Fire season has become a hard-to-breathe reality in much of the western United States. But the blazes may affect more than air quality or even human lungs. A recent study attributes as many as 7,000 preterm births in California to wildfire smoke exposure.

Researchers from Stanford University analyzed data on 3 million births in the Golden State between 2007 and 2012, matching it to data on wildfire smoke plumes within the period to determine the number of “smoke days” experienced during the pregnancies.

The analysis revealed associations between smoke exposure and preterm birth, even when the data was adjusted to exclude other factors that can lead to premature babies, such as race and income. Every day of wildfire exposure upped the risk by half a percentage point on average, and each week — the average each expectant mother was exposed to during the period — worked out to a 3.4 percent increase in preterm births.

While exposure during the first trimester didn’t increase risk, the second trimester was associated with the most wildfire-related preterm births. The researchers attributed 3.7 percent of all preterm births during the period to wildfire smoke — and in 2008, when 1.6 million acres of California burned in more than 6,200 wildfires, the researchers attributed 6.3 percent of all preterm births to the smoke.

“Our findings have important implications for understanding the costs of growing wildfire smoke exposure, and for understanding the benefits of smoke mitigation measures,” the researchers write. The types of particles in wildfire smoke are about 30 times narrower than the average strand of human hair, and inhaling them is associated with everything from asthma to heart attacks.

Human-caused climate change is predicted to increase droughts, dry spells and lightning storms, and California will bear much of the brunt. In 2020 alone, an estimated 4.3 million acres burned in the state — a 268 percent rise from the highest fire year the researchers studied.

Fire risk mitigation and avoiding smoke exposure may help, Stanford environmental economist Marshall Burke says in a news release.

“There’s no safe level of exposure to particulate matter,” says Burke, who co-wrote the study.

The paper was published in the journal Environmental Research.