Woodsmoke particulate matter generates more DNA damage than traffic-generated particulate matter per unit mass in human cell lines.

Wood Smoke Is Toxic Pollution

Wood smoke contains thousands of chemicals, including known irritants, carcinogens, suspected carcinogens, mutagens, teratogens (substances that are linked to birth defects) and metals, as well as toxic and irritant gases such as carbon monoxide, nitrogen oxides and sulfur dioxides. The following is a brief overview of only some of the health-damaging toxins in wood smoke.


An illustration showing different toxins emerging from a chimney and raining down on neighboring homes. Polycyclic Aromatic Hydrocarbons (PAHs)

PAHs were the first chemical carcinogens to be discovered. They are formed during the combustion of organic matter.


Many of the toxic effects of wood smoke are thought to arise in part from its high level of PAHs. As this study points out, the PAH content is “much higher” in wood smoke compared to vehicle exhaust, and “a higher mutagenic and carcinogenic potential” exists for wood smoke compared to traffic exhaust.



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. 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.


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 benzo(a)pyrene as burning one kilogram (2.2 lbs) of wood.


Benzo(a)pyrene has also been shown to cause 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.”


PAHs have detrimental effects on fetal and child development.Photo of a pregnant woman having a medical exam.PAHs Affect Fetal Development

Aside from cancer, PAHs have also been shown in multiple studies to have detrimental effects on fetal and child development, including lower birth weight and premature births.


The World Health Organization, for instance, has noted that exposure to benzo(a)pyrene in the womb may interfere with fetal growth due to its anti-estrogenic effects, which may disrupt the developing endocrine system. It has been 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.”


In addition, research has found that PAHs also have anti-androgenic effects in the womb that may affect the development of reproductive organs. The developing male reproductive system is particularly sensitive to benzo(a)pyrene.


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.


Photo of a wood stove with a basket of logs in front.Residential wood burning is the most significant source of  mutagenic and carcinogenic PAHs in many countries around the world.Residential Wood Burning: Largest 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 and Canada it’s responsible for more than 90% of ambient PAHs. 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.” It was found that residential wood burning was responsible for 62% of the total PAH levels in winter, and that, “The large contribution of the residential wood combustion to the PAH concentration highlights its possible impact on human health, as considered in previous toxicological studies.”


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.


As Oregon State University’s Superfund Research Program tells us, if you want to reduce your exposure to PAHs, “avoid smoke from wood fires, whether from home heating or for recreational purposes.”


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.


Illustration of the molecular structure of dioxin.A Significant Source of 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. Residential wood burning is a significant source of dioxins in the United States and is estimated to be the third largest contributor of dioxins to the environment in Europe . In Fresno, California, wood burning was shown to be the primary source of dioxins in the environment.


Exposure to benzene can lead to blood disorders, including anemia and excessive bleeding, as well as leukemia.Photo of blood samples in a laboratory.A Surprisingly Large Contributor 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 known to cause leukemia.


Residential wood burning contributes a surprisingly large amount of benzene to the environment. For example, this 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.


Eight Tons of Benzene in One Community

In the area around Klamath Falls, Oregon (population approximately 21,200), wood stoves are estimated to emit eight tons of benzene during the wood burning season. As the State of Oregon Department of Environmental Quality points out, wood burning emits more benzene than any other source of home heating. For example, if all the wood burning appliances were to be changed to non-wood-burning heating sources, benzene emissions from residential heating would be near zero.


An Australian study found evidence that use of a wood stove in the year before or during pregnancy increased the chance of a child developing acute lymphoblastic leukaemia (ALL). It was 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 be contributing to an increased risk as well, since there are other carcinogens in wood smoke, including PAHs.


 Illustration of the chemical formula for acroleinAldehydes

There are several types of aldehydes in wood smoke, including acetaldehyde, which is a probable carcinogen, formaldehyde, which is a known carcinogen, and acrolein.


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.


Acrolein is a highly 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 impairs vascular repair, which can lead to increased risk of cardiovascular disease.


 An illustration of the the periodic table information for arsenicArsenic

Levels of arsenic in the air have been highly correlated with levels of wood smoke. The use of treated wood in fireplaces and wood stoves, along with trash, may be so common that it has been suggested that arsenic may be used as a source tracer to measure the proportion of wood smoke in the air.


In Nelson, New Zealand, where wood burning is common in winter, 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.


Arsenic that is released by burning becomes attached to fine particles in the air that may travel for days over long distances, eventually winding up in soil or water.


An illustration of the the periodic table information for mercury.Mercury

Wood burning is also a significant source of mercury emissions into the atmosphere. Mercury, which is a neurotoxin, is classified as a persistent bioaccumulation toxic (PBT) by the US EPA.


Black Carbon

Recent research has found that black carbon increases antibiotic resistance in bacteria and promotes 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).


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