Photo: "It is a myth that eco-labelled wood stoves under optimal firing conditions do not pollute."

All Wood Stoves Pollute —

Including New Certified Ones

The wood stove industry works hard to convince everyone that buying more wood stoves will solve our wood smoke pollution problems. But this simply does not work.


EPA-certified wood stoves are not nearly as clean and efficient as many believe. Even if used with perfectly dried wood and operated exactly as recommended ― which, in the real world, they almost never are ― they are still far more polluting than other heating options.


photo of a modern wood stoveNew certified wood stoves still emit far more pollution than other kinds of heating.Emissions Tests Don’t Reflect Actual Usage

Laboratory emissions tests are conducted using kiln-dried wood arranged to exact specifications with fixed spacing between the pieces of lumber. This perfectly configured “crib” is burned under controlled conditions that provide optimal testing numbers that do not reflect real-world usage.


Furthermore, the large amount of emissions released during the start-up period is not taken into consideration during testing. Emissions only count once the stove has reached a stable burn phase. On a catalytic stove especially, this can lead to significantly more pollution being emitted than testing indicates, because the catalyst is bypassed during the start-up.


For example, a report for the US EPA compared the certified emissions level of wood stoves installed in the Oregon communities of Portland and Klamath Falls. It was shown that actual emissions varied greatly from the stoves’ official certified values. For instance, the catalytic stove with the lowest certification value, 1.6 grams/hr, had actual in-home emissions of 24.1 grams/hr.


Another study found “substantially greater” emissions when wood stoves were operated in homes by householders compared to testing conditions. Even when the highest emitting stove was excluded, “emissions under real-life operating conditions were approximately 16 times higher than those discharged during the authorization test (g/kg).”


A report (PDF download) from the Danish Ecological Council points out, “It does not seem logical that an eco-labelled stove is allowed to pollute 25 times more than a 10 year old truck.” The report further states, “In measurements of ultra fine particles in the chimney exhaust of a modern eco-labelled wood stove under ideal operational conditions a higher pollution level than the measurement limit of the equipment was measured… It is therefore a myth that eco-labelled wood stoves under optimal firing conditions do not pollute.”


Certification tests also vary by country. A study that sent the same wood stove to different countries in North America and Europe for certification testing concluded, “the environmental acceptance of a specific wood stove, based on emission testing in different countries, gives a random outcome.”


Photo of a crib of wood burning during a wood stove emission test. Crib test image: US EPACertified Wood Stove Emissions Increase over Time

Emissions from both newer non-catalytic and catalytic wood stoves increase over time due to physical degradation of the stoves from use. Within five years the particulate emissions from a catalytic stove may reach the level of an older, uncertified conventional wood stove. According to a report for the US EPA, “over the normal life of the catalyst, the average performance of the heater will be similar to that of a non-catalyst heater that does not change its emission performance as significantly with time.”


Since the effects of degraded catalytic components, including greatly increased emissions, largely occur outside the user’s home, there is little incentive for owners to spend the money to replace them.


Wood That Is Too Wet Increases Emissions

When wood is too wet, emissions will be higher, even in a newer stove. Several studies were carried out to measure moisture in woodpiles around the United States and Canada. Residential woodpiles in New York and Vermont had moisture content ranging from 17% to 41%. In Portland, Oregon woodpiles were found with a moisture content of over 100% — there was more water than wood. Due to regional differences in relative humidity, the  moisture content of seasoned wood can vary significantly.


Wet firewood for sale in front of a store in Northern Califonia.Photo of a display of wet and moldly-looking firewood for sale in front of a store.Wood That Is Too Dry Increases Emissions

It has also been noted that dry softwood (11 – 14% moisture) has higher particulate emissions compared to wood with 20 – 30% moisture.


Very dry wood also creates more toxins. As this research review article notes, “Combustion may also be too intense when using very dry fuel in well insulated stoves, thus resulting in so-called ‘air-starved’ conditions. Such combustion may contain the highest emissions of carcinogenic polycyclic aromatic hydrocarbons (PAHs) and is dominated by solid agglomerated soot (black carbon) particles, which shows similarities to diesel soot.”


As another study points out, “Traditional recommendations to wood stove and boiler users and manufacturers are to avoid slow, low temperature combustion (i.e., moist fuel and poor insulation). Our study illustrates how excessively high heat release rates are also undesirable, because of the emissions of particulate PAHs.”


Wood Stove Efficiency Ratings Mislead

Efficiency ratings can also be misleading. According to a report for the EPA, “there are numerous ways to measure and report efficiencies. There is no universally, or even generally, accepted standardized method to measure or report efficiencies, and in fact it is still an area of contention. The contention is often exacerbated by the competitiveness of marketing claims.” A wood stove’s efficiency will also vary in real-world usage, depending upon how the stove is used and on the type, size and moisture content of the wood that is burned.


Certified Wood Stoves and Higher Levels of Carcinogenic Toxins

In addition, the advocacy group Families for Clean Air, among others, have pointed out troubling evidence that newer, certified stoves do not reduce levels of toxic chemical pollutants such as dioxins and furans. In fact, they emit more. These are some of the most dangerous chemicals to which one can be exposed, and some of the most persistent in our environment.


An EPA study compared catalytic stoves against conventional models and concluded that the catalytic stove emissions were more mutagenic than emissions from the conventional stoves.


Another study has shown that stoves with catalytic converters emit substantially higher levels of chlorophenol, dioxins and furans.


Photo of lots of brown wood smoke pouring from a house's chimney and onto neighboring houses all down the block.Emissions from an EPA-certified wood stove in British ColumbiaAn animal study found that modern technology appliances (which included a newer wood stove as well as pellet-burning appliances) had lower PM1 emissions, “but they induced the highest inflammatory, cytotoxic and genotoxic activities” in the lungs. Similarly, a laboratory study comparing smoldering combustion with that of highly efficient wood chip emissions found that the particles derived from the most efficient combustion caused more cellular damage. The study authors noted that different combustion conditions produce particles with “highly different” toxicological properties.


A recent study that compared emissions from different kinds of wood-burning appliances found that the “advanced” wood stoves emitted amounts of non-methane hydrocarbons, nitrogen oxides and PAHs that were similar to or higher than those from traditional wood stoves. In fact, the newer wood stoves were found to emit even more carcinogenic benzo(a)pyrene than the traditional stoves.


A study of indoor pollution from fireplaces and wood stoves found that levels of carcinogenic PAHs in a sample of wood-burning homes was always higher than the target yearly value for outdoor levels established by the European Commission. The home in the study with the lowest measured levels of particulates had high levels of PAHs, suggesting that particulate levels alone are “not a good indicator of indoor air quality.”


Residential Sources References

See also: Wood Stove Changeouts

For more information about common household wood-burning devices, please visit the following links: