That evidence shows up every day in the emergency room at the children's hospital in Oakland, Calif.
Today, 15-year-old Damaria Wood is among the many kids coming in with asthma.
This is where the EPA's concerns about the health effects of ozone pollution or smog meet the real world of children struggling to breathe.
When the children come in, "they're gasping for breath, many of them," said Mindy Benson, an asthma program manager from Children's Hospital in Oakland.
Eighteen-month-old Walter Abram has had asthma attacks since soon after he was born.
"When he has his attacks, you can see his ribs," said Qiana Abram. "He's hurting. I cry for him."
In the Emergency Department of Children's Hospital, some 5,000 children each year come in for emergency treatment of asthma. The causes are complicated, but there's no doubt air pollution plays a big role.
"On days of high levels of ozone, people with asthma are more likely to have symptoms or shortness of breath," said University of California Dr. Homer Bouchley.
The ill effects of ozone in the air go well beyond asthma. Studies done by researchers at Yale and Johns Hopkins show that smog can kill.
In the 95 major urban areas studied, an increase in daily ozone levels was associated with more than 3,700 additional deaths each year from cardiovascular and respiratory illnesses.
But the battle against ozone, the main ingredient in smog, has had some success. Los Angeles and other California cities are not nearly as smoggy as they once were.
"We've made enormous progress in fighting ozone. It's gone down 75 percent in Southern California since 1990," said Karen Caesar of the California Air Resources Board. However, she adds, "we're still not where we need to be."
While cleaning up the air can be challenging and expensive, the costs of air pollution are not counted only in dollars.