Children who bite their fingernails may be damaging their IQ, a study suggests.
Many people continue to bite their nails as adults
Researchers in Russia say children who chew their nails are at greater risk of lead poisoning.
This is because lead can gather under their nails simply by playing in dusty conditions, both indoors and outdoors.
It has long been known that exposure to lead may contribute to developmental problems in some children. Previous studies have suggested it may also damage the nervous system.
Lead is found naturally in soil and dust. As a result, it is sometimes consumed via fruit or vegetables that have not been washed properly. Many men, such as plumbers, painters and printers, are exposed to lead at work.
But scientists at the Ural Regional Centre for Environmental Epidemiology in Ekaterinburg, believe that biting finger nails may explain why some children also show high levels of the chemical.
They assessed children living in a number of cities in the Urals. They found that as many as two out of three children in some areas had worryingly high levels of lead.
Levels varied depending on whether the children lived in homes that overlooked busy roads or if they had a habit of eating soil, snow or paint.
But they also found a link between high levels of lead and children who regularly bit their nails.
They found more than 69% of girls and 62% of boys involved in the study bit their nails or other objects like pencils.
All of the children involved in the study lived in highly industrialised cities with high levels of lead.
However, children living in the UK and other western countries may also be at risk.
Dr Erik Millstone, a senior lecturer at the University of Sussex, said: "The government stopped collecting figures on blood lead levels a number of years ago.
"However, I wouldn't be surprised if something in the order of 6% to 10% of children under the age of six had blood lead levels at which there is evidence of adverse effects."
Lead levels in soil across Britain are quite low. However, old paintwork is often a source of this potentially damaging chemical.
"Many homes built before the mid 1960s have old leaded paint," Dr Millstone said.
"When this is sanded down it can create contaminated lead dust.
"For that reason we should not be trying to stop children putting their fingers or other objects into their mouth, but rather focus on the importance of handling old leaded paint with care."