Air pollution is bad for everyone, but it is especially bad for children.

Children take in more air per pound of body weight than adults. When a child is exercising at maximum levels, he or she may take in 20-50 percent more air than an adult would in the same activity, says Michael T. Kleinman, a professor in the Department of Community and Environmental Medicine at the University of California, Irvine. That means if ground-level ozone or particulate levels are high, children breathe in more harmful pollutants.

The fact that children are growing and developing brings additional problems. Lungs contain more than 40 different kinds of cells, Kleinman says, and air pollution can damage cells that are used in the development of new functional parts of the lung. If those cells are damaged, the lung’s growth may be stunted and its function compromised as the child matures.

Children also typically spend more time outdoors than adults, often during afternoon hours when air pollution is highest. Studies show that a typical adult spends 85-95 percent of his or her time indoors while children spend less than 80 percent of the time indoors.

The results of a recent Children’s Health Study conducted by professionals at the University of Southern California reveal that children with asthma are at much greater risk for increased asthma symptoms when they live in communities with higher levels of ozone and fine particles and participate in three or more competitive sports. Kleinman stresses that he does not want to discourage participation in sports. “Appropriate exercise and prudent exposures of children and adults should be encouraged,” he says, “even in an environment that may always contain some amount of air pollution.”