“Water, water everywhere—nor any drop to drink.” The plight of Coleridge’s ancient mariner is that of present-day populations in some 70 countries on six different continents. From China to Chile, unsafe levels of naturally occurring arsenic—a potent human toxicant and carcinogen—have been detected in the drinking water. And nowhere is the problem more pronounced than in Bangladesh. Beginning in the 1970s, handpumped tube wells were installed throughout the country in an effort to provide rural communities with clean water for the prevention of cholera and other water-borne diseases. It wasn’t until the early 1990s, however, that geological surveys revealed substantial contamination of the aquifers from which these wells drew their water. In the decades since, as many as 77 million people in Bangladesh alone are believed to have been chronically exposed to raised concentrations of the toxic metalloid. That exposure has been associated with a wide variety of adverse health effects, including reduced cognitive function, peripheral neuropathy, respiratory complications, diabetes and cardiovascular disease, and cancers of the skin, lung, kidney and bladder, among other problems.“Now it’s just a matter of adding new items to the list”, says Habibul Ahsan, director of the Center for Cancer Epidemiology and Prevention at the University of Chicago and principal investigator on the Health Effects of Arsenic Longitudinal Study (HEALS). A prospective cohort study of 30 000 men and women in Bangladesh, HEALS has yielded important new findings about the health risks of arsenic exposure.“Like tobacco, arsenic has multisystemic effects”, says Ahsan, a native of Bangladesh, who reported in a 2010 article in The Lancet that just under a quarter of all chronic disease related deaths in the HEALS cohort could be attributed to arsenic-contaminated well water. Given that well water is the only pathogen-free water to which most Bangladeshis have access, he says,“it’s not as though they can easily switch to something else”.

Hence the importance of determining exactly how much arsenic people can safely consume—and using that information to tailor sustainable mitigation strategies.“What’s the minimum dose and how long does someone have to be exposed?” he says.“We still don’t know the answer to that.”