Wood smoke contains thousands of chemicals, including known irritants, carcinogens, suspected carcinogens, mutagens, teratogens (substances that are linked to birth defects), metals, and more.
The following is a brief overview of only a few of the health-damaging toxins in wood smoke.
Polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons, which are often referred to as PAHs, were the first chemical carcinogens to be discovered. They are formed during the combustion of organic matter.
Compared to other sources of air pollution, wood burning is a particularly large contributor of PAHs to our environment.
As one study pointed out, the PAH content is “much higher” in wood smoke compared to vehicle exhaust, and because of this “a higher mutagenic and carcinogenic potential” exists for wood smoke compared to traffic exhaust.
It’s still not common knowledge that wood smoke—whether visible or invisible—is toxic.
The carcinogen benzo(a)pyrene
The PAH benzo(a)pyrene, which is in wood smoke, is listed as one of twelve “Level 1 Compounds” under the US-Canada Binational Toxics Strategy (PDF).
Once emitted, benzo(a)pyrene can travel as much as 1,000 km, and once deposited, it can remain in bodies of water and on land for several years before degrading.
An Italian study found that benzo(a)pyrene levels were highest in “peripheral areas” where the major source of pollution was residential wood burning, rather than vehicles. The exception to these findings was in urban Milan, in an area where emissions were found to largely come from wood-burning pizzerias as well as cars.
Links to tumors and cancer metastasis
PAHs such as benzo(a)pyrene are also in tobacco smoke, and their role in promoting cancer in smokers has been widely studied. You would have to light 27,333 cigarettes to emit as much (PDF) benzo(a)pyrene as burning one kilogram (2.2 lbs) of wood.
Benzo(a)pyrene has also been linked to breast tumors in rodents, and laboratory research and epidemiological studies have provided further evidence of its role in promoting human breast cancer.
Research also suggests that it promotes cancer metastasis in those who have breast cancer. Women who have detectable PAH-DNA adducts have been shown to have an increased risk of hormone receptor-positive breast cancer. (A DNA adduct is a segment of DNA bound to a cancer-causing chemical.)
A toxicology study on human lung epithelial cells found that, “strikingly,” PAHs that were adsorbed onto wood smoke particles were even more potent in activating changes in gene expression than benzo(a)pyrene individually applied in suspension.
The researchers concluded, “As PAHs initiate multiple adverse outcome pathways and are prominent carcinogens, their role as key pollutants in wood smoke and its health effects warrants further investigation.”
The toxicity of residential wood combustion has been shown in both epidemiological and toxicological studies.
PAHs affect fetal development
Aside from cancer, PAHs have also been linked in multiple studies with detrimental effects on fetal and child development, including lower birth weight and premature births.
The World Health Organization, for example, has noted that exposure to benzo(a)pyrene in the womb may interfere with fetal growth (PDF) and disrupt the developing endocrine system. They noted that higher DNA adduct levels have been found in infants compared to their mothers, suggesting “an increased susceptibility of the developing fetus to DNA damage.”
PAHs and pulmonary immune response
It has also been suggested that PAHs alter pulmonary macrophages, which are instrumental in mounting an immune response to respiratory infection. This may be one of the reasons why exposure to wood smoke increases susceptibility to respiratory infections.
Woodsmoke contains at least five chemical groups classified as known human carcinogens by the International Agency for Research on Cancer (IARC), others categorized by IARC as probable or possible human carcinogens, and at least 26 chemicals listed by the US EPA as hazardous air pollutants.
Residential wood burning is a large source of PAHs
Residential wood burning is the most significant source of PAHs in many countries around the world, including, for example, the United States and Sweden.
In Denmark it is responsible for more than 90% of ambient PAHs.
In Canada, government figures from 2022 attribute 92% of PAH emissions to home firewood burning.
In Australia, an inventory of pollution sources has found that residential solid fuel burning, which is almost entirely from wood, is the largest source of environmental PAHs.
A study in Seiffen, Germany found that the “presence of short-term events of extremely high PAH concentrations were directly attributed to a plume of chimney exhaust emitted from houses located close to the sampling place.” They noted that residential wood burning was responsible for 62% of the total PAH levels in winter.
Residential wood burning was found to contribute almost half the annual carcinogenic potential from PAHs in the air in Athens, Greece.
…in-vitro toxicity studies suggest that not all particles are equal. For example, wood smoke PM, compared with diesel PM, induces higher levels of oxidative damage to human umbilical endothelial cells and stimulates increased monocyte adhesion.
Increased mutagenicity and cancer potency during wood-burning season
A study in New Zealand compared winter and summer ambient levels of PAHs in three urban areas where most of the particulate pollution in winter is from residential wood burning. In the area around Alexandra, for instance, daily PAH concentrations were 0.45 nanograms/m3 in summer, but 128.9 nanograms/m3 in winter.
During winter, 74% of PM10 extracts from all three locations showed significant mutagenicity in the Ames Test, compared to 25% in summer.
Wood-burning homes have higher PAH levels
A Swedish study found that wood-burning homes had 3- to 5-fold higher levels of several hazardous PAHs, and roughly 4 times the total PAH cancer potency, compared to non-wood-burning homes.
The median indoor level of benzo(a)pyrene was found to be 500% higher than the Swedish health guideline, which was also exceeded outdoors on all days.
Free radical formation and DNA damage
Once in the atmosphere, PAHs react with light and oxygen to form secondary compounds. PAH photoreaction products and PAHs can absorb light energy to form reactive oxygen species (ROS) and free radicals. These can cause DNA damage that is associated with age-related diseases, including cancer. PAHs exposed to sunlight may promote skin aging and skin cancers.
Hydroxylated polycyclic aromatic hydrocarbons (OH-PAHs) are oxidation products of PAHs.
Research has shown that OH-PAHs are “present in significant amounts in wood smoke particles,” and that they may be even more toxic and carcinogenic than unaltered PAHs.
[Residential] wood burning appeared to be also a notable source of arsenic, cadmium, and lead.
Wood smoke and dioxins
Dioxins and furans refer to two closely related families of chemicals with similar physical and biological characteristics. These are some of the most toxic chemicals to which you can be exposed.
They are persistent organic pollutants (POPs) that remain in the environment and in animal and human bodies long past exposure. They are also endocrine disrupting chemicals capable of causing reproductive harm.
An Australian study found that dioxin levels rose ten times above background levels when wood stoves were in use (PDF).
Residential wood burning is a significant source of dioxins in the United States and has been estimated to be the third largest contributor of dioxins to the environment in Europe.
Wood burning is a surprisingly large source of benzene
Exposure to benzene can lead to blood disorders, including anemia and excessive bleeding, as well as to damage to the immune system. Benzene is also classified as a Group 1 carcinogen by the IARC and as a Category A carcinogen by the US EPA (both meaning there is sufficient proof to conclude it is carcinogenic), and is linked to leukemia.
Residential wood burning contributes a surprisingly large amount of benzene to the environment.
For example, a study of a typical residential area in Finland showed that wood combustion contributes as much as 70% to local benzene sources. “Wood combustion was clearly the most important source for many compounds (e.g., benzene).”
A study of chemicals in wood smoke determined that hardwood burned in a wood stove emits 1 gram of benzene per kg of wood burned. (The study also noted that, in addition to benzene, wood stoves are also a “notable source” of toluene and xylenes.) Other studies have shown similar results.
Wood stoves emit 8 tons of wintertime benzene in Oregon town
In the area around Klamath Falls, Oregon (population approximately 22,000), wood stoves emit an estimated 8 tons of benzene during the wood burning season (PDF), based on an analysis of air toxics monitoring by the state’s Department of Environmental Quality.
According to the DEQ, if all the wood-burning appliances were to be changed to non-wood-burning heating, benzene emissions from residential heating would drop to near zero.
An Australian study found evidence that use of a wood stove in the year before or during pregnancy may increase the chance of a child developing acute lymphoblastic leukemia (ALL). The study’s researchers noted that earlier studies have found a correlation between childhood ALL and exposure to ambient levels of benzene in the air. They also noted that other compounds in wood smoke aside from benzene could contribute to an increased risk as well, since there are other carcinogens in wood smoke, including PAHs.
Aldehydes in wood smoke
There are several types of aldehydes in wood smoke, including acetaldehyde, which is a probable carcinogen, formaldehyde, which is a known carcinogen (PDF), and acrolein.
Wood burning is a large source of formaldehyde
Wood burning is a surprisingly large emitter of formaldehyde. For instance, the NSW EPA in Australia reports that residential wood burning is responsible for 38% of formaldehyde emissions in the Sydney region.
A study in Yakima, Washington found that residential wood burning was responsible for 73% of formaldehyde emissions and 69% of acetaldehyde.
Acrolein in wood smoke
Acrolein is a toxic aldehyde found in high concentrations in both tobacco smoke and wood smoke.
It is an irritant that can affect the eyes, nose, throat and lungs, and can pose a special problem for people who have asthma or bronchitis. It is also known to suppress the immune system and is implicated in demyelinating diseases such as multiple sclerosis.
There is also evidence that breathing in acrolein may impair vascular repair, which can lead to increased risk of cardiovascular disease.
Airborne arsenic levels rise
Arsenic exposure is linked to increased risk of cancer and other serious health effects. When people burn waste wood that was treated with an arsenic-containing preservative, arsenic is released in the smoke.
Levels of arsenic in the air have been highly correlated with residential wood burning. The burning of treated waste wood in fireplaces and wood stoves may be common enough that it has been suggested arsenic could be used as a source tracer to measure the proportion of wood smoke in the air.
In Nelson, New Zealand ambient arsenic levels have been shown to rise substantially in correlation with wood burning levels. The New Zealand Ambient Air Quality Guideline for arsenic is 5.5 nanograms/m3 annual average. But the wintertime level has been shown to rise as high as 90 nanograms/m3 in correlation with wood burning.
Researchers in Aukland, New Zealand found a significant correlation between the smell of wood smoke in the neighborhood and the presence of arsenic in study participants’s beards.
An air pollution study in Flanders, Belgium, found that residential wood burning was a “notable” source of arsenic, as well as cadmium and lead.
In the UK, where wood heating has become a growing problem, researchers in London and Manchester have been finding arsenic in the air associated with wood burning.
Woodsmoke contains thousands of chemicals, many of which have well-documented adverse human health effects…
Mercury in wood smoke
Wood burning is also a significant source of mercury emissions. Mercury, which is a neurotoxin, is classified as a persistent bioaccumulation toxic (PBT) by the US EPA.
Black carbon
Black carbon has been shown to increase antibiotic resistance in bacteria and promote the spread of infections in the respiratory system.
A study found that indoor concentrations of black carbon were about 2.5 times higher in a home with a wood stove than in a home without. It was also noted that black carbon from wood smoke was also present in non-wood-burning homes, due to infiltration from outside.
Another study found that an average of 78% of black carbon particles from residential wood burning outside eventually wound up inside nearby homes, demonstrating “the minimal shielding a home provides” from outdoor wood smoke.
Black carbon is also a short-lived climate pollutant (see our Climate page for more information).
Wood smoke is toxic pollution references