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Persistent, low-level exposure to lead over decades is statistically linked to some 400,000 premature deaths in the United States each year, far more than previously thought, researchers said Monday.

Compared to people with little or no lead in their blood, those with high levels -- at least 6.7 milligrammes per decilitre (mg/dl) -- were 37 percent more likely to die early, according to a new study in The Lancet Public Health, a leading medical journal.

The risk of succumbing to coronary heart disease doubled in such cases, the study found.

"Low levels of lead exposure are an important, but largely ignored, risk factor for death from cardiovascular disease," mainly heart attacks and strokes, said lead author Bruce Lanphear, a professor at Simon Fraser University in Canada.

The new research challenges "the assumption that specific toxicants -- like lead -- have 'safe levels'," he said in a statement.

Lanphear and his team reviewed two decades of health data for more than 14,000 adults in the US, covering the period 1990-2011.

The participants all had blood tests at the outset to measure past and current exposure to lead, as well as a urine test for the metal cadmium.

People can be exposed to lead via fuel, paint and plumbing, as well as around smelting sites or by handling lead batteries. Lead contamination can also occur in drinking water, as well as foods stored in lead-tainted containers.

Safety regulations have significantly reduced the risk of lead exposure in recent decades, especially in developed countries, but the heavy metal can persist in the body for many years.

Lead was undetectable in the blood of nearly one in 10 of the volunteers tested. At the other extreme, a fifth were found to have at least five mg/dl of lead flowing through their veins.

The largest lead concentrations found in the study were 10 times higher.

- Lead-laden jet fuels -

Overall, 18 percent of US participants who died from all causes during the period reviewed were found to have more than one mg/dl of lead in their blood.

The study concluded that nearly 30 percent of all deaths due to cardiovascular disease -- basically, heart attacks and strokes -- "could be attributable to lead exposure".

"Lead represents a leading cause of disease and death, and it is important to continue our efforts to reduce environmental lead exposure," Lanphear said.

The researchers called for more aggressive measures to retire contaminated housing, phase out lead-laden jet fuels, replace lead pipes in plumbing, and reduce emissions from smelters and lead battery factories.

"Lead has toxic effects on multiple organ systems and relatively low levels of exposure previously thought to be safe," Philip Landrigan, a professor at New York's Icahn School of Medicine, said in a comment, also in The Lancet Public Health.

"A key conclusion to be drawn from this analysis is that lead has a much greater impact on cardiovascular mortality than previously recognised."

The authors controlled for other factors that might contribute to cardiovascular disease, such as smoking, excessive alcohol consumption, poor diet and lack of exercise.

They were not, however, able to factor out the possible impact of exposure to arsenic or air pollution.

AFP