Lead is linked to a number of health problems
Young children's exposure to lead in the environment is harming their intellectual and emotional development, according to UK researchers.
The researchers say the toxic effects of lead on the central nervous system are obvious even below the current so-called safe level of lead in the blood.
They are recommending the threshold should be halved.
A spokesman for the Health Protection Agency said levels of exposure should be kept to the minimum.
Lead has been removed from paint and petrol by law in the UK, but it is still widespread in the environment.
The study from the University of Bristol Centre for Child and Adolescent Health set out to see if there was any effect on the behaviour and intellectual development of children who had ingested just below the so-called safe level of 10 microgrammes per decilitre (or tenth of a litre) of blood.
The study is published in the journal, Archives of Diseases in Childhood.
SOURCES OF LEAD
Lead water pipes
Soil around the home
Paint on children's toys
Children's bead necklaces
The Bristol researchers took blood samples from 582 children at the age of 30 months.
They found 27% of the children had lead levels above five microgrammes per decilitre.
They followed the children's progress at regular intervals and then assessed their academic performance and behavioural patterns when they were seven to eight years old.
After taking account of factors likely to influence the results, they found that blood lead levels at 30 months showed significant associations with educational achievement, antisocial behaviour and hyperactivity scores five years later.
With lead levels up to five microgrammes per decilitre, there was no obvious effect.
But lead levels between five and 10 microgrammes per decilitre were associated with significantly poorer scores for reading ( 49% lower) and writing (51% lower).
A doubling in lead blood levels to 10 microgrammes per decilitre was associated with a drop of a third of a grade in their Scholastic Assessment Tests (SATs).
And above 10 microgrammes per decilitre children were almost three times as likely to display antisocial behaviour patterns and be hyperactive than the children with the lower levels of lead in their blood.
The effects of lead toxicity in children were first described in 1892 in Brisbane, Australia.
Since then acceptable levels of lead in the blood have fallen sharply.
In 1991, the US Centres for Disease Control and Prevention, revised their level of concern for blood levels down to ten microgrammes per declitre.
The World Health Organisation estimates that globally half of the urban children under the age of five have blood levels exceeding this limit.
Professor Alan Emond, who led this study, said a third of the children in his study had levels only half of this but were still exhibiting adverse effects.
He said: "Lead in the body is one of many factors that impacts on education, but this is a reminder that environmental factors are important and paediatricians must test more children with behavioural problems for lead."
"We did our blood survey when the children were about two and a half years old.
"We think this is quite close to the peak age for lead ingestion when the children are putting everything in their mouths as they explore their environment.
"This is a normal phase that we grow out of, but for children who have developmental problems, like autism, it may go on for a longer time so they may be particularly vulnerable. "
A Health Protection Agency spokesman said: "The Agency's advice is that exposures to lead should be kept to the minimum that is reasonably practical.
"This has been the policy in the UK and of health agencies throughout the world for many years.
"Measurements have shown that levels of lead in children and adults have decreased markedly over the last two decades or more, primarily because of these policies."