NORDEN, Calif.—On a frosty evening in the Sierra Nevada, smoke curling from the chimney of the Clair Tappaan Lodge is a welcome sight to chilly snowshoers and cross-country skiers. Gathering by the massive stone hearth at this landmark Sierra Club mountain hostel, guests relax in the warmth and aroma of the crackling log fire.

Those same woodsy scents waft across the wintry north, as millions of fireplaces and wood stoves are lit by people seeking an environmentally friendly source of heat and ambience. But recent research raises new concerns over the toxic substances borne aloft in wood smoke.

The tiny airborne specks of pollution known as particulate matter, or PM, produced by wood-burning stoves appear to be especially harmful to human health. Small enough to penetrate deep into the lungs, they carry high levels of chemicals linked to cardiopulmonary diseases and cancer, and they can damage DNA and activate genes in hazardous ways comparable to cigarette smoke and car exhaust.

“We found that wood smoke PM has similar toxicity and effects on DNA as that of vehicle exhaust particles,” said University of Copenhagen researcher Steffen Loft, who led a new study of air pollution from wood stoves.

Another new study, conducted in Canada, found that infants and toddlers living in areas with a lot of wood stoves and fireplaces were significantly more likely to get ear infections, one of the leading causes of childhood trips to the doctor.

Early humans began building wood fires hundreds of thousands of years ago, providing protection from predators, expanding sources of food and allowing migration to colder climates. Because wood is a “natural” material and has been an integral part of human existence for so long, many view it as a benign, cheap and renewable energy alternative.

“It’s the cave man’s television,” said John Walsh, an engineer who heats his 3,000-square-foot home with a wood stove during the brisk winters in Bozeman, Mont., describing how the graceful gyre of flames has enthralled people through the ages.

 Walsh, who burns mostly lodgepole pines killed by pine beetles, enjoys the exercise of cutting and splitting the logs, as well as saving about $2,000 in energy bills a year. In addition, “wood heat is carbon neutral,” he said, because “burning it releases the same amount of carbon as having it decay.”

Wood-burning fits in with a rustic ethic. In Northern California’s nine-county Bay Area Air Quality Management District, the most frequent violations of the region’s fireplace and wood-stove restrictions tend to come from bucolic Sonoma County, home to vineyards, ranches and farms.

“These are places that are somewhat rural,” said Bay Area AQMD spokesperson Aaron Richardson, “and there does tend to be a kind of a culture of relying on wood for additional heating needs.”

However, that woodsy “link to the land” is also linked to potentially serious health risks. Vented outdoors, the smoke can pose a bigger threat to people in the community than to those sitting fireside.

Exposure to the particulates in smoke irritates the lungs and air passages, causing swelling that obstructs breathing. Wood smoke can worsen asthma, and is especially harmful to children and older people. It also has been linked to respiratory infections, adverse changes to the immune system, and early deaths among people with cardiovascular or lung problems.

“We know there’s a lot of bad stuff released when wood is burned,” said Dr. John Balmes, a professor of medicine at the University of California, San Francisco, and professor of environmental science at the University of California, Berkeley School of Public Health. “It’s actually not that far away from tobacco smoke and smoke from fossil fuel combustion engines. They’re in the same ball park.”

Environmental Health News commissioned this story by InvestigateWest, a non-profit journalism studio focused on the environment, public health and social justice in western North America.