Air pollution is a 'pervasive public health threat' in the US. A latest study by the American Lung Association has found that about half of Americans, around 44%, have been living in communities where the air can be dangerous to breathe.

But still, the group's 'State of the Air 2015' report card that was released on Wednesday, noted that most of the nation has much cleaner air as compared to a decade ago. The report mentioned that the eastern half of the country has made the most progress.

Janice Nolen, assistant vice-president for national policy at the American Lung Association, told TODAY, that they have seen great improvement in year-round particle levels. Nolen said that in a lot of ways, it's encouraging, but still there are many challenges.

California is one of the main challenges. It has warm climate, sun-soaked days, a hilly landscape that 'holds things in place' and plenty of cars, trucks and tractors spewing fumes. The report found that in general, heat and drought in the West have been creating situations 'ripe' for pollution.

Nolen said Los Angeles has shown great improvements even though it still is looming large in the 'most polluted' lists. There are many other cities that have cleaned up in the recent years include Birmingham, Alabama; Pittsburgh, Pennsylvania; and Knoxville, Tennessee.

The 16th annual report card was based on the levels of ground-level ozone and particle pollution nationwide from 2011 to 2013.

Nolen said breathing enough of those pollutants can end you up with wheezing and coughing, or worse. Such pollutants can affect the heart as well as the lungs, and can shorten one's life.