Children living in smoggy areas have retarded lung development, US research shows.
The pollutants came from vehicle exhausts
Those who grew up in the most polluted areas were five times more likely to have poor lung function, the National Institutes of Health team found.
This increases the risk of lung disease, heart attacks and death, they told the New England Journal of Medicine.
They said it was the longest study ever on air pollution and children's health.
The team from University of Southern California Keck School of Medicine had previously found that children who were exposed to air pollution scored more poorly on respiratory tests.
In the latest study, they tested the same children at the age of 18 when the lungs have almost completely matured.
Among the 1,759 teenagers tested, those who had grown up in the most polluted areas had the worst lung function - less than 80% of the lung function expected for their age.
High levels of air pollution increased the risk of retarded lung development five-fold.
The air pollutants identified by the researchers had come from vehicle exhaust and the burning of fossil fuels.
Lead researcher Dr John Peters said: "When we began the study 10 years ago we had no idea we would find effects on the lung this serious.
"Lung development in teenagers determines their breathing capacity and health for the rest of their lives.
"The potential long-term effects of reduced lung function are alarming.
"It's second only to smoking as a risk factor for mortality."
The researchers do not know how air pollution hampers lung development, but they believe lung inflammation in response to daily irritation by air pollutants may play a role.
They plan to follow the teenagers into their early 20s, when their lungs will have stopped developing entirely.
Professor Andrew Peacock of the British Thoracic Society said: "Air pollution is a major public health problem.
"This research adds to the growing body of evidence that poor air quality can have a damaging effect on the lungs of our children.
"More needs to be done to help reduce children's exposure to air pollution.
"The government must work together with others to minimise vehicle emissions and pollution levels in this country."