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

Newer certified wood stoves are not nearly as clean as many believe. Even if used with perfectly dried, shaped and sized wood and operated exactly as recommended in laboratory-perfect conditions―which, in the real world, they virtually never are―they are still far more polluting than a non-wood-burning heat source.

A crib of wood burning in a test
A crib test. Image: US EPA

Emissions tests don’t reflect real-life emissions

Laboratory emissions tests are conducted using kiln-dried wood arranged to exact specifications with fixed spacing between the pieces of lumber. This specially configured “crib” is burned by expert technicians under carefully controlled conditions. This provides optimal emissions data for certification, but little hint as to how the stoves will actually perform in real life.

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. The catalytic stove with the lowest certification value, 1.6 grams per hour, had actual in-home emissions of 24.1 grams per hour (PDF).

Another study also 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).”

Conditions in a testing lab are not like in a house

A wood burning industry consultant writing in an industry trade publication in 2017 (no longer online, but available archived) noted:

Certification numbers for wood heaters are not predictive of their actual performance in homes or of the relative ranking of the performance of various wood heaters under real world conditions.

The industry consultant cited “a host of variables that make in-home usage quite different from the controlled test conditions in the laboratory.”

These variables include the arrangement and sizes of the wood pieces in the stove, how the wood is added during refueling, the shape and configuration of the chimney and how much creosote has built up in it, whether an exhaust fan or clothes dryer is used in the house at the same time as the wood stove, the weather and barometric pressure, the age of the stove, the house’s elevation, and more.

Bottom line is that the uncertainties inherent in needed measurements combined with the variability in the operation of the wood heater do not allow for a very accurate determination of a certification value.
…EPA’s certification program to ensure new wood heaters meet clean air requirements is dysfunctional. It is easily manipulated by manufacturers and testing laboratories.
NESCAUM. Assessment of EPA’s residential wood heater certification program.

Start-up emissions aren’t counted

Wood stove emissions testing also misleads by omitting parts of the burn cycle. 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.

Emissions up to 7,842% higher during the first hour

A 2021 report by the Northeast States for Coordinated Air Use Management (NESCAUM) on the failings of the US EPA’s wood stove certification program (PDF) found that both catalytic and non-catalytic wood stoves had greatly increased emissions during the first hour of operation:

For cordwood stoves, 42 percent had first-hour emission rates that were more than three times higher than the emission standard. On average, first-hour emissions were 616 percent higher than the appliances certification value. The first-hour values ranged from 132 percent to 7,842 percent higher.

Dysfunctional wood stove certification program

The NESCAUM report found that:

EPA’s certification program to ensure new wood heaters meet clean air requirements is dysfunctional. It is easily manipulated by manufacturers and testing laboratories. EPA has done little to no oversight and enforcement…
At its core, EPA’s [wood stove certification] program as currently run allows the continued sale and installation of high-emitting devices, many of which will be in homes located in overburdened communities already suffering from environmental and other inequities. Once installed, these units will remain in use, emitting pollution for decades to come.

A 2023 report issued by the US EPA Office of Inspector General came to a similar conclusion, finding that the EPA’s wood stove certification program is ineffective and doesn’t protect the public from polluting wood stoves.

For the over 250 certified wood heaters reviewed, this report finds a systemic failure of the entire certification process, including EPA’s oversight and enforcement of its requirements.
NESCAUM. Assessment of EPA’s residential wood heater certification program.

Averaging emissions disguises intensive pollution peaks

The government oversight agency also noted that the EPA regulates wood stoves based on average emissions over time, but averaging emissions disguises the intensive peaks in pollution that occur at different parts of the burn cycle, “resulting in periods of high-impact exposure to PM2.5 pollution.”

A random outcome

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.”

Eco-certified stoves: 750 times more polluting than a heavy-duty truck

A 2021 report from the European Environmental Bureau and Green Transition Denmark found that, per gigajoule (GJ) of energy produced, a modern European Eco-certified wood stove emits 750 times more fine particle pollution than a modern heavy goods truck. The report used emissions data reflecting perfect use of the stoves in laboratory conditions. A real-world emissions comparison would almost certainly be much higher.

The report’s authors point out that “both the EU EcoDesign requirements and the more ambitious Nordic Ecolabel fail—under optimal laboratory conditions—to reduce new stoves’ particle emissions to acceptable low levels.”

They also point out that “burning just one kg of wood [in an EcoDesign certified wood stove] will pollute 500,000 cubic meters of completely clean air to up the level of the current WHO air quality guideline for fine particulate matter.”

Wood stoves cause flooding of particulate matter into homes

A study of indoor emissions from certified wood stoves found they cause “flooding” of PM2.5 and PM1 into the homes of wood burners when used as recommended. The study’s authors noted:

The PM that is released into the home is not an aberration from normal use, but results directly from it. This is because real-world operation cannot occur without opening the stove door.

Based on these findings, the researchers recommended that certified wood stoves be sold with a warning that they pose a health risk under normal usage.

More indoor pollution than on a polluted urban street

Green Transition Denmark issued a report that includes information on the high indoor, as well as outdoor, emissions (PDF) from modern certified wood stoves when used as recommended with dry wood.  

They note that newer eco-labeled wood stoves have been shown to cause indoor particle pollution levels that are “many times higher than along the most polluted streets in Denmark.”

…both the EU EcoDesign requirements and the more ambitious Nordic Ecolabel fail—under optimal laboratory conditions—to reduce new stoves’ particle emissions to acceptable low levels.
European Environmental Bureau and Green Transition Denmark. Where there’s fire, there’s smoke: Emissions from domestic heating with wood.

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 can be comparable to that of an older, uncertified conventional wood stove. According to a report prepared for the US EPA, “over the normal life of the catalyst (PDF), 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 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 wood stove.

Several studies were carried out to measure moisture in woodpiles around the United States and Canada (PDF). 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 humidity and storage conditions, the moisture content of seasoned wood can vary significantly.

A pile of wet firewood for sale at a store
Wet firewood for sale at a store in California.

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 a research review article noted, “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.”

Wood stove efficiency ratings mislead

Efficiency ratings can also be misleading. According to a report for the US 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.

Smoke from a certified wood stove blankets neighboring homes.
Smoke from an EPA-certified wood stove blankets neighboring homes in Parksville, British Columbia. Photo courtesy of Bill Lewin.

Certified wood stoves emit a lot of ultrafine particles

Although not often measured or considered in regulatory guidelines, there is growing evidence that ultrafine-sized particles are particularly harmful.

Newer wood stoves emit more ultrafines

Newer certified wood stoves have been shown to emit more ultrafine-sized particles than conventional wood stoves.

Ultrafine levels exceed limits of measuring equipment

Green Transition Denmark measured ultrafine particle emissions from a modern certified wood stove (PDF) using dry wood arranged carefully to recommended specifications.

They found that “even under optimal firing conditions in a good eco-labeled wood stove, [ultrafine] particle emissions increase instantaneously to the maximum limit of the measurement equipment of 666,000 particles per cm3 (corrected for removal of 25% in the probing tube).”

They noted that “in reality emissions were markedly higher” than what the equipment was able to measure.

…modern eco-labeled wood stoves have [been] shown to cause particle pollution inside houses that are many times higher than along the most polluted streets in Denmark; important details that the wood stove industry ‘forget’ to mention.
Green Transition Denmark. Pollution from residential burning.

Higher levels of carcinogenic toxins

A study demonstrated that, compared to other wood stoves, those with catalytic converters emit substantially higher levels of chlorophenol, dioxins and furans.

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

An 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 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 always a good indicator of indoor air quality.