Context: Approximately 100 million people throughout the world consume water contaminated with arsenic at levels above carcinogenic thresholds, including 40 million in Bangladesh alone, with up to one-fourth of deaths attributed to arsenic exposure in the worst-affected regions. There are no proven therapies for treating chronic arsenic toxicity or for preventing arsenical cancers. Selenium has been known to counter arsenic toxicity in a variety of animal models. The investigators have recently shown in animals and humans that this effect is mediated by the formation of [(GS)2AsSe]- , the seleno-bis(S-glutathionyl) arsinium ion, which is then rapidly excreted via the hepatobiliary system. Concurrently, two Phase II studies in China and Bangladesh have suggested clinical benefit to selenium supplementation in arsenicosis patients.

Objective: To assess whether daily selenium supplementation counters arsenic toxicity in patients exposed to drinking water arsenic. If proven effective, selenium supplementation might be safely and cost-effectively implemented in the worst-affected localities.

Inclusion Criteria:

• Resident of Bangladesh in an arsenic-affected region (Chandpur)
• Age between 12 and 55
• Exposure to arsenic in home drinking water greater than 50 ug/L.
• Arsenical melanosis on the torso confirmed by epiluminescence microscopy

Exclusion Criteria:

• Recent history or plans to consume selenium-containing supplements
• Anticipated change in home drinking water supply during study period