Raleigh, N.C. — As the weather heats up, North Carolinians are used to hearing about Code Orange and Code Red ozone action days. But so far, 2009 has seen only one day when ozone exceeded federal standards.

Debbie Credle, who has had asthma for 10 years, said she checks the air quality forecast every before leaving for work at WakeMed, where she helps asthma patients.

"It's very important. You need to know what your air quality's going to be before you leave home," Credle said.

Two months into the 2009 ozone season, though, North Carolina has seen only one day when ozone levels violated tough, new federal standards. The cooler, wetter weather has played a role in that, but state officials said it's indicator of a bigger trend:

"Air quality is improving," said Tom Mather, a spokesman for the state Division of Air Quality.

Nitrogen oxide is the main cause of ozone pollution. Since 1999, emissions of nitrogen oxide from North Carolina's power plants are down almost 75 percent.

Reasons for that include the Clean Smokestacks Act, which has reduced pollution from the state's power plants, state officials said.

"Our cars and trucks are much cleaner. They have to meet better standards, and they're burning much cleaner fuel," Mather said.

However it happens, it's important to keep working for cleaner air, Credle said.

"It's nice to know that these things are making a difference, and we need to make a difference," she said.