Wildfire smoke may be contributing to premature births.

As wildfires ravage the West, burning through millions of acres, they are producing blankets of smoke that are spreading far beyond the boundaries of the fires themselves. Now, new research indicates that the air pollution is endangering some of the most vulnerable: the unborn.

The findings, published this month in Environmental Research, suggested that from 2007 to 2012 in California, about 7,000 preterm births, or nearly 4 percent of all such births during those years, were associated with exposure to wildfire smoke.

It is the latest sign of the potential health risks of smoke from wildfires, which can include not only the soot and ash from burning trees and undergrowth but also the chemicals that are released when homes, cars and countless other things go up in flames when wildfires race through towns and neighborhoods.

Wildfire smoke can blunt the body’s immune response, causing anything from mild but annoying sore throats or coughing to serious cardiovascular and respiratory problems. Research published this month found that exposure to wildfire smoke last summer could be associated with thousands of additional Covid-19 infections and hundreds of deaths in the pandemic.

And by nearly every metric, wildfires in the United States are worsening. They are growing larger, spreading faster and reaching higher elevations. Their plumes are also reaching farther. Last month wildfire smoke from Canada and the West stretched across the United States, prompting health alerts in cities as far east as Toronto and Philadelphia.