A warming planet means more hot days than before, and the phenomenon could cause three to nine more days per year when ozone levels are unhealthy by the 2050s, scientists warned Thursday.

Automobile exhaust spews nitrogen dioxide and causes a buildup of unhealthy ground level ozone in the atmosphere. The problem tends to get worse when the temperature rises and the air stagnates.

The dangers of ozone pollution can include lung damage in adults and asthma in small children.

"In the coming decades, global climate change will likely cause more heat waves during the summer, which in turn could cause a 70 to 100 percent increase in ozone episodes, depending on the region," said Lu Shen, a graduate student at the Harvard John A. Paulson School of Engineering and Applied Sciences and lead author of the study in Geophysical Research Letters.

California, the southwestern United States and the northeast region could see up to nine more days of dangerous ozone levels than present, the study found.

Much of the rest of the United States will likely experience an increase of 2.3 days, according to the model devised by researchers, based on observed relationships between temperature and ozone to predict future episodes.

"Typically, when the temperature increases, so does surface ozone," said co-author Loretta Mickley.

"Ozone production accelerates at high temperatures, and emissions of the natural components of ozone increase. High temperatures are also accompanied by weak winds, causing the atmosphere to stagnate. So the air just cooks and ozone levels can build up."

High ozone spells danger for people who have chronic lung disease. They are often warned to stay inside on unhealthy ozone days.

Unhealthy ozone levels range from 100 to 500 parts per billion, according to the US Environmental Protection Agency.

Ways to reduce ozone pollution include driving less and cutting down on electricity use, the EPA says.

"This research gives us a much better understanding of how ozone and temperature are related and how that will affect future air quality," said Mickley.

"These results show that we need ambitious emissions controls to offset the potential of more than a week of additional days with unhealthy ozone levels."