About 1 in 12 Americans are suffering from asthma, a rise in 12.3 percent since 2001, according to a report issued by government Tuesday.
The extra 4.3 million cases over the past decade also cost more to the US economy -- from $53 billion for medical expenses and lost productivity in 2001 to $56 billion in 2009, the Centers for Disease Control and Prevention said.
But officials say they do not understand why more people are getting asthma, especially when fewer Americans are smoking and the nation is taking steps to cut back on air pollution.
"Despite the fact that outdoor air quality has improved, we've reduced two common asthma triggers -- second-hand smoke and smoking in general -- asthma is increasing," said Paul Garbe, chief of CDC's Air Pollution and Respiratory Health Branch.
"While we don't know the cause of the increase, our top priority is getting people to manage their symptoms better."
Asthma can affect people of all ages but tends to be more prevalent among the poor, said the CDC Vital Signs report, issued as the United States marks Asthma Awareness Month.
African-American children have the highest rate of asthma, at 17.6 percent.
But all ethnic and demographic groups saw a rise in incidences of asthma over the 2001-2009 study period, which used data from the National Health Interview Survey and the Behavioral Risk Factor Surveillance System.
The overall prevalence of asthma in the US population went from 20.3 million people in 2001 (7.3 percent) to 24.6 million people in 2009 (7.3 percent).
Asthma can be triggered by "tobacco smoke, mold, outdoor air pollution, and infections linked to influenza, cold-like symptoms, and other viruses," the CDC said.
Most asthma sufferers can eliminate their symptoms if they take prescription drugs such as inhaled corticosteroids, and if they can "modify their environment to reduce or eliminate exposure to allergens and irritants," the CDC said.
Medical expenses were about $3,300 per person annually for treating asthma, but among people without health insurance, two in five said they could not afford medication.