November 4, 2010

Arsenic In Drinking Water Could Lead To Stroke

A study of Michigan residents suggests that people who live in areas with moderately elevated levels of arsenic in the drinking water may have an increased risk of stroke.

The findings do not prove that drinking water arsenic is responsible for the heightened risk.  The study's lead researcher told Reuters Health that the findings also do not suggest that water with arsenic levels that meet the guidelines of the U.S. Environmental Protection Agency (EPA) are a stroke hazard.

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

Arsenic is naturally found in rock, soil, water, the air and the food supply.  It is also released into the environment through wood preservative, paints, dyes and fertilizers.

High arsenic exposure can lead to cancer, and chronic exposure to moderate levels have been linked to high blood pressure and type 2 diabetes.  However, the possible health effects of this type of exposure are not completely understood.

The EPA set the maximum allowable level of arsenic in drinking water at 10 parts per billion (ppb).  The Centers for Disease Control and Prevention (CDC) said about 80 percent of U.S. drinking water supplies have an arsenic level below 2 ppb, but 2 percent exceeded 20 ppb.

Researchers looked at whether variations in arsenic levels in Michigan's water supplies were related to residents' risk of being hospitalized for stroke.

Lead researchers Dr. Lynda D. Lisabeth of the University of Michigan told Reuters Health that the relationship between arsenic and high blood pressure and diabetes make it feasible that drinking water with arsenic could contribute to stroke.

She said that it is also possible that low-level arsenic exposure could accelerate atherosclerosis, a hardening and narrowing of the arteries that leads to heart attacks and stroke. 

The team 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.

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 countries.  The average arsenic level in drinking water was below the EPA cutoff.

Lisabeth and her colleagues found that there were a little over 294,000 hospitalizations for stroke across the state between 1994 and 2006.  Rates were generally higher in counties that had higher arsenic levels.  However, the researchers found that those counties had higher rates of hospitalizations for ulcers and hernias, two conditions that are not linked to arsenic.

The researchers say that some county-level factors other than drinking water with arsenic may explain the higher stroke risk.

However, when the team focused on one county with historically elevated arsenic levels in the water, they found higher stroke rates in zip codes with the highest arsenic levels.

There were 14,033 hospitalizations for stroke among county residents during the study period.  The risk of stroke hospitalizations among the 20 percent of zip codes with the highest arsenic levels was more than double that in the 20 percent of zip codes with the lowest drinking-water arsenic levels.

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

One of the limitations that the team faced is that no data on individual residents had been recorded, including their personal exposure to arsenic and medical history.

The team also factored in certain variables that varied by county and zip code, such as residents' median income and the percentage of black residents, both of which are more prone to having a stroke.

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

She stressed that most people do not need to 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."

She added that those who get water from private wells may want to have their water arsenic level tested.

Lisabeth said that if arsenic levels in private wells exceed 10 ppb then they should use reverse osmosis filters for removing it.

Researchers have estimated that about 140 million people around the world are drinking water with arsenic levels above 10 ppb.


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