Ozone concentrations below the current U.S. clean-air standard may still temporarily impair lung function, even in healthy young people, a small study suggests.

Ozone has been linked to inflammation in the lungs, and to breathing problems. Researchers found that when 31 young adults exercised at various levels of ozone exposure over several hours, participants' average lung function began to deteriorate when ozone concentrations stood at 70 parts per billion (ppb).

That is below the current U.S. National Ambient Air Quality Standard (NAAQS), which states that people should be exposed to ozone levels of no more than 75 ppb, averaged over 8 hours.

The findings essentially suggest that if healthy young adults exercise outside at ozone levels of 70 ppb for several hours — taking a long hike, for instance — about half will suffer respiratory symptoms like coughing or pain during deep breathing, explained lead researcher Dr. Edward S. Schelegle, of the University of California, Davis.

Typically, summer ozone levels hover between 50 and 60 ppb in the U.S., according to a 10-year study by the U.S. Environmental Protection Agency.

The findings, reported in the American Journal of Respiratory and Critical Care Medicine, are based on exercise tests of 31 non-smokers between the ages of 18 and 25.

Schelegle and his colleagues had the men and women work out moderately on a stationary bike or treadmill on five separate days. On each day, participants exercised in six 50-minute bouts over 6.6 hours; over the course of each day, they breathed either filtered air, or air containing ozone concentrations of 60, 70, 80 or 87 ppb.

The researchers found that on average, exercisers' lung function deteriorated after several hours of exposure to ozone levels of 70 ppb. There was, however, significant variation from person to person; 19 percent showed a fairly strong sensitivity to ozone concentrations of 70 ppb, while some showed far lesser reactions.

Some others were particularly sensitive; 16 percent showed significantly decreased lung function when ozone levels stood at 60 ppb.

"One thing that is often lost..." Schelegle told Reuters Health in an email, "is that the magnitude of response to ozone varies greatly from one person to the next, and that it is important for even healthy people to be conscious of their respiratory symptoms when exercising for prolonged periods in areas where ozone levels may be elevated, even if the levels are below the current NAAQ Standard."

This may be especially important for people with asthma or other lung conditions, he noted.

As for whether the clean-air standard for ozone should be lowered, Schelegle said that the data from this study and other recent ones need to be analyzed before any recommendations can be made.

In an editorial published with the study, Dr. James S. Brown of the U.S. Environmental Protection Agency, which sets the NAAQ Standard, agrees on the importance of ongoing research.

"Only with continued research efforts," he writes, "will we be able to better characterize the susceptibility in some healthy individuals, as well as asthmatic individuals, to the effects of short-term (ozone) exposures."