People who live in areas with moderately elevated levels of arsenic in the drinking-water supply may have a somewhat increased risk of stroke, a study of Michigan residents suggests.

The findings, published in the journal Stroke, do not prove that drinking-water arsenic is responsible for the elevated risk. Nor do they suggest that water with arsenic levels that meet guidelines from the U.S. Environmental Protection Agency (EPA) - which most U.S. drinking-water supplies do - are a stroke hazard, the study’s lead researcher told Reuters Health.

However, the study does call for more in-depth research to determine whether arsenic in the water supply is contributing to some strokes.

Arsenic is an element found naturally in rock, soil, water, the air and the food supply. It is also released into the environment through industrial activities; for instance, arsenic is used as a wood preservative and in some paints, dyes and fertilizers.

High arsenic exposure can lead to cancer, and chronic exposure to even moderately elevated levels has been linked to high blood pressure and type 2 diabetes. But the possible health effects of such modestly increased exposures are not yet fully clear.

The EPA has set the maximum allowable level of arsenic in drinking water at 10 micrograms per liter (or 10 parts per billion). According to the Centers for Disease Control and Prevention, about 80 percent of U.S. drinking-water supplies have an arsenic level below 2 parts per billion (ppb), but 2 percent exceed 20 ppb.

For the new study, researchers looked at whether variations in arsenic levels in Michigan’s water supplies were related to residents’ risk of being hospitalized for stroke.

The links between arsenic and high blood pressure and diabetes make it plausible that drinking-water arsenic could contribute to strokes, lead researcher Dr. Lynda D. Lisabeth, of the University of Michigan in Ann Arbor, told Reuters Health in an e-mail.

It’s also possible, she said, that chronic, low-level arsenic exposure could accelerate atherosclerosis, a hardening and narrowing of the arteries that can lead to heart attacks and stroke.

The researchers focused on Michigan because an estimated 230,000 people in the southeastern part of the state are exposed to drinking water that exceeds the EPA’s standard - making it one of the nation’s most densely populated regions with moderately elevated drinking-water arsenic.

Lisabeth’s team used state government data on water samples collected between 1983 and 2002 to estimate the arsenic exposure among residents of Michigan’s 83 counties. In most counties, the average arsenic level in drinking water was below the EPA cutoff, with the median, or midpoint, level being 1.8 micrograms per liter of water.

Using a hospital database, Lisabeth and her colleagues found that there were just over 294,000 hospitalizations for stroke across the state between 1994 and 2006. In general, rates were slightly higher in counties with relatively higher arsenic levels; however, the researchers also found that those counties had higher rates of hospitalizations for ulcers and hernias, two conditions with no known relation to arsenic.

That, the researchers say, suggests that some county-level factor other than drinking-water arsenic may explain the higher stroke risk.

However, when the investigators focused on one county with historically elevated arsenic levels in the water - Genesee County - they found higher stroke rates in zip codes with the highest arsenic levels. And there was no similar connection to ulcer and hernia hospitalizations.

Among county residents, 14,033 hospitalizations for stroke occurred during the study period. In the 20 percent of zip codes with the highest arsenic levels (between roughly 19 and 22 micrograms per liter), the risk of stroke hospitalization was more than double that in the 20 percent of zip codes with the lowest drinking-water arsenic levels.

The findings, Lisabeth said, suggest an association between drinking-water arsenic and stroke risk, but do not prove cause-and-effect.

One of the main limitations of the study is that it had no data on individual residents, including their personal exposure to arsenic in drinking and their risk factors for stroke.

The researchers were able to factor in certain variables that varied by county and zip code - like residents’ median income and the percentage of black residents, both of which are related to stroke rates; arsenic in drinking water was still related to stroke risk with those factors considered.

However, Lisabeth said, studies with information on individuals, including “detailed exposure assessment,” are still needed.

For now, she stressed that most people need not worry about potentially harmful levels of arsenic in their drinking water.

“These results do not provide any evidence that the (EPA) guideline is inadequate,” Lisabeth said, “so I would not be concerned about the safety of public water supplies.”

However, she added, people who get water from private wells may want to have the water tested if they live in an area that, like southeast Michigan, is known to have elevated levels of arsenic.

If levels in private wells exceed 10 micrograms per liter, reverse osmosis filters are recommended for removing the arsenic, Lisabeth noted.

While most public drinking water in the U.S. meets the EPA standard, high arsenic levels are a bigger threat in other parts of the world. Researchers have estimated that about 140 million people worldwide drink water with arsenic levels above 10 ppb; Bangladesh has been hardest hit, with millions being exposed to high levels of naturally occurring arsenic in well water.

SOURCE: Stroke, online October 14, 2010.
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By Amy Norton

NEW YORK