The new study, which published Monday in The Lancet Public Health
, tracked more than 14,000 adults over a period of about 20 years. It found that those individuals with an initial blood lead concentration at the 90th percentile had a 37% increase in all-cause mortality and a 70% increase in cardiovascular disease mortality compared to those with a blood lead concentration at the 10th percentile.
While a link between lead exposure and high blood pressure has been known for decades, the magnitude of the effect on cardiovascular mortality -- particularly at low levels of lead exposure -- was greater than anticipated, according to Dr. Bruce Lanphear
, a professor of health sciences at Simon Fraser University and a leading author of the study.
"Nobody had even tried to estimate the number of deaths caused by lead exposure using a nationally representative sample of adults," Lanphear said. "But if we're underestimating the impact of lead exposure on cardiovascular disease mortality and other important outcomes beyond IQ, then it might have a big impact on the way we make investments in preventing lead poisoning exposure."
According to the study, the 10th percentile corresponded to a blood lead concentration of 1.0 micrograms per deciliter, while the 90th percentile corresponded to a concentration of 6.7 micrograms per deciliter. The Centers for Disease Control and Prevention
currently recommends that public health officials take action when blood lead concentrations rise above 5 micrograms per deciliter.
The researchers relied on a nationally representative sample of 14,289 adults ages 20 years and older who participated in the Third National Health and Nutrition Examination Survey between 1988 and 1994. The survey is administered by the US Centers for Disease Control and Prevention every year.
Of the initial 14,289 survey respondents, 4,422 had died by 2011. The researchers calculated that approximately 18% of those deaths could have been prevented by reducing blood lead concentrations to 1.0 micrograms per deciliter.
"I think it fits in well with the literature," said Dr. Philip Landrigan
, Dean for Global Health at the Icahn School of Medicine at Mount Sinai and an expert on environmental pollution in children, who was not involved in the study.
"The literature has been showing for many years that lead causes hypertension, stroke, and cardiovascular disease," he added. "This study now shows that the cardiovascular toxicity of lead extends down to lower levels than were previously examined."
"There's no safe threshold," Lanphear said. "Once we found that there was a risk across the entire range of exposures, we could estimate the number of attributable deaths. And instead of it being 40,000 deaths, which is what had previously been estimated, we found that it was about 10 times that."
While the toxic effects of lead exposure have been known for centuries, the vast number of people potentially affected by the cardiovascular effects of lead exposure were surprising, even to the study's researchers.
"When you start looking at the risk across the entire range of people exposed, all of a sudden the number of affected people balloons," said Lanphear. "Mostly it's a numbers thing -- there are so many people in the low- to moderate-risk groups that, as long as there are some risks with low-level exposure, many more people are going to die or develop heart disease."
In the 1970s, after decades of rising levels of lead contamination in the US, Congress banned lead-based paint for residential use in 1978 and amended the Clean Air Act to establish new guidelines for automobile gasoline requirements.
Since then, the CDC has reduced the acceptable blood concentration from 40 micrograms per deciliter in the early 1970s, to 10 micrograms per deciliter in the early 1990s, to 5 micrograms per deciliter just a couple of years ago, according to Landrigan.
But despite creating these thresholds, the CDC still cautions that no safe blood lead level has yet been identified.
"The CDC can't tell health departments around the country that they have to get every child's blood level down to zero," Landrigan said. "It's an impossible task."
Lead exposure is thought to contribute to cardiovascular disease by a number of different pathways. Lead has long been known to damage the epithelial cells lining the body's blood vessels, which increases the likelihood of developing plaques that can then cause a heart attack, said Landrigan.
"Basically, lead causes endothelial damage, which increases the risk of plaque formation and arteriosclerosis.
"It also causes kidney damage, which causes hypertension, and the two probably act synergistically with each other."
Even with current regulations in place, approximately 90% of Americans are stilled exposed to the contaminant, particularly those who work in certain occupations such as construction or those who live in areas more vulnerable to contamination runoff, according to Lanphear.
"Aviation gas, or gas that is used in single piston jet engines in regional airports, still uses leaded gas," Lanphear said. "And if you live around a regional airport, your blood lead levels will be a little higher than if you live further away from the regional airport."
The study also controlled for other relevant risk factors, including age, sex, ethnicity, income, level of physical activity, diabetes status, and body mass index. They found that the association between lead exposure and cardiovascular disease remained present even after controlling for these factors.
The researchers also relied heavily on one blood concentration measurement taken at the beginning of the study period, one of the study's main limitations, according to Lanphear.
"Our reliance on a single blood test as opposed to serial blood tests means that we have underestimated the impact of lead exposure on cardiovascular disease," he said. "There are some things in the study design itself that we really couldn't change."
Ultimately, both Landrigan and Lanphear believe that the results of the study will likely impact how physicians think about lead exposure as a risk factor for cardiovascular disease, particularly among older adults.
"My guess is that internists and family doctors who are caring for adults are, after the publication of this study, going to think more about lead as a risk factor for heart disease," Landrigan said.
But taking precautions at home -- such as hiring professionals when removing paint from homes built before 1978 -- still remain among the most effective ways to prevent the negative health impacts of lead exposure, according to Lanphear.
"We've made tremendous progress in reducing these exposures in the past four to five decades," Lanphear added. "But our blood levels are still 10 to 100 times higher than our pre-industrial ancestors."