Lead may raise the risk of death from many causes at levels much lower than those widely thought to be safe, researchers suggest.
Lead is linked to a range of health problems
The World Health Organization recommends children should not be exposed to blood levels of more than 100 microgram/litre.
But US scientists found increased risks at levels up to five times lower.
The Tulane University findings, in the journal Circulation, suggest the safety limits should be reassessed.
The researchers examined data on 13,946 adults whose blood lead levels were measured between 1988 and 1994. They also looked at death rates and cause of death for this group up to the end of 2000.
They found that the risk of death from all causes, and cardiovascular disease, increased progressively at higher lead levels.
Compared to people with a blood lead level below 19 micrograms/litre, those with a level of between 36 and 100 micrograms/litre had:
- A 25% higher risk of death from any cause
- A 55% higher risk of death from cardiovascular diseases
- An 89% higher risk of death from heart attack
- Two-and-a-half times the risk of death from stroke
Chief researcher Dr Paul Muntner said the study also found evidence that lead blood levels as low as 20 micrograms/litre were associated with a raised risk of cardiovascular death.
He said the public health implications of the findings were potentially significant, as 38% of US adults were estimated as having blood lead levels higher than 20 micrograms/litre in 1999-2002.
Dr Muntner said: "Future research is needed to identify the level of lead exposure that is not associated with major health outcomes.
"Although markedly reduced, the current blood lead levels may not be low enough, and we believe that practical and cost-effective methods for reducing lead exposure in the general population are needed."
A spokesman for the UK's Health Protection Agency said the UK had cut lead emissions by 97% since the 1970s.
Measures had included a ban on the use of lead pipes for drinking water in new installations, and the phasing out of lead-based petrol.
However, he said there was evidence to suggest that even exposure to small amounts of lead could pose a risk - particularly to the development of children's brains.
He said: "We do not believe that any exposure to lead is entirely harmless, and it has been government policy for many years to reduce exposure wherever reasonably practical.
"As a result of these efforts, blood lead levels in the UK have fallen dramatically in recent decades and surveys indicate that the great majority of UK children are now well below the target level."
"It is reasonable to expect further reductions in blood lead levels, as older legislation continues to have an effect and newer actions - such as lowering limits for lead in drinking-water - are introduced."
Ellen Mason, a cardiac nurse at the British Heart Foundation, said: "Because lead does not stay around in the bloodstream for long it makes it difficult to tell whether it is linked to heart disease.
"Further studies are needed to establish whether a link exists and whether it would take short or long term exposure to lead to put the heart at risk."
The researchers found no association between blood lead levels and death from cancer.