Asthma has complicated Todd Anderson's life ever since he was 3 years old. Doing sports—even stairs—had been difficult. Cigarette smoke, pollen, disinfectants or a passing whiff of perfume could set off a severe asthma attack. "It feels like your chest is clenching up like a fist," says Mr. Anderson, a 29-year-old music teacher in the New York City schools.

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