By Lisa Rapaport

(Reuters Health) – People may be more likely to develop acute lower respiratory infections after even brief increases in air pollution, and babies and toddlers are especially vulnerable, a U.S. study suggests.

Researchers looked at more than a decade of data on patients in Utah who were treated for serious lower respiratory tract infections like bronchitis, pneumonia, influenza and pertussis to see if there was any connection between these cases and exposure to so-called PM 2.5, tiny particles that include dust, dirt, soot and smoke.

During the study, average daily concentrations of PM 2.5 were 10 micrograms per cubic meter of air (ug/m3), well below the 35 ug/m3 daily outdoor limit set by the U.S. Environmental Protection Agency.

Each short-term 10 ug/m3 increase in PM 2.5 levels, however, was associated with 15 percent higher risk of an acute respiratory infection for kids 0 to 2 years old, the study found. These infections accounted for 77 percent of the cases in the study, and patients were typically treated at hospitals or clinics about one to four weeks after air pollution temporarily spiked.

“It takes two to three weeks in most cases for the infections to occur and become serious enough that people seek healthcare services and, thus, report their infection,” said lead study author Benjamin Horne, a researcher at the Intermountain Heart Institute in Salt Lake City, Utah.

Growing evidence links both short-term and longer-term exposure to air pollution to an increased risk of respiratory and cardiovascular diseases as well as premature death, researchers point out in the American Journal of Respiratory and Critical Care Medicine. But research to date in young children who are especially prone to these infections has had mixed results.

It’s possible that some previous studies failed to find a clear link between short-term increases in air pollution and respiratory infections because these studies were too small, or didn’t include wide variation in air pollution, or failed to account for the weeks it takes for infections to develop after the exposure to PM 2.5, Horne said by email.

Current U.S. standards, last updated by the EPA in 2013, limit PM 2.5 exposure outdoors to an average of 35 ug/m3 over 24 hours or an average of 12 ug/m3 over the course of a year.

Nearly 60 percent of children in the U.S. live in counties with average PM 2.5 levels above that standard, the authors note.

In the current study, a total of 146,397 people were included in the analysis. The maximum PM 2.5 concentration measured over the study period was 123 ug/m3.

Most of the infants and toddlers with respiratory infections had bronchiolitis, which develops when small breathing tubes in the lungs get infected and clogged with mucus and is often caused by respiratory syncytial virus (RSV).

Among older children from 3 to 17 years old, each short-term 10 ug/m3 increase in PM 2.5 levels was associated with 32 percent higher odds of an acute lower respiratory infection.

For adults, each short-term 10 ug/m3 increase in PM 2.5 was linked to 19 percent higher likelihood of a respiratory infection.

Unlike infants and toddlers, older children and adults most often developed influenza and these infections typically developed about one week after exposure to elevated air pollution.

The study wasn’t a controlled experiment designed to prove whether or how brief exposure to increased air pollution, even at overall low levels, might cause respiratory infections.

Still, the results suggest that parents – especially parents of newborns and toddlers – need to protect their children as much as possible from exposure to polluted air, said Kian Fan Chung, a researcher at the National Heart and Lung Institute and Imperial College London in the UK who wasn’t involved in the study.

“This study clearly delineates another deleterious effect of environmental pollution in the young,” Chung said by email.

“Clearly, high levels of PM2.5 increase the risk of catching a respiratory virus infection in the young and very young, a group particularly susceptible to this effect of pollution,” Chung added. “For these children – particularly the very young – living in areas of high pollution, one should consider reducing the levels of pollution at home and use air purifiers, and on days of high pollution they should not go outside.”

SOURCE: American Journal of Respiratory and Critical Care Medicine, online April 13, 2018.