By age 18, the lungs of many children who grow up in smoggy areas are underdeveloped and will likely never recover, according to a study in this week’s issue of the New England Journal of Medicine.

The research is part of the Children’s Health Study, the longest investigation ever into air pollution and kids’ health.

Between 1993 and 2001, study scientists from the Keck School of Medicine of USC tracked levels of major pollutants in 12 Southern California communities while following the pulmonary health of 1,759 children as they progressed from 4th grade to 12th grade.

The 12 communities included some of the most polluted areas in the greater Los Angeles basin, as well as several low-pollution sites outside the area.

Keck School researchers previously found that children who were exposed to more air pollution scored more poorly on respiratory tests. In this latest study, researchers analyzed the same children’s respiratory health at age 18, when lungs are almost completely mature.

“Teenagers in smoggy communities were nearly five times as likely to have clinically low lung function, compared to teens living in low-pollution communities,” said W. James Gauderman, associate professor of preventive medicine at the Keck School and lead author of the study.

People with clinically low lung function have less than 80 percent of the lung function expected for their age – a significant deficit that would raise concerns during a doctor’s exam.

“When we began the study 10 years ago, we had no idea we would find effects on the lung this serious,” said John Peters, Hastings Professor of Preventive Medicine in the Keck School, director of the Southern California Environmental Health Sciences Center and senior author of the study.

Study technicians traveled to participating schools every year and tested children’s lung function, a measure of how well their lungs work. As an example, someone with sub par lung function cannot exhale and blow up a balloon as quickly or as big as someone with good lung function.

Researchers correlated the students’ lung health measurements with levels of air pollutants monitored in the communities during the same time period.

They found greater deficits in lung development in teenagers who lived in communities with higher average levels of nitrogen dioxide, acid vapor, particulate matter with a diameter of less than 2.5 micrometers (about a tenth the diameter of a human hair) and elemental carbon.

“These are pollutants that all derive from vehicle emissions and the combustion of fossil fuels,” Gauderman said.

Deficits in lung function have both short- and long-term effects.

“If a child or young adult with low lung function were to have a cold, they might have more severe lung symptoms, or wheezing,” Gauderman said. “They may have a longer disease course, while a child with better lung function may weather it much better.”

Potential long-term effects are more alarming. “Low lung function has been shown to be second only to smoking as a risk factor for all-cause mortality,” Gauderman said.

Lung function grows steadily as children grow up, peaking at about age 18 in women and sometime in the early 20s in men. Lung function stays steady for a short time and then declines by 1 percent a year throughout adulthood.

As lung function decreases to low levels in later adulthood, the risk of respiratory diseases and heart attacks increases.

Researchers are unsure how air pollution may retard lung development.

Gauderman believes chronic inflammation may play a role, with air pollutants irritating small airways on a daily basis. Scientists also suspect that pollutants might dampen the growth of alveoli – tiny air sacs in the lungs.

The research team will continue to follow the study participants into their early 20s, when their lungs will mature and stop developing entirely. The team seeks to find out if the participants begin to experience respiratory symptoms and if those who moved away from a polluted environment show benefits.

The California Air Resources Board, National Institute of Environmental Health Sciences and Hastings Foundation supported the research.