People exposed to low levels of arsenic may significantly increase their chance of developing cancer, according to a recent study.

Researchers at the National Institutes of Health found that mice exposed to low doses of arsenic in drinking water, similar to what some people might consume, developed lung cancer.

Arsenic levels in public drinking water cannot exceed 10 parts per billion (ppb), which is the standard set by the U.S. Environmental Protection Agency. However, there are no established standards for private wells, from which millions of people get their drinking water.

"This is the first study to show tumor development in animals exposed to very low levels of arsenic, levels similar to which humans might be exposed," Michael Waalkes, lead author on the paper and director of the National Toxicology Program (NTP) Laboratory, said in a statement. "The results are unexpected and certainly give cause for concern."

For the study, the concentrations given to the mice in their drinking water were 50 ppb, 500 ppb, and 5,000 ppb. 50 ppb is the lowest concentration that has been tested in an animal study, and researchers say that because of differing rates of metabolism, mice need to be exposed to greater concentrations of arsenic in drinking water than humans to achieve the same biological dose and similar health effects.

In the study, the mice were given arsenic three weeks before breeding and throughout pregnancy and lactation. Arsenic was then given to the offspring after weaning, and all through adulthood at concentrations relevant to human exposure. The researchers looked at the tumors in the adult offspring.

Researchers found that more than half of the male offspring mice developed significant increases in benign and malignant lung tumors at the two lower doses. Female offspring also developed benign tumors at the lower concentrations. Interestingly, the researchers did not find significant increases in lung tumors in either sex at the highest dose.

"Although this is only one study, it adds to a growing body of evidence showing adverse health effects from very low exposures to arsenic, raising the possibility that no level of arsenic appears to be safe," Linda Birnbaum, researcher and director of the National Institute of Environmental Health Sciences and NTP, said in a statement.

The findings were recently published in the Archives of Toxicology.