Exposure to wildfire smoke may alter the immune system for years, new research found, as the tiny particulate matter in the smoke that penetrates into the lungs and into the bloodstream could linger for a long time.

When exposed to wildfires, people are also inhaling noxious fine particles measuring less than 2.5 microns, or a fifth the size of a particle of dust or pollen. Researchers have had a hard time quantifying exposure to those tiny particles.

A new study, published in the journal Allergy, found exposure to high levels of that tiny particulate matter, abbreviated as PM2.5, impairs the immune system of children. The researchers tested the blood of 36 children exposed to wildfire smoke blown into Fresno in 2015.

In their results, they found changes in a gene involved in the development and function of T cells, an important component of the immune system. The alteration made the gene less capable of producing T regulatory cells, potentially putting the children at greater risk of developing allergies or infection.

“T regulatory cells act as peacekeepers in your immune system and keep everything on an even keel,”Mary Prunicki, an allergy researcher and lead author, told WIRED. “You have fewer of these good, healthy immune cells around when you’re exposed to a lot of air pollution.”

As with wildfires, controlled fires to clear out underbrush, known as prescribed burns, also can cause health effects. Thirty-two children exposed to smoke from prescribed burns had immune changes, too, but the effect wasn’t as strong as it was for children exposed to wildfire smoke, the study showed.

The research did not follow those children to see if their altered immune systems led to worse health outcomes, but an ongoing study at the University of California, Davis, raises some similar concerns. This one focused on rhesus macaques that live in an outdoor enclosure at the California National Primate Research Center and were exposed to 10 days of PM2.5

At three years of age, researchers examined 50 monkeys that had been exposed to wildfire smoke. They produced less of an immune-related protein as compared to monkeys not exposed to smoke as babies. That protein triggers inflammation to fight pathogens. A closer examination o revealed immune-related genetic changes as well.

“Clearly, the toxicants in air pollution are having a permanent effect on the DNA of immune cells,” Lisa A. Miller, principal investigator, told WIRED. “It’s a change that stays with that cell for its entire life.”

The National Interagency Fire Center predicts an “above normal” potential for wildfires this summer for Northern California. People can take precautions to limit their exposure when wildfire smoke blankets their area. Some cities provide “clear air centers” like a wildfire version of the evacuation shelters used during hurricanes.