For many the smell of wood smoke from a fireplace elicits fond memories of hearth and home. There is a lack of awareness, however, that wood smoke has become a major source of air pollution in the United States. Combustion of organic matter such as wood and yard debris releases a variety of harmful substances, including particulates, carcinogens, carbon monoxide, respiratory irritants and toxins. Many people--infants and children, pregnant women, senior citizens, and those suffering from allergies, asthma, bronchitis, emphysema, pneumonia, or other heart or lung diseases--are at risk from the pollution released by wood smoke.
Compounds released during the combustion process interfere with normal lung development and function. Indoor and outdoor air quality can be degraded significantly by the use of poorly designed, non-certified wood stoves. Poor burning processes, lack of maintenance, improper stove installation, and burning wet wood create excessive amounts of pollution. Fires left smoldering to keep a house warm during the night can also be particularly harmful. Smoldering wood burns slower and incompletely, thereby releasing more smoke and gas into the air.
Wood smoke contains tiny particles of creosote, soot, and ash that can remain airborne for up to three weeks. Small particles of solid and liquid matter suspended in the air are called particulate matter, or "PM." PM10 are those particles 10 microns or less in diameter. (In comparison, a human hair is approximately 70 microns in diameter.) PM2.5, or "fine" particulate matter, are those particles 2.5 microns or less in diameter. Inhaling fine PM causes coughing, irritation, and permanent scaring of the lungs. This type of damage decreases lung function, increases the potential for respiratory illness, and may contribute to cancer, heart disease, and changes in DNA, leading to auto-immune diseases. Because of the health threats associated with particulate air pollution, the federal government regulates all particulate matter as one of the six major air pollutants.
Particulate pollution from wood stoves is primarily produced in the winter when stagnant air and temperature inversions limit air movement. At this time smoke is unable to rise and disperse, and this pollution becomes trapped close to the ground in our breathing space. Areas with valleys and poor air circulation can be strongly affected. The small size of these particles allows them to seep into houses through closed doors and windows.
Many of the small particles from wood smoke are too small to be filtered by the nose or upper respiratory system. Therefore, they are able to penetrate deep within the lungs, and they collect in the most remote portions of the lungs called the alveoli, which are tiny air sacs where oxygen enters the blood stream. Due to their ability to evade the defenses of the body, these particles are efficient vehicles for transporting toxic gases, bacteria, and viruses into the lungs, and ultimately the blood stream. Some toxic compounds are cancer-causing and can attach to the smallest smoke particles and enter the lungs at the same time. Particulate matter can clump together, blocking tiny veins as well as invoking harmful structural and chemical changes in the lungs.
A report released by the Washington State Department of Ecology based on research conducted by the University of Washington in Seattle and the EPA in Boise, Idaho, found that indoor PM10 levels from wood smoke in homes without woodstoves can reach 50-70 percent of the outdoor PM levels. The PM released from wood heating can also cause biological mutations (chromosome defects and genetic damage) in cells of the lungs. Mutagens and carcinogens are not exactly the same and not all mutagenic substances cause cancer. Mutations brought about by wood smoke, however, potentially lead to cancer formation. In 1988 an EPA study found that biological mutations in bacteria exposed to winter air samples increased with higher concentrations of fine particulate matter and were most numerous at the times of coldest temperatures, weekends, and holidays when wood stoves were used the most.
The cancer threat from air pollution is another serious public heath concern. In 1985 the EPA started a research program to clarify the sources of air pollution and to estimate their future cancer risk (Washington State Department of Ecology 1997). Their research determined that motor vehicles and wood stoves were the major sources of particulate air pollution and associated cancer risk in the urban airsheds studied. According to the EPA, many of the substances identified in wood smoke are suspected human carcinogens or co-carcinogens. These compounds include many polycyclic aromatic hydrocarbons (PAHs), such as benzo(a)pyrene, and various aldehydes, alkenes, and semi-volatile organic compounds.
For information about the health risks from exposure to air toxics. See EPA’s Health Risk Assessment brochure
Carbon monoxide is a colorless, odorless gas that is also produced when wood is burned. Once in the blood stream, it reduces the ability of blood to carry oxygen to body tissues. Respiratory toxins and irritants, including nitrogen dioxide, are also released during wood combustion. These compounds impair the respiratory system and reduce its ability to fight infection.
Wood Smoke vs. Cigarette Smoke
Although many people associate tobacco smoke with certain health risks, research indicates that second hand wood smoke has potentially even greater ability to damage health. A comparison between tobacco smoke and wood smoke using electron spin resonance revealed quite startling results (Rozenberg 2001, Wood Smoke is More Damaging than Tobacco Smoke). Tobacco smoke causes damage in the body for approximately 30 seconds after it is inhaled. Wood smoke, however, continues to be chemically active and cause damage to cells in the body for up to 20 minutes, or 40 times longer.
Some of the components in wood smoke are free radicals, which steal electrons from the body, leaving cells unstable or injured. Some of these cells may die, while others may be altered and take on different functions. These changes lead to inflammation, which causes stress on the body. EPA researchers suggest that the lifetime cancer risk from wood stove emissions may be 12 times greater than the lifetime cancer risk from exposure to an equal amount of cigarette smoke. (Rozenberg 2001, What’s in Wood Smoke and Other Emissions).
What You Can Do
In order to mitigate the negative impacts caused by wood smoke, homeowners should consider converting their wood fireplaces to gas stoves. This type of heating is not only healthier for household members and the community, but it is a more efficient way to heat a home. Wood stoves often draw warm air from a room up the chimney along with smoke, resulting in less heat in a room. Gas stoves, on the other hand, provide a better, more efficient heating effect, with virtually no emissions.
If wood stove heating is the only option, replacing an older appliance with EPA-certified wood burning technology can reduce wood smoke emissions by up to 85 percent, according to the Hearth Products Association. Proper stove installation and/or regular chimney sweeping can often eliminate the high level of indoor wood smoke produced from back drafting. Wood stove users should be mindful of the type of wood they use--hard, dry wood burns cleaner than soft, wet wood. Using small pieces of wood to build a fire allows it to burn hotter, which results in less smoke. Research also indicates that using a fire starter to build a fire in a wood stove can reduce particulate emissions by more than 69 percent (Hearth Products Association). For more tips on using efficient woodstoves, see Wood Stoves and Air Pollution (Fact Sheet ARD-36).
View a table listing some of the major pollutants associated with smoke from various sources , including on-road and off-road mobile sources, stationary and area sources, and a variety of fuels.
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