There's one possible reason so many Americans, especially those living in inner cities, have high blood pressure: lead exposure.

Researchers tested people’s bones for evidence they took in lead over the years. The heavy metal accumulates in bone and stays there for decades as people drink lead-tainted water, breathe in dust carrying lead or get it some other way.

But if something causes its release — anything from simple aging to pregnancy or thyroid disease — it can raise blood pressure.

The researchers found that veterans who had lead in their bones were more likely to have high blood pressure that could not be lowered sufficiently even by a cocktail of blood pressure drugs.

It’s possible that lead exposure from decades past is helping feed the growing epidemic of high blood pressure, said Sung Kyun Park of the University of Michigan School of Public Health, who led the study team.

“Lead is a widespread environmental toxicant. As you have seen in Flint and many other cities, there may be continued exposure,” Park told NBC News.

Lead in drinking water

Residents of Flint, Michigan are still struggling to get a water supply free of lead and other contaminants. Researchers say many cities across the country with aging infrastructure are also delivering lead-tainted water to residents, or are at risk of doing so.

“Since the lead problems in the drinking water in Flint, Michigan, have surfaced, the issue has become more troubling, especially in older U.S. cities,” Park said.

Old paint in homes and buildings is another significant source of lead, and people who were alive before lead was removed from gasoline in the U.S. in the 1990s will have breathed in lead from car emissions, as well as from industrial pollution.

Even though lead is known to raise blood pressure, Park said, he had seen no studies checking to see if patients with high blood pressure might have lead in their bodies.

So he and colleagues tested 475 veterans with high blood pressure being treated at a Boston Veteran’s Affairs center.

They performed x-rays on their legs — it’s an easy-to-reach spot — and also tested their blood for lead.

Lead is cleared from the blood fairly quickly and they didn’t find any link between lead levels in blood and hypertension. But when they checked the shin bones, they found that men with hard-to-control high blood pressure also were more likely to have higher levels of lead in their bones.

Some of the men couldn’t get their blood pressure down even when they took three different blood pressure drugs. Doctors often worry that patients are not taking their medication properly, are eating too much salt or other unhealthy foods, and that’s why the drugs don’t seem to help.

But Park and his team think lead could be another possible reason. They found a man’s risk of drug-resistant high blood pressure went up 19 percent for each 15 microgram per gram increase in lead levels in the shin bone.

“This is the first study, to our knowledge, that points to cumulative low‐level lead exposure as a potential risk factor for resistant hypertension,” they wrote in their report, published in the Journal of the American Heart Association.

“Laws limiting lead exposure have been on the books for decades, but in recent years it is recognized that lead remains an environmental toxin that is still with us,” Park said.

The men that Park’s team tested were almost all white, so more groups will have to duplicate the work in different people to see if the association holds, he said.

But there is much evidence to show that lead raises blood pressure and that many people have lead stored up in their bodies.

Lead causes brain damage in young children, but it also accumulates in their bodies as they grow up. Adults usually don’t show immediate effects of lead exposure unless they get huge doses, but it can stay in their teeth and bones.

Aging, menopause, pregnancy, breastfeeding, kidney disease, stress, thyroid problems and other conditions can all release the lead from the bones.

Nearly half of all Americans have high blood pressure, according to the American Heart Association. African-American men are especially susceptible to high blood pressure.

The American Academy of Pediatrics says 37 million homes in the United States still contain lead-based paint. People living in lower-income areas are the most likely to live in homes that still have lead-based paint, which flakes off into dust that can be inhaled. Low-income areas are also more likely to have lead-contaminated water supplies.