Babies who are breastfed have lower arsenic exposure than babies who are fed formula, according to a new study.

The powder and water used to make the baby formula may be sources of arsenic, which occurs naturally in the environment and in large doses is linked to serious health problems, the researchers write in the journal Environmental Health Perspectives.

It's not clear, however, whether the low levels of arsenic exposure in the study will turn out to be harmful, the researchers say.

Parents who need to use formula to feed their infants shouldn’t feel badly about the new findings, said Kathryn Cottingham, one of the study’s lead authors. Instead, they should pay attention to the water they’re using to make baby formula.

“People who don’t know what’s going on in their water should test their water,” said Cottingham, who works at the Center for Children's Environmental Health and Disease Prevention Research at Dartmouth College in Hanover, New Hampshire.

She and her coauthors note that arsenic occurs naturally in bedrock and is a common contaminant of well water. While the U.S. regulates how much arsenic is allowed in public drinking water, there’s no regulation of private wells. And in New Hampshire, where this study was done, private wells supply water to 40 percent of the population. About one in 10 wells in the state have arsenic levels higher than the 10 micrograms per liter that's allowed in public drinking water.

Past research has shown that breast milk doesn’t contain high levels of arsenic, even when mothers have been exposed to high levels of the element.

Baby formula powder, however, may have low levels of naturally occurring arsenic, the researchers say.

For the new study, the researchers analyzed urine samples from six-week-old babies of New Hampshire women who were pregnant and recruited for the study starting in January 2009.

Out of the 72 babies in the study, 70 percent received only breast milk, 13 percent received only formula and 17 percent received a combination of the two.

“Overall, exposure levels in this age group are low and that’s great news,” Cottingham said.

They did find that infants fed only formula had the highest concentrations of arsenic in their urine, followed by those who received formula and breast milk. Exclusively breastfed infants had the lowest levels of arsenic in their urine.

Based on samples of the participants’ tap water and published data, the researchers say about 70 percent of the arsenic exposure in their study came from the formula powder.

“In conclusion, our findings suggest that breastfed infants have lower exposure to arsenic than formula-fed infants, even when drinking water arsenic concentrations are low,” the researchers write.

They also caution that their study has limitations, including not having enough information to make estimates of arsenic exposure for individual infants. They also only had a handful of babies fed exclusively formula.

Cottingham also said they can’t say that formula-fed infants or those exposed to higher levels of arsenic will have worse health outcomes later in life.

“As the (group of babies) ages, we’ll be able to follow and see if there are any associations with that time period,” she said.