Arsenic as a chemical element occurs in both grey and yellow crystalline forms. The most common commercial product is white arsenic (arsenious oxide), which is usually prepared as a by-product of the roasting of various ores. The highly toxic nature of many arsenic compounds has led to their extensive use in insecticides and weed killers.
Arsenic occurs in almost all soils in small amounts. It is also found in variable quantities in natural waters, depending on geographical location. Air also contains arsenic, but usually in very small concentrations. Environmental pollution by arsenic may arise from agricultural practices (weed-killers, fungicides, sheep dips, rodenticides, insecticides) and from industry. Atmospheric emissions of arsenic are associated with metal smelters refining arsenical ores and with the use of arsenic-containing pesticides; reports from Czechoslovakia and India indicate that coal burning is another possible source.
Ambient air concentrations of arsenic are of the order of 0,02 micro g/m3; but these may be much higher in the vicinity of arsenic-emitting sources. Arsenic in topsoils, since it is exposed to atmospheric oxygen, is usually present in the less reactive, pentavalent form. Industrially produced arsenic is in the more toxic trivalent form. Arsenic may be present in small amounts in drinking-water, and a tentative safety limit of 0,05 micro g/l is recommended by WHO. Arsenic occurs naturally in foods and beverages and is normally present in relatively high concentrations in crustacea and other shellfish. It was detected in shrimp as long ago as 1935 in amounts of 42-174mg/kg. The naturally occurring form found in marine animals is probably not very toxic. Arsenic can occur as a contaminant if employed as a pesticide.
In Bangladesh, almost 97 per cent of the population have access to drinking water through more than 4 million tubewells and regard this source as a safeguard against diarrhoeal diseases. However, high arsenic concentrations have now been identified in large numbers of the wells in rural areas. Of 20 000 tubewells tested to date, 19 per cent were found to be contaminated by unacceptable levels of arsenic (more than 0.05 mg per litre) and 2 200 cases of arsenicosis have been identified. The cause of arsenic contamination in groundwater has not yet been established with certainty but natural geological changes are presumed to be the primary cause.