Certified Wood Stove Performance in the Real World Continues to Disappoint

For decades, the wood stove industry has tried to convince consumers and regulators that changing out older wood stoves for new certified wood stoves will reduce hazardous wood smoke pollution.

Unfortunately, much like the tobacco industry’s claims about low-tar cigarettes, things haven’t played out as promised in the real world. The latest evidence comes from New Zealand.

There, multiple towns in the region of Otago went so far as requiring that all wood stoves with a certified emission rate greater than 1.5 g/kg had to be removed by January 2012. Since then, all new installed wood stoves have been required to be “ultra low emission wood burners” with a certified emission rate of less than 0.7 g/kg.

Despite this effort, these towns continue to have serious air quality problems due to wood burning.

It’s just one more example in a long history of disappointing wood smoke reduction efforts that have focused on certified wood stove changeout programs and “better” burning practices.

The problem is that even when used in ideal, carefully controlled laboratory conditions with perfectly sized and perfectly dried pieces of wood, modern certified wood stoves are far more polluting than most people think they are.

For example, the European Environmental Bureau and Green Transition Denmark found that, per unit of energy produced, a perfectly used modern European “Eco-certified” wood stove burning dry wood emits as much fine particle (PM2.5) pollution as 750 modern diesel trucks.

Here in the U.S., a scathing 2021 report by the Northeast States for Coordinated Air Use Management (NESCAUM) detailed problems with the EPA’s wood stove certification program, which they described as “dysfunctional.” They found there’s no guarantee that new certified wood stoves are any cleaner than the ones they replace.

In 2023, the EPA’s Office of Inspector General issued the findings of their own investigation. They concluded that the EPA’s “ineffective residential wood heater program” doesn’t protect the public from harmful wood stove emissions.

In the real world, wood stoves aren’t used in the laboratory controlled manner in which they’re tested—so their emissions are even higher.

In Libby, Montana, the wood stove industry teamed up with the U.S. EPA and the state to organize a large wood stove exchange that took place from 2005–2008. At a cost of over $2.5 million, the majority of older wood stoves in the Libby area were replaced with EPA-certified ones. Participants in the exchange were also given information on recommended burning practices with their new wood stoves.

The industry proclaimed Libby to be a great success. But was it?

After this highly publicized wood stove changeout program, wood stove emissions accounted for approximately 81% of Libby’s wintertime PM2.5 pollution—roughly the same percentage as before the exchange.

Meanwhile, over in Australia, the Office of the Commissioner for Sustainability and the Environment in the Australian Capital Territory (ACT) seems to have finally learned the lesson from all these failed wood stove changeout efforts.

In a report issued in 2023, they concluded that “even tightened standards are unlikely to reduce the smoke-related health impacts of wood heaters.” They found that the ACT’s “Burn Right Tonight” program, which has been running since 2011, may, rather than helping, be “paradoxically” encouraging people to believe that wood burning is less harmful than it actually is.

Their recommendation? To protect public health, wood stoves should be phased out.

Wood Stove Industry Charged with Making Deceptive Environmental Claims

You’ve probably encountered it: greenwashing by the wood stove industry. They make hand-wavy claims about how their products are “environmentally friendly,” that wood burning is “carbon neutral,” and even “good for the environment.” 

These misleading marketing claims, which are intended to persuade consumers to buy new “certified” wood stoves, have been repeated so many times that many people have fallen for them. It helps that these claims are what consumers want to hear—so they can feel better about doing something they want to do anyway. It’s the same marketing strategy successfully used by the tobacco industry to sell “light” cigarettes.

But in Denmark, officials have finally seen enough. They stepped in and charged that 23 companies had violated the Marketing Practices Act with their deceptive environmental claims about wood burning.

Denmark’s Consumer Ombudsman ruled that wood stoves, firewood, and wood pellets may no longer be marketed as environmentally friendly or carbon neutral—because these claims are false. 

In a statement, the Consumer Ombudsman wrote:

When companies market wood-burning stoves and firewood, they must not give consumers the mistaken impression that heating by burning wood is environmentally friendly. It is misleading and therefore contrary to the Marketing Act. 

The Ombudsman shined a long overdue spotlight on the Swan certification scheme, which is comparable to EPA certified wood stoves here in the United States, noting that these certified wood stoves still emit “environmentally harmful particles…The marketing must therefore not give the consumer the impression that burning wood in a Nordic Swan-labeled stove is less harmful than it is.”

They also ruled that wood-burning stoves can no longer be marketed as being carbon neutral, because it is misleading. Burning wood emits CO₂ as well as other climate pollutants. In fact, for the same amount of heat, burning wood releases more CO₂ than fossil fuels.

The Danish Ombudsman stated that while virtually the entire industry had engaged in marketing their products as being less harmful to the environment than they actually are, they found one company’s claims to be so deceptive that they referred the matter to the police.

EPA Misses Historic Opportunity to Reduce Wood Smoke Pollution

Image: U.S. EPA

On February 7, following a multi-year process, the U.S. Environmental Protection Agency announced an update of the National Ambient Air Quality Standards for Particulate Matter. These standards set the maximum allowable levels of fine particulate matter (PM2.5). 

Scientists acknowledge that PM2.5 is one of the most hazardous of all air pollutants, and the standards are meant to protect the public by ensuring air pollution is kept below levels that are known to harm health. Under the Clean Air Act, the EPA is supposed to review, and revise if necessary, the standards every five years based on current scientific evidence. 

First for the good news: the EPA announced it is planning to lower the annual PM2.5 standard from 12 μg/m³ (micrograms per cubic meter) to 9 μg/m³.

However.

There are actually two PM2.5 standards. The second (and arguably more important standard) is measured over a 24-hour period. The 24-hour standard hasn’t been changed since 2006, the year the Dow Jones Index first crossed 12,000 (it’s now above 38,000). Since then, there has been a deluge of scientific evidence linking PM2.5, even at extremely low levels, to serious health problems and premature death.

Last year, thousands of people and organizations, including ours, submitted comments to the EPA urging them to lower the 24-hour PM2.5 standard. We pointed out in our submission that lowering the 24-hour standard was critical for protecting public health from wood smoke pollution.

During the rule-making process, the EPA acknowledged that the 24-hour standard is the more important one in communities affected by wood smoke pollution. They noted that in areas affected by localized sources of pollution, including wood burning, that the “24-hour standard is generally the controlling standard,” meaning it is the more relevant one for protecting air quality. 

The majority of the EPA’s own Clean Air Scientific Advisory Committee (CASAC) recommended lowering the 24-hour standard. They specifically expressed concern that the annual standard did not protect people in areas affected by wood smoke pollution.

So how did the EPA respond?

By deciding to leave the 24-hour standard unchanged at a whopping 35 μg/m³, the same level that was set back in 2006.

To put that in perspective, the World Health Organization recommends a 24-hour standard of 15 μg/m³.

This is not good news for the significant numbers of Americans affected by pollution from wood burning. 

According to the EPA’s own National Emission Inventory, more PM2.5 pollution in the U.S. comes from wood burning than from motor vehicles or from industrial processes. Wood burning is the largest source of PM2.5 pollution in many communities, such as the San Francisco Bay Area.

During the 18 long years since the 24-hour PM2.5 standard was last updated, the scientific evidence of harms from PM2.5 has only grown stronger. Will we have to wait another 18 years before the EPA finally lowers the 24-hour PM2.5 standard? 

And more importantly: how many people will needlessly suffer or die from wood smoke pollution in the meantime?  

Do Wood Stove Changeout Programs Actually Work?

We recently came across a page on the British Columbia Lung Association website that touts the success of their long-running wood stove changeout program. The page claimed that the program, which began in 2008, has “reduced particulate matter emissions by over 400 tonnes per year.”

This claim struck us—to put it politely—as surprising, so we wrote to the Lung Association asking them for details. They never replied.

What made us so skeptical of this claim? For one thing, an in-depth evaluation of the British Columbia wood stove exchange program published in 2014 noted that 6 years after the program began, “…there has not yet been a clear reduction in fine particulate matter pollution coming from residential wood stoves in BC.” Another study that compared pollution levels in homes before and after the program concluded, “There was not a consistent relationship between stove technology and outdoor or indoor air quality indoor concentrations of PM2.5.”

Perhaps things had turned around since those studies were published? Apparently not, since the BC Lung Association’s own 2018 “State of the Air” report shows that there were no significant decreases in PM2.5 (particulate matter smaller than 2.5 microns in diameter) from 2008 to 2017.

One might reasonably ask: What’s going on here?

Our educated guess is that the BC Lung Association’s claimed reduction in PM2.5 of 400 tons per year is likely a calculated reduction based on taking the certification values of EPA wood stoves and multiplying them by the number of wood stoves that have been changed out through the program.

This is like claiming that you lost 20 pounds based on counting how many calories you ate last month rather than actually weighing yourself.

So why wasn’t the program as effective in reducing wood smoke pollution as it was projected to be? The answer is simple: EPA-certified wood stoves do not perform as well in the real world as they do under the laboratory test conditions that are used to produce their certification values. Indeed, it is widely acknowledged by the EPA—and even by the hearth products industry—that wood stove certification values do not correlate well with the in-home performance of wood stoves.

One study found that certified wood stoves operated by homeowners in their own homes produced up to 30 times more particle pollution than their certification value.

The Trouble with EPA Certified Wood Stoves

There are many reasons why EPA certified wood stoves perform differently in the real world than they do in the test lab. In the real world, people burn wood that isn’t perfectly dried. They “damper down” their wood stove overnight or when their home gets too warm. They overfill the firebox with logs. In reality, it takes a lot of work to burn wood properly—and the end user doesn’t have a lot of incentive for doing so, since the majority of the pollution they create disappears up their chimney and out into the neighborhood.

While EPA certified wood stoves have been aggressively promoted as a panacea, in reality an improperly operated EPA certified wood stove emits more pollution than a properly operated conventional wood stove. Even if EPA certified wood stoves performed as well in real life as their lab certification values would suggest, a single stove would emit more fine particle pollution than hundreds of homes using a gas furnace (electric heat pumps produce almost no particle pollution).

Largely due to industry influence, the EPA test method is gamed to present an unrealistically optimistic picture of wood stove performance. For example, rather than logs, it uses kiln dried lumber that is arranged in a crib formation and specifically excludes the massive amount of pollution that is created when the wood stove is started up.

Wood Stove Changeout Programs: Disappointing Results

The sad reality is that the effectiveness of wood stove changeout programs that incentivize the sales of new EPA certified wood stoves has never been adequately demonstrated in real world studies. Yet in spite of this, due to pervasive industry influence, wood stove changeout programs have become the de facto response by air quality regulators to the problem of wood smoke pollution.

The Hearth, Patio and Barbecue Association is the main lobbying organization for the wood stove industry. On a website they host dedicated to wood stove changeout programs, there is a section dedicated to “success stories.” While these success stories include details about capturing “congressional earmarks” to fund new wood stove sales, they curiously do not include data showing actual air quality improvements (other than, once again, calculated estimates).

The one program that is constantly cited in favor of wood stove changeout programs was subsidized by the wood stove industry and took place in Libby, Montana. From 2005–2007, 95% of all the non-certified wood stoves in the community were replaced. In the year following the changeout, PM2.5 levels fell by 27%. But this was less than half the reduction that would be expected if the new, certified stoves actually emitted 70% less particulates than the old stoves, a figure cited by the EPA.

Notably, the relative contribution of wood smoke to PM2.5 levels was unchanged following the changeout, raising the question of whether the PM2.5 reduction was due to changes in wood stove technology or to another factor, such as newer cars on the road. Moreover, a study of air quality at two schools found that “…the changeout did not result in a measurable improvement on school indoor air quality.” When researchers looked at air quality inside houses following the changeout, 5 of 21 houses actually showed increased PM2.5 levels.

A Better Solution to Wood Smoke Pollution

If changeout programs that subsidize sales of new EPA certified wood stoves don’t work, what does?

Somewhat ironically, the 2014 report on the British Columbia program showed the way forward, stating: “For many communities, the elimination of wood burning for heating may be the desired path to reduce exposure to pollutants.” In other words, if you want to eliminate wood smoke pollution, don’t encourage or subsidize the use of new wood stoves. Instead, subsidize only cleaner heating devices, such as electric mini-split heat pumps or gas heaters.

A changeout program in Launceston, Australia attempted to do just that, offering incentives to encourage homes that were heating with wood to switch to electric heat. Even though only half of the homes that were using wood heat converted to electric heating, particle pollution levels fell by 40%.

Despite the evidence that the British Columbia wood stove exchange program has not reduced particulate matter pollution from wood smoke, the 2014 report on the British Columbia wood stove exchange program states that “…the program has been very successful.” If PM2.5 levels haven’t dropped, how is success measured? Apparently by the number of stoves that are exchanged, even if air quality isn’t improved. This might be a win for the wood stove industry, but it isn’t a win for the residents of British Columbia who must live with the harmful effects of wood smoke pollution.

It’s not difficult to see why the wood stove changeout scheme rolls on: it gives regulators an easy way out by allowing people to continue burning wood. In reality, however, the idea makes as much sense as using public funds to subsidize light cigarettes for tobacco users instead of encouraging them to quit smoking.

(Photo: Kim Nilsson)