High arsenic exposure is known to be a risk factor for skin cancer, but a new study suggests that even more-moderate exposure through drinking water may boost the risk of pre-cancerous skin growths.

The study, reported in the American Journal of Epidemiology, focused on an area of Bangladesh with naturally high levels of arsenic in well water. It's common for water there to have arsenic levels upwards of 100 micrograms per liter of water.

By comparison, the upper limit for arsenic in U.S. public drinking water is 10 micrograms per liter, and most local systems have levels well below that.

Arsenic is an element found in rock, soil, water and air. It is also used as a wood preservative and can be found in some paints, dyes and fertilizers.

High arsenic exposure has been linked to several cancers, including cancers of the bladder, lungs and skin.

In particular, studies have shown that people who drink water with arsenic levels of at least 100 micrograms per liter have a heightened risk of pre-cancerous skin growths.

But the risk at lower arsenic exposures has not been clear, according to the researchers on the new study, led by Dr. Habibul Ahsan of the University of Chicago.

So Ahsan's team followed more than 10,000 Bangladeshi adults for about six years, looking at the relationship between arsenic exposure and the odds of developing arsenic-related skin lesions.

In all, 866 men and women developed new skin lesions—pigment changes or thickened areas of skin that are often precursors to arsenic-related skin cancer.

People whose well water topped 200 micrograms per liter were almost three times as likely to develop a skin growth as those whose water arsenic levels were below 10 micrograms per liter.
In the latter group, about six percent developed a skin lesion, versus 15 percent in the high-arsenic group.

But there was also an increased risk at more-moderate levels. People whose well water had arsenic concentrations between 50 and 100 micrograms per liter were 69 percent more likely to develop skin lesions than those with water levels below 10 micrograms per liter.

"The findings of this study have important public health implications for arsenic in drinking water," Ahsan and his colleagues write.

"We found that arsenic exposure through drinking water was associated with increased risk of skin lesion incidence, even at water concentrations less than 100 micrograms per liter."

Right now, both the World Health Organization and the U.S. Environmental Protection Agency (EPA) consider 10 micrograms per liter the upper limit for arsenic in drinking water. It's estimated that 140 million people worldwide drink water with levels higher than that.

But researchers still aren't sure what a "safe" level of arsenic exposure is. Some studies, but not all, have linked even moderately elevated levels of arsenic in drinking water, or in the body, to increased risks of high blood pressure, type 2 diabetes and stroke.

The EPA is currently considering lowering the 10-microgram limit.

It's estimated that 13 million Americans live in areas where the public water supply exceeds the current EPA standard. And unregulated private wells might also contain too much arsenic, particularly in certain areas of the West, Midwest and New England where the groundwater contains high concentrations of the toxic chemical.

Experts suggest that people have private well water tested for arsenic. If the level exceeds 10 micrograms per liter, it can be treated with special filtration systems.

There's also some evidence that diet can help counter the toxic effects of higher arsenic exposure.
In a previous study of the same Bangladeshi adults, Ahsan's team found that those who ate the most root vegetables and gourds, like pumpkin and squash, had a lower risk of skin lesions than people whose diets were heavier in meat or other types of vegetables.

Those findings do not prove that the foods are protective. But the researchers said it was important to keep searching for ways to mitigate the toxic effects of arsenic because once people are chronically exposed to high levels, their health risks remain higher-than-normal even after their arsenic exposure is reduced.