The United States may face a 70 percent increase in unhealthy summertime ozone levels by 2050 as the climate warms, according to a government-funded study released on Monday.

The study, published in the U.S. Journal of Geophysical Research-Atmospheres, found that warmer temperatures, higher atmospheric levels of methane, and other atmospheric changes related to a changing climate spur chemical reactions that increase overall levels of ozone.

"It doesn't matter where you are in the United States, climate change has the potential to make your air worse," lead researcher Gabriele Pfister of the U.S. National Center for Atmospheric Research (NCAR) said in a statement.

"A warming planet doesn't just mean rising temperatures, it also means risking more summertime pollution and the health effects that come with it," said Pfister.

Ozone pollution is not emitted directly, but instead forms as a result of chemical reactions that take place between nitrogen oxides and volatile organic compounds in the presence of sunlight. These gases come from human activities such as combustion of coal and oil as well as natural sources such as emissions from plants.

Unlike ozone in the stratosphere, which benefits life on Earth by blocking ultraviolet radiation from the Sun, ground-level ozone can trigger a number of health problems, ranging from coughing and throat irritation to more serious problems, including aggravation of asthma, bronchitis, and emphysema. Ozone pollution also damages crops and other plants.

To examine the impacts of climate change on ozone pollution, Pfister and her colleagues looked at two scenarios. In one, emissions of nitrogen oxides and volatile organic compounds would continue at current levels through 2050. In the other, emissions would be cut by 60 to 70 percent.

The researchers found that if emissions continue at present-day rates, the number of eight-hour periods in which ozone would exceed 75 parts per billion (ppb), a threshold considered unhealthy, would jump by 70 percent on average across the United States by 2050.

Overall, the researchers said that 90 percent of the time, ozone levels would range from 30 to 87 ppb in 2050 compared with an estimated 31 to 79 ppb in the present.

However, sharp reductions in nitrogen oxides and volatile organic compounds could reduce ozone pollution even as temperatures rise.

According to the study, 90 percent of the time, ozone levels would then range from 27 to 55 ppb, and the number of instances when ozone pollution would exceed the 75 ppb level dropped to less than 1 percent of current cases.

"Our work confirms that reducing emissions of ozone precursors would have an enormous effect on the air we all breathe," Pfister said.

The work was funded by the U.S. National Science Foundation, which is NCAR's sponsor, and the U.S. Department of Energy. (Xinhua)