />needed in at least five categories: (1) developing standardized procedures to <br />permit correlation of biologically observable effects with suitable chemical <br />forms of arsenic; (2) conducting studies under controlled conditions with <br />appropriate aquatic and terrestrial indicator organisms to determine the <br />effects of chronic exposure to low doses of inorganic and organic arsenicals <br />on reproduction, genetic makeup, adaptation, disease resistance, growth, and <br />other variables (3) measuring interaction effects of arsenic with other common <br />environmental contaminants, including carcinogens, cocarcinogens, and <br />promoting agents; (4) monitoring the incidence of cancer and other <br />abnormalities in natural resources from areas with relatively high arsenic <br />levels, and correlating these with the possible carcinogenicity of arsenic <br />compounds; and (5) developing appropriate models of arsenic cycling and <br />budgets in natural ecosystems. <br />iv