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Arsenic, a toxic substance that can enter the body through water and food, may thicken the walls of the heart's main pumping chamber, a new study shows. That damage can eventually lead to heart failure.

"This is the first U.S. study to measure heart function as related to arsenic exposure," said Dr. Ana Navas-Acien, senior author of the in Circulation: Cardiovascular Imaging. "Few people, including clinicians, are aware that metals exposure matters."

Rocks are a major source of arsenic exposure, which has been linked to various types of cancer. It seeps into groundwater and then into tap water, especially water from private wells, which the Environmental Protection Agency doesn't oversee. Overall, more than 2 million Americans may be exposed to unsafe arsenic levels.

"We recommend that everybody using a well test their water for arsenic," said Navas-Acien, a physician-epidemiologist at the Mailman School of Public Health at Columbia University in New York City. "If you don't have a well, you are safer because the EPA regulates the level of arsenic in community water systems."

In addition, arsenic from past use of pesticides and soil contamination can travel from the soil to crops and food, particularly rice and apple juice made from concentrate, Navas-Acien noted.

Previous research in Mexico and Bangladesh has linked arsenic to heart damage, but those were studies of people exposed to high levels of arsenic.

The current study included 1,337 American Indians in Arizona, Oklahoma, North Dakota and South Dakota exposed to low to moderate levels of arsenic in drinking water. The participants were about 31 years old on average and did not have cardiovascular disease or diabetes at the start of the five-year study.

Researchers analyzed data from heart ultrasounds and urine samples and found that a twofold increase in arsenic bumped up the odds of a thickened left ventricle by nearly 1.5 times among the group overall and by more than 1.5 times among those with elevated or high blood pressure.

Left ventricle hypertrophy is usually a sign of high blood pressure. It indicates that stress is causing the chamber to work harder. Although it may take years for symptoms to develop, the heart requires more blood flow and rhythm disturbances can become more likely.

"What most interests me is that we see this effect even independent of blood pressure," said Dr. Gervasio Lamas, chief of cardiology at Mount Sinai Medical Center in Miami Beach, Florida. He was not involved in the study.

Indeed, participants with normal blood pressure had 1.2 times the odds of left ventricle thickening with a twofold increase in arsenic.

Navas-Acien hopes to get funding for a follow-up analysis looking at 10 years of data.

"Fortunately, our population is now exposed to lower levels of arsenic because communities have been able to intervene," she said. Even so, it will be important "to see how exposure reductions over time translate into health benefits, or if the effects of past exposures persist."

Environmental contaminants are a hidden factor in heart trouble.

They are "the elephant in the room that cardiologists do not recognize," Lamas said. "We don't look for them and don't have good treatments. Arsenic, lead and cadmium are three common toxicants that all of us are exposed to in the course of daily life and cause heart and blood vessel disease."

Although the study looked only at tribal communities, researchers say the results likely apply to people in other parts of the country exposed to arsenic in their water. Navas-Acien suggests those residents to remove arsenic.