The ongoing water crisis in Flint, Michigan, is a reminder that lead exposure remains a toxic, irreversible threat to children’s brains, even in otherwise wealthy countries. After the city switched to a new water supply to cut costs, thousands of children suffered from lead exposure and the mayor declared a state of emergency in 2015 over the disaster.

New research published March 12 in The Lancet suggests the water crisis likely harmed adults too — in particular, their hearts.

The study estimated that more than 400,000 — or 18 percent — of all deaths in the US every year can be linked to lead exposure from all sources. Some 250,000 of those deaths are from cardiovascular disease, while 185,000 were related to coronary artery disease.

That’s about 10 times more than the current estimates of lead-related deaths — and it suggests, lead study author Bruce Lanphear of Simon Fraser University, said, “Lead exposure appears to be a major but largely ignored risk factor for cardiovascular disease and death from cardiovascular disease — especially coronary heart disease.”

“This is huge,” said Mona Hanna-Attisha, a pediatrician in Flint whose research helped expose the water crisis there. “It’s time our policies and practices caught up with the science, and we truly invested in lead elimination not only for our children today but also for decades to come.”

The participants in the study, who were adults in the late 1980s, were exposed to lead through sources like paint, gasoline, water, or soil, potentially starting in childhood and lasting throughout their lives. And there was a strong association between people with higher blood levels of lead and a higher risk of death, especially from cardiovascular complications.

While smoking, a lack of exercise, and an unhealthy diet are certainly important contributors to cardiovascular disease, health researchers are now warning that lead exposure needs to be considered too.

“These findings throw one important risk factor into the mix [for cardiovascular problems], which has largely been overlooked until now,” said Philip J. Landrigan, dean of global health at the Icahn School of Medicine at Mount Sinai, who wrote the study’s related commentary.

Lead damages brain cells — but it can wreak havoc on the blood vessels too

Lead is a naturally occurring heavy metal found in mineral deposits in the Earth’s crust. It’s also a poisonous substance for humans, accumulating in our teeth and bones. At higher levels, lead can register in blood tests.

People are typically exposed to lead by breathing in lead-contaminated dust, drinking water from leaded pipes, or eating from cans that have been soldered with lead. For children, there are other risks: eating soil, or paint chips, for example. “Children’s innate curiosity and their age-appropriate hand-to-mouth behavior result in their mouthing and swallowing lead-containing or lead-coated objects,” the World Health Organization summed up.

Inside the body, lead can slip into human cells easily and wreak havoc. In children, particularly under the age of 10 or babies in the womb, the metal can pass through the blood-brain barrier and kill off brain cells. Children absorb up to five times as much lead as adults, and with their smaller bodies and developing nervous systems, it doesn’t take much to sicken them.

“That is why children who have been exposed to lead in early life have loss of IQ or shortening of attention span or other cognitive or mental health problems,” Landrigan explained.

There’s also no real cure for lead poisoning. Chelation therapy can reduce the amount of lead circulating in the bloodstream — but this hasn’t been proven to improve children’s intellectual abilities or behaviors after lead damage has been done.

Since the 1970s, lead exposure has been declining worldwide after the metal was eliminated from paint and gasoline in the wake of research exposing its health effects.

Researchers have also known that lead can enter blood vessels in both adults and children, harming the endothelial cells that line the vessels. This process hardens arteries and causes plaque to form in blood vessels, increasing blood pressure and the risk of heart disease and stroke. Meanwhile, lead can damage kidneys, which play an important role in regulating blood pressure — also increasing a person’s risk of heart disease and stroke, Landrigran explained.

This is why the Environmental Protection Agency, the World Health Organization, and the National Toxicology Program out of the National Institutes of Health have all determined that high levels of lead exposure increase the risk of high blood pressure and coronary heart disease, Lanphear said.

But lead’s potential harms to adults and their hearts have gotten relatively little attention among the public and policymakers. Lanphear and his co-authors hope to change that.

A high level of lead exposure was associated with a 70 percent increase in cardiovascular disease mortality risk

For the study, the researchers gathered data on a nationally representative sample of 14,000 adults who were enrolled in the National Health and Nutrition Examination Survey between 1988 and 1994, and followed up until December 31, 2011. The study participants had undergone a slew of medical tests, including quantifying the lead levels in their blood. Their health data was also linked up with records about their cause of death, when relevant.

Lead in the blood is typically measured in µg/dL (micrograms per deciliter) or in parts per billion. Researchers used to think 5 µg/dL — or 50 parts per billion, about the same concentration as 100 tablespoons in an Olympic swimming pool — was a safe blood lead level. But agencies like the Centers for Disease Control and Prevention have recently determined there’s actually no known safe blood concentration for children.

Lanphear and his colleagues believe the same may be true for adults. In the study, they found a strong correlation between lead in the blood and a higher risk of death from cardiovascular complications: Comparing the group with the lowest level of lead exposure (1 µg/dL or 10 parts per billion) to the group with the highest (6.7 µg/dL or 67 parts per billion), the researchers found a 70 percent increase in cardiovascular disease mortality risk and a doubling of mortality from coronary heart disease. This indicates that more lead exposure may lead to more heart trouble, and also that there is no safe threshold for lead exposure.

Even after the researchers controlled for potential confounding factors — including age, sex, ethnic origin, where people lived, smoking status, diabetes, alcohol intake, and even household income — the association held.

Still, this was an observational study, which can only tell us about relationships between phenomena, and not whether one caused the other. And at least one important potential confounding factor was overlooked, said Stephen Lim, director of science at the Institute for Health Metrics and Evaluation, which measures health effects of lead exposure on blood pressure and cardiovascular disease globally.

Lead is correlated with other known risk factors for cardiovascular disease — most importantly, that lead exposure tends to be higher in lower-income communities like Flint, Lim said. “[They] did not control for community-level socioeconomic factors, and as a result, the magnitude of the effect found in the study may be confounded by the relationship of higher lead exposure in lower socioeconomic communities.” And it’s possible this might have exaggerated the magnitude of lead’s effects on heart health.

“That lead increases the risk of cardiovascular disease is not surprising, [but] what is surprising is the magnitude of the effect,” Lim said.

“Today, lead exposure is much lower because of regulations banning the use of lead in petrol, paints, and other consumer products, so the number of deaths from lead exposure will be lower in younger generations,” Lanphear said.

Landrigan suggested that blood lead testing should become the norm in adult medicine the way pediatricians now often screen children for lead exposure.

Flint’s Hanna-Attisha said the study should be further fuel for policymakers working to eliminate lead exposures from our environment. “We spend billions each year treating cardiovascular disease — medications, hospitalizations, procedures — yet with lead elimination, we know how to prevent a portion of this health burden, not even considering preventing all the other evils of lead exposure,” she said.

There are many ways the EPA and federal regulators can continue to reduce people’s lead exposure, Lanphear said, by enforcing stricter standards on allowable levels of lead in the air, water, dust, and soil. EPA Administrator Scott Pruitt has declared a “war on lead” in drinking water in the wake of the Flint water crisis, but the agency has dragged its feet on new regulations for lead in paint.

“Ongoing sources... need to be phased out, remediated, or banned,” Lanphear said, adding that manufacturers should further reduce lead levels in foods, drinks, and cosmetics.

With limited treatments available for lead, he emphasized, “the ideal solution is prevention.”