(CBS) Asthma in America is up. Between 2001 and 2009, the number of people diagnosed with asthma rose by 4.3 million, according to a new report issued by the Centers for Disease Control and Prevention. That means one in 12 Americans has been diagnosed with the potentially deadly respiratory disorder.

The increase in asthma rates affected all demographic groups, but black children were especially hard hit. They showed an almost 50 percent increase in asthma rates during the time period.

PICTURES - Asthma dangers: 8 things that trigger deadly attacks

The rise puzzles researchers, especially given the measures taken to reduce air pollution in recent years.

"Despite the fact that outdoor air quality has improved - we've reduced two common asthma triggers, secondhand smoke and smoking in general - asthma is increasing," Dr. Paul Garbe, chief of CDC's Air Pollution and Respiratory Health Branch, said in a written statement. "While we don't know the cause of the increase, our top priority is getting people to manage their symptoms better."

Asthma symptoms include wheezing, breathlessness, chest tightness, and coughing. The condition is lifelong and incurable, but symptoms can often be controlled by taking medication - and taking steps to avoid smoke, mold, dust mites, and other common triggers of asthma attacks.