Living too near a busy road could stunt a child's lung development, US research involving 3,677 children suggests.
Exhaust fumes contain toxins
Children who lived within 500 metres of a major road, such as a motorway, were shown to have lung impairment in tests.
Many children live and go to schools near to busy roads and could be at risk, the University of Southern California authors warn in The Lancet.
Experts already know toxic traffic fumes can trigger lung conditions such as asthma.
But the latest work suggests pollution can stop the lung from growing to its full potential - even in children who are otherwise healthy.
As background air quality did not alter the picture, children living in the countryside but close to a main road would also be at risk, the researchers add.
Children living close to big roads in cities with high levels of background air pollution were likely to be at a greater risk of lung problems however because of the double effect on their lungs, they suggest.
They examined the lung function of 3,677 children annually from the age of 10 until they reached 18 - when the lungs are fully developed.
Those who had lived within 500 metres of a motorway had much poorer lung function at the age of 18 than those who had lived 1,500 meters away or more, even when factors such as smoking in the home were taken into account.
Dr John Peters and colleagues say fumes from cars, bikes and lorries are to blame.
They recommend more work to identify the exact culprits in vehicle exhausts.
Scientists do not know exactly how air pollution hampers lung development, but they believe lung inflammation in response to daily irritation by air pollutants may play a role.
Stephen Holgate, Medical Research Council clinical professor of immunopharmacology at the University of Southampton, said: "The finding builds on accumulating knowledge that the chemicals that are contained in the exhaust emissions of modern vehicles adversely effects the development of the lung through childhood.
"This is probably through their powerful oxidant effect on lung development in the first five to eight years of life."
Oxidative stress is caused by the toxic effect of highly reactive oxygen molecules, called free radicals, which damage cells and DNA.
Liberal Democrat health spokesman Norman Lamb said the work reinforced the case for reducing vehicle emissions.
"This report will be disturbing reading for many parents.
"As road usage continues to increase, this issue will have to be taken seriously."
A government spokeswoman said they would consider the evidence and whether further investigations were needed.