Air pollution may cause asthma, not just trigger it, according to an article published in the journal Lancet.

Researchers followed 3,500 9 year of children in 12 Southern California towns for five years. Half of the towns had clean air and half had polluted air. All of the children led active lives participating in sports like baseball, basketball and soccer.

During the study, 265 of the children developed asthma, the vast majority of them from the polluted towns.

These results support the idea that pollution may cause asthma, not just trigger it. That is a big change in our thinking about this epidemic disease.

Asthma affects about 20 million Americans. Nearly half are kids and the number is growing. The disease costs billions of dollars in lost time from work and school as well as medical costs.

Rules and regulations have cut pollution in lots of cities, but there may still be enough dirty air to cause asthma.