Ozone exposure early in life raises the risk a child will develop asthma and wheezing by age 4, a recent analysis found.

The study, published in JAMA Network Open, analyzed data on 1,188 children in Washington state, Minnesota, New York, California and Tennessee who were drawn from three cohorts in the National Institutes for Health’s Environmental Influences on Child Health Outcomes program. All of the children were exposed to modest ozone pollution between birth and age 2, and 81.9 percent had mothers with no history of asthma.

Among the children studied, 12.3 percent had asthma by ages 4 to 6, and 15.8 percent had wheezing. Among 8- to 9-year-olds, 9.4 percent had asthma and 8.3 percent had a “persistent” wheeze.

Overall, the children were exposed to a mean of 26.1 parts per billion (ppb) of ozone in their early years. The Environmental Protection Agency’s threshold for eight-hour ozone exposure is 70 ppb.

Each rise of 2 ppb of ozone was associated with a 31 percent higher risk of early asthma and a 30 percent higher risk of wheezing, the researchers found. Unexpectedly, a secondary analysis showed no association between early ozone exposure and asthma/wheezing in 8-to-9-year-olds. Though the finding was “puzzling,” it is still important, study leader Logan Dearborn said in a news release.

“Even if we only see the effects early in life, there are still all kinds of associated health care costs and stresses for families,” said Dearborn, a doctoral student at the University of Washington during the research. “There are all sorts of larger contextual factors about having this chronic disease at any point in life.”

While ozone in the stratosphere protects humans and other life from the sun’s ultraviolet rays, it is considered a pollutant at ground level. According to the EPA, ground-level ozone is most likely to reach unhealthy levels on hot, sunny days, though levels can also spike in colder weather.

Because ozone is the air pollutant that most often exceeds federal guidelines, the researchers wrote, it’s worth understanding its effects. “This analysis underscores the importance of better understanding the role of early-life exposure to” ozone, they concluded.