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Wood Heat Concerns

A. Particulate Emissions

Particulates from wood heaters are a serious health concern, particularly in areas where high concentrations of particulates from wood smoke builds up. The cleanest burning modern stoves have drastically reduced indoor and outdoor particulates. Scores of mitigation strategies can be employed in those areas to improve air quality, including banning new installations altogether. Incentive programs provide the government with leverage to steer consumers towards the cleanest burning appliances and, for example, to only incentivize the cleanest pellet appliances in urban areas.

The combustion of biomass releases a wide variety of pollutants into the air, including carbon monoxide, volatile organic compounds, toxics (such as benzene and acrolein), and small incompletely combusted particles. Particulate matter with diameters under 2.5µm (PM2.5) are considered a hazard to human health in concentrations found in many U.S. cities. For reference, one µm (micrometer) is one thousand times smaller than a mm (millimeter). PM2.5 can increase the risk of asthma and lung disease. 1The emission of all particulates from stoves is measured in grams per hour (g/hr) and is he primary way that consumers – and policymakers – can assess the level of emissions of PM2.5. 2

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Figure 20: Particulate matter diagram (Minnesota Pollution Control Agency)

Older adults, young children and those suffering from heart or lung diseases face an increased risk of complications from breathing particles found in the smoke from older, inefficient wood stoves. The dangers of indoor and outdoor wood smoke are likely to be greater in low-income communities, where wood burning is more common, and older, more heavily polluting wood heating appliances are often more prevalent.

There are several ways to mitigate the adverse health effects of wood smoke: changing out old polluting wood burning appliances for the much cleaner burning modern models (see Chapter 5, Section A: Changeout Programs) restricting installation of all but the very cleanest burning (under 1 g/hr) appliances in urban areas and valleys prone to inversions, 3 expanding property line set-backs and other restrictions for outdoor wood boilers (See Chapter 6, Section E: Outdoor Wood Boilers), clean burning education for the users of wood heating appliances to reduce user error derived wood smoke, and other policies designed to only allow the cleanest wood burning appliances.

Pellet stove technology has reached a point where scores of models have very low (under 1 g/hr) particulate emission rates and are generally not considered to pose a significant health threat. The average fuel oil boiler emits twice as much PM2.5 per unit of heating value (British thermal units) than a pellet stove. 4 A life cycle particulate analysis, as opposed to a combustion particulate analysis, may point to a more comparable emissions profile between fossil fuels and the cleanest pellet stove and boiler emissions since it takes into account all of the emissions released in the extraction and refinement of fossil fuels. In Europe, pellet stove and boiler particulate levels are considered reasonable and not a barrier to installations even in urban areas. 5

Strategies for Reducing Residential Wood Smoke
Air Quality Planning Division, EPA

Wood Smoke Health Resources

EPA certified wood stoves manufactured today typically emit 2 – 4 grams per hour, roughly ten times less than the 15 to 40 g/hr that older stoves typically emits. However, about 80% of the residential wood stoves manufactured prior to the EPA standards are still in operation 6, potentially posing health risks to local populations while at the same time negatively influencing public opinion about the viability of wood heat as a clean renewable energy technology. Wood stoves tend to last a long time and are replaced less frequently than other major appliances, so many of the older stoves are still used regularly for heat. For this reason, the replacement of older stoves (see Chapter 5, Section A: Changeout Programs) and the removal of loopholes that allow unregulated new stoves to stay on the market (see Chapter 6, Section F: Exempt Woodstoves) are vital to improving air sheds across the country.

The EPA jump-started cleaner residential stoves in 1988 with their New Source Performance Standards, and is only now revisiting these regulations in 2011. The original standards dictated a maximum of 7.5 g/hr for non-catalytic wood stoves and 4.1 g/hr for catalytic wood stoves. Many stove types were exempt from regulation for various reasons (see Chaper 6: Appliance Types). The proposed New Source Performance Standards are likely to at least partially follow the lead of Washington State, where, in 1995, officials sent a strong message to the industry and Federal Government by setting stricter standards for wood stoves sold in the state. The current emissions standard in Washington is 4.5 g/hr for non-catalytic wood stoves and pellet stoves and 2.5 g/hr for catalytic wood stoves. The burgeoning residential biomass boiler market in Europe provides clear evidence that wood stoves have the capacity to become much cleaner (see Chapter 5, Section D: European Incentives).

Like all forms of renewable energy, wood heat is well suited for some areas of the country, but not so well suited for others. Strict regulations on new installation of cordwood appliances in certain major urban non-attainment areas like Los Angeles and Denver, as well as valleys subject to inversions is often a necessary air quality step, though the cleanest pellet appliances may still be permitted. These local restrictions also prevent incentive programs, like the federal tax credit, from incentivizing installations in places that already have very poor air quality.

For purposes of this report, incentive programs that involve government funding offer a clear opportunity to mitigate health impacts of high emitting wood burning appliances by leveraging public funding to maximize benefits of wood heat while minimizing the potential health impacts. Strategies include making only the cleanest appliances eligible or by only incentivizing pellet stove installation in urban areas, but not installation of wood appliances unless they meet or exceed air quality restrictions and are accompanied by wood moisture requirements. In Europe, extensive incentives for qualified pellet appliances has likely steered many consumers towards these very clean burning appliances. The same could hold true in the United States.

1 EPA. Burnwise. Last updated Feb. 8, 2011. <>
2 Hereafter the grams per hour emissions of PM2.5 will be referred to as g/hr.
3 Gulland J. The Argument in Favour of Wood Heating. The Fuelwood Project. February 2007. Pg 7
4 U.S. EPA. Technology Transfer Network Clearinghouse for Inventories & Emissions Factors. WebFIRE. <> SCC 210400400.
5 U.S. EPA Residential Wood Calculator. Contact Roy Huntley. #2 fuel oil 115.11 lb PM/MMBtu compared to pellet stove 48.96 lb/MMBtu.
6 Fiedler, Frank. The state of the art of small-scale pellet-based heating systems and relevant regulations in Sweden, Austria and Germany. Solar Energy Research Center, Dalarna College.
7 Broderick, D.R.; Houck, J.E. Projected Residential Wood Burning Appliance Installations; OMNI Environmental Services Inc.: Beaverton, OR, 2005.

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