Reporting in the July 6 issue of the New England Journal of Medicine, the researchers found a 13 percent reduction in children's "forced vital capacity" -- a measure of lung function -- for every one micrometer-squared increase of carbon content in their lungs.
"Higher levels of exposure [to carbon particles] are associated with lower levels of lung function," concluded one of the study's authors, Dr. Jonathan Grigg, who was a senior lecturer in pediatrics at the University of Leicester in England at the time of the study.
Black carbon particles are a major component of air pollution, according to background information in the study. This type of pollution comes largely from motor vehicle emissions. Past research has suggested that carbon pollution might harm children's lung function.
To confirm the association, the researchers recruited 114 healthy children from Leicester -- a medium-sized city with air quality that falls within the United Kingdom's acceptable range.
All of the children were between 8 and 15 years old, had normal levels of physical activity and had been living in their current homes for at least one year.
The researchers collected sputum (mucous) samples from the children and tested their lung function using a method called spirometry. They were able to get sufficiently sized samples from 64 of the children.
They tested the sputum samples to measure the carbon content.
Grigg and his colleagues found that the more carbon present in the sputum, the greater the reduction in lung function.
For each one micrometer-squared increase in carbon content, there was a 17 percent reduction in forced expiratory volume in one second, and a 13 percent reduction in forced vital capacity. Forced vital capacity measures how fast and how hard you can exhale.
According to Grigg, the most likely explanation for this decrease in function is that these particles restricted the growth of the children's lungs.
The association held true even after the researchers controlled for other factors such as exposure to second-hand smoke, body mass index and gender.
To ensure that reduced lung function wasn't causing an increase in carbon deposits, the researchers also measured the carbon content found in the lungs of nine children with asthma, who have naturally lower lung function than healthy children.
Surprisingly, the researchers found less carbon in the lungs of children with asthma. Grigg said the researchers suspect this may be because people with asthma may process inhaled particles differently than people with healthy lungs do, and that in people with asthma, carbon may deposit itself in different cells.
Dr. Jonathan Field, director of the pediatric allergy and asthma clinic at New York University Medical Center/Bellevue in New York City, said that because people with asthma take medications that relax the airways, they may be able to better expel these particles. Or, he said, "if there's already airway constriction, there may be less particles coming in."
The damage to children's lungs is cumulative, Grigg stressed. "Particle pollution exerts a small negative effect on lung function growth, but since the effect is continuous, it may have a large negative effect when acting over several years," he said. "This study is another piece in the jigsaw showing that air pollution from traffic has adverse consequences," he said.
However, that doesn't mean that people living near busy traffic need to move, Grigg said. On an individual basis, the pollution-linked lung function changes are quite small, he said.
"We can't avoid inhaled particulate pollution. These data are important when doing the cost/benefit analysis for different, less-polluting fuels. This study reminds us that there is a health cost of burning fossil fuels, even though at present these fuels bring many benefits," he said.
People can take simple steps to minimize their exposures, however. Field said people who live in a high-pollution area and have respiratory symptoms should minimize their outside exposure on high-ozone or high-pollution days. People with respiratory symptoms who are already mulling a move may want to consider living in a less-trafficked, less-polluted area, away from busy streets.
The study also highlights the need for a move away from gas-dependent vehicles and toward less-polluting "hybrid" models, Field said.
More information
To learn more about air pollution and its effects on your health, visit the American Lung Association.
