Smoke from wood fireplaces, stoves raises new health concerns


Story by Cheryl Katz
Environmental Health News

Photos by Michael Macor, San Francisco Chronicle

March 14, 2011

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.

Small enough to penetrate deep into lungs, particulates in wood smoke contain chemicals capable of damaging DNA and activating genes in ways comparable to cigarettes and car exhaust.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.”


Until recently, little was known about the specific harmful agents in wood smoke. While the components of particulate matter produced by vehicles and tobacco have been studied fairly extensively, the University of Copenhagen project is one of the first to characterize the minute particles and droplets of pollution released by wood fires.

The researchers analyzed the particulate matter in air samples from a Danish village where most homes were heated by wood-burning stoves, and compared it to background particles in air outside the smoke area. They also sampled emissions at the stoves’ flues.

The results, published in the January issue of Chemical Research in Toxicology, determined that wood smoke was more likely to comprise extremely small “fine” and “ultrafine” particles – PM 2.5 (.0001 inch) or less, which can lodge deep in the lungs or even pass into the bloodstream. The wood smoke particles had higher levels of polycyclic aromatic hydrocarbons, including some of the same ones found in tobacco smoke and classified by the U.S. Environmental Protection Agency as “probable human carcinogens.”

When added to cultures of human lung cells, the wood smoke particulates induced large amounts of reactive oxygen species – powerful oxidants able to injure cells, damage DNA and trigger oxidative stress. Although they are a normal product of metabolism, overexposure can set off an inflammatory reaction that interferes with the body’s ability to fight infection and can spread to other parts of the body.

The Danish researchers found that wood smoke particulates were more powerful than the background air particles at damaging DNA, causing potentially cancerous changes, and activating genes linked to inflammation and oxidative stress, which is a possible mechanism for atherosclerosis, asthma and other diseases.

The new findings provide important information on the toxic agents in wood smoke, Balmes said. In addition to the fine particles, wood smoke contains gases such as nitrogen oxides, and carcinogenic compounds like benzene, formaldehyde and dioxins.

Identifying the specific harmful components in particulate matter is key to understanding how to protect human health, said Doug Brugge, a Tufts Medical School professor who studies the effects of particles produced by traffic.

“The evidence is pretty clear that fine and ultrafines get into the lungs much better,” he said, describing the particles as so tiny that as many as 100,000 packed together would be the size of a sugar cube. The tiniest – ultrafine particles with diameters of 100 nanometers (.000004 inch) or less – appear to be especially toxic.

“We know there’s a lot of bad stuff released when wood is burned. It’s actually not that far away from tobacco smoke and smoke from fossil fuel combustion engines. They’re in the same ball park.” Dr. John Balmes, University of California, San Francisco“There is growing evidence that the ultrafines are able to pass through the lungs and get into other parts of the body," Brugge said. Because they are light, they may be able to travel greater distances.

This may be the case with the otitis media – a painful middle ear infection that is responsible for many a midnight trip to the emergency room with a screaming baby, and source of the greatest number of antibiotic prescriptions for children under five.

A study conducted at the University of British Columbia and reported in the January issue of Epidemiology analyzed visits to the doctor for 45,000 children aged two and under in Vancouver and surrounding areas, comparing it to data on wood smoke pollution levels during the same period. The study is the first to show a connection between ear infections and neighborhood wood stoves and fireplaces.

Children with the highest exposures to wood smoke "were 32 percent more likely to visit the doctor for otitis media" compared to children with the lowest exposures, said lead author Elaina MacIntyre, a scientist at the University of British Columbia’s School of Environmental Health.

MacIntyre cautioned that the association may be due to some other factor not accounted for in the study, but said the correlation between wood smoke exposure and otitis media was as strong as the well-known link between tobacco smoke and this disease. The study proposes that substances in the smoke suppress children’s ability to fight off common upper-respiratory tract viruses and bacteria, which then migrate to the ear, causing infections.

Overall, few numbers are available on the amount of disease and deaths attributed to wood smoke pollution, say Loft and other researchers.

“In the Third World, more than a million women and children die annually… due to massive exposure [from cooking on indoor] open wood fires,” Loft pointed out. “However, that cannot be translated to use of wood stoves in the developed world. We lack proper population-based data to estimate the risks of that like we can for traffic emissions.”


Data on fine particulates from all sources show a considerable health toll. In California, the state Air Resources Board estimates that they contribute to about 9,200 premature deaths from cardiopulmonary disease every year. Wood smoke is a major component of fine particulates in many areas, especially in winter. And the smoke going up the chimney can find its way back inside: more than 70 percent of indoor particulate concentrations come from sources outside the house, a University of Washington study shows.

Many municipalities, such as Denver, Albuquerque and the San Francisco Bay Area, have taken steps to clean up wood smoke. In the Bay Area for instance, home to an estimated 1.7 million fireplaces and wood stoves, burning wood is prohibited on winter “Spare the Air” days when air quality is forecast to be poor, and all new wood-burning devices sold or installed in new or remodeled homes must meet EPA emission standards.

Southern California, which has some of the worst fine particle pollution in the nation, will begin banning fires on poor-air-quality days next winter. The South Coast Air Quality Management District estimates that winter residential wood-burning in the region belches out 10 tons of fine particulates per day. Other cities, such as Fairbanks, Alaska and Tacoma, Wash., offer cash incentives for residents to replace their old wood-burning appliances with new, less-polluting ones.

“Parents should be aware that wood smoke is an important risk factor in the development of childhood respiratory infections." Elaina MacIntyre, University of British ColumbiaIn one of the most successful programs, nearly all the wood stoves in Libby, Mont., were replaced with new models that burned as much as 15 times cleaner. The fine particulate levels in this mountain town, which has a long tradition of heating with wood, dropped an average of 20 percent, and residents reported fewer cases of respiratory infections, bronchitis and children's wheezing, according to recent studies by University of Montana.

So is it safe to cozy up to that inviting hearth at the mountain lodge or your own little fireplace or woodstove at home?

“If you don’t have asthma I think it’s okay,” said Balmes. “If it’s an efficient fireplace with good updraft, it doesn’t expose the people sitting near the fireplace. Rather, “it’s the people downwind” who are exposed, he said.

MacIntyre cautioned that families with young children in wood-burning areas should keep the windows closed and use a HEPA air filter.

“Parents should be aware that wood smoke is an important risk factor in the development of childhood respiratory infections and that wood-burning increases the risk of these infections, not only for their own children, but also for children in their neighborhood,” she said. “If parents choose to continue using their fireplace or wood stove it is important that they ensure proper maintenance and consider upgrading to cleaner technologies wherever possible.”

According to Walsh, who uses a high-efficiency, EPA-approved, tightly sealed wood stove in his Bozeman home, “If your stove is designed properly and you have a proper chimney, you don’t really have a lot of reason to be concerned about smoke.”

At the Sierra Club's Clair Tappaan lodge, guests use the fireplace almost every night. The wood is dried for a season or two and cut into small pieces to keep the smoke to a minimum, said general manager Peter Lehmkuhl.

“There’s just something about a fire,” he said. “It’s very meditative to sit there and think about your day and stare at that fire and reflect on where you are. Dealing with a very primal element there, the fire.”

 For more information on cleaner-burning fireplaces and wood stoves, go to