ATLANTA, Georgia (CNN) -- Twice a day, 7-year-old Hannah Austin exhales all the air from her lungs. She then takes a puff of a low-dose steroid from a purple inhaler, holds her breath for a few seconds and exhales.
Hannah Austin's asthma was diagnosed last summer after she grew short of breath while swimming.
Like nearly 7 million other children in the United States, Hannah, a second-grader from Smyrna, Georgia, has asthma. This simple exercise with the inhaler allows her to breathe easier.
But on a day when the air quality is poor, she often struggles to catch her breath.
"We know that environmental pollutants have a very significant impact on children with asthma," said Dr. Avril Beckford, a pediatrician in Austell, Georgia. See how the air harms little lungs »
Children are more vulnerable to the effects of air pollution because their lungs don't fully form until they are adolescents, the American Academy of Pediatrics noted.
The leading pediatricians group added that "because children spend more time outdoors than do adults, they have increased exposure to outdoor air pollution."
"If you live near a polluted area of a city, it's like the child is smoking," said best-selling author and pediatrician Bill Sears. "We all know what smoking does for the lungs."
Sears called the long-term effect of air pollution on a developing child devastating. "Children do not grow as well because they do not breathe as well. The brain really needs a lot of oxygen. They don't think as well. They don't learn as well."
Hannah's asthma was diagnosed last summer. Her mother, Drew Austin, became alarmed when she noticed that Hannah was short of breath while swimming.
"When her asthma is really bad, she just gets lethargic and starts coughing," Austin said.
Coughing, wheezing or whistling when exhaling, and shortness of breath are some of the most common symptoms of asthma in children.
Sears warned that the symptoms can lead to poor sleep habits. "When the child wakes up in the morning with a runny nose and baggy eyes, you can tell they didn't sleep well because they were coughing in the night," he said.
Proper diagnosis and treatment can help manage asthma symptoms. Experts also recommend that people with asthma avoid indoor and outdoor allergens and irritants.
Indoor triggers include dust mites, mold, furry pets, tobacco smoke and certain chemicals.
Outdoor irritants range from pollen to cold air to air pollution.
Michael Chang, an atmospheric research scientist at the Georgia Institute of Technology in Atlanta, estimated that 50 percent of air pollutants are created by cars and trucks.
He explained many parts of the U.S. are now transitioning to higher temperatures and more humid summers.
"We don't have the winds that blow things out of the air," Chang said. "The stuff we put into the atmosphere lingers longer."
He compared the air quality in many big cities to a chemical soup of thousands of compounds, including ground-level ozone and fine particulate matter. The Environmental Protection Agency describes ground-level ozone as the primary component of smog. It includes motor vehicle exhaust, industrial emissions, gasoline vapors and chemical solvents.
Chang's office is responsible for monitoring air quality and issuing smog alerts in Atlanta. He also tries to educate residents by telling them to pay attention to the warnings.
"Jogging late in the afternoon during the summer is not the best time," Chang said. "Ground-level ozone is at its worst at that time of day."
Beckford goes one step further, warning parents to not choose a house, school or playground that is close to a busy road or a highway.
She tells her young patients to get their exercise earlier in the day, when the air is cleaner.
Beckford also urges children such as Hannah to take control of their asthma by taking their medicine as directed and learning to use their inhalers properly.
She may be only 7, but Hannah has found one more way to control her symptoms and prevent an asthma attack. She watches the news and checks local Web sites to monitor the daily air quality, to see whether it is safe for her to play outside.
Her mother said, "I hate to have her environment controlling what she can do. I want her to be able to control and make those choices." E-mail to a friend
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