Many people who use wood-burning stoves are unaware of the associated health hazards from indoor air pollution, especially for children and older adults. Cleaner, safer forms of domestic heating are now available that provide the added bonus of lower greenhouse gas emissions.
The cheerful glow of a wood-burning stove creates a cozy atmosphere on a cold winter’s night, but the aesthetic appeal of wood burners comes at a high price for human health.
The Environmental Protection Agency (EPA) warns that wood smoke is largely responsible for poor air quality during winter months in many residential areas across the United States.
Burning wood, in addition to producing toxic gases such as nitrogen oxides and carbon monoxide, also generates tiny, solid particles called particulates.
“The particle pollution is especially dangerous because these particles are so tiny that they can travel deep into the lungs, causing irritation and inflammation,” said Dr. John M. James, medical specialist and spokesperson at the Asthma and Allergy Foundation of America.
“Immediate exposure can cause coughing, wheezing, shortness of breath, chest tightness, and asthma attacks,” he told Medical News Today.
“Chronic exposure can lead to an overall decline in lung function and chronic bronchitis,” he added.
According to the EPA, particulate matter (PM) can also cause heart attacks, stroke, irregular heart rhythms, and heart failure, especially in people who are already at high risk for these conditions.
“Wood smoke can irritate your lungs, cause inflammation, affect your immune system, and make you more prone to lung infections, likely including SARS-CoV-2, the virus that causes COVID-19.”
– Environmental Protection Agency
The smaller the particulates, the more easily they pass from the lungs into the bloodstream and throughout the body.
The most harmful particles are therefore less than 2.5 micrometers (thousandths of a millimeter) across and are known as PM2.5.
In addition to domestic wood-burning, other sources of PM2.5 include power stations, the engines of motor vehicles, and rubber tires as they wear down.
The particles lodge in the lungs, heart, brain, and other organs, where they can have serious impacts on health, especially for vulnerable individuals such as children, older adults, and people with pre-existing conditions.
According to the British Lung Foundation, PM2.5 can cause chronic obstructive pulmonary disease (COPD) and lung cancer. There may also be links between PM2.5 exposure and diabetes, as well as brain diseases such as Alzheimer’s and Parkinson’s.
Low birth weight has also been found in babies of mothers who have been exposed to PM2.5 during pregnancy.
A recent study found that increased exposure to PM2.5 may also be associated with poorer reasoning and problem-solving abilities, and a higher risk of anxiety and depression.
The EPA reports that indoor PM levels can exceed those outdoors. Sources include any kind of combustion, from cigarette smoking, cooking, and candles, to open fireplaces, wood stoves, and unvented gas or kerosene heaters.
Wood is a relatively cheap fuel, which makes it an especially appealing way to heat homes when oil and gas prices are soaring.
Wood stoves have also become popular in recent years for their aesthetic appeal. However, they are a major source of particulates, both indoors and outdoors.
A recent review found that open fires generate the most particulates for each unit of heating, followed in descending order by multi-fuel stoves, and then stoves that burn wood pellets.
A study in 2020 found that indoor levels of PM2.5 were three times as high in households that used wood stoves compared with those that did not. The research suggested that opening the stove door to add more fuel caused spikes in indoor particulate levels.
Oil and gas boilers also produce particulates, but the total contribution of wood-burning in stoves and open fires to overall particulate pollution may come as a surprise.
In the United Kingdom, for example, a government report estimates that the use of wood as a fuel accounted for 70% of PM2.5 emissions from domestic combustion in 2020.
Emissions of PM2.5 from this source increased by 35% between 2010 and 2020 to reach 17% of total PM2.5 emissions.
This suggests that in 2020, wood-burning in U.K. homes produced more particulate pollution than all road traffic, which accounted for 13% of PM2.5 pollution.
Wood stoves are a popular way to heat homes in rural areas, where they can cause high levels of indoor and outdoor particulate pollution. Low-income communities may be especially vulnerable.
Households that had not cleaned their chimney recently, and those that were not using high quality stoves, had considerably higher indoor PM2.5 levels.
To minimize indoor air pollution, the EPA recommends that people:
- Avoid the use of unvented stoves, fireplaces, or fuel-burning space heaters indoors.
- Choose a wood stove that has been certified as compliant with EPA emission standards, and ensure that the door fits tightly.
- Use an appropriate fuel in stoves and fireplaces, such as well-seasoned, dry wood.
However, there is no such thing as a pollution-free wood burner.
“Even the best designed and best-operated wood-burning (or composite wood pellet-burning) stove nevertheless produces some air pollution,” said Kevin M. Stewart, director of environmental health advocacy and public policy at the American Lung Association.
“Even if venting of exhaust is ideal and all products of combustion are released into the outdoor air, those emissions frequently remain in close proximity to the residence that emits them, and it is common for some of those emissions to become re-entrained into the air that is brought into the home,” he told MNT.
He pointed out that the pollutants vented from a stove can also enter neighboring homes.
Even stoves that meet the highest environmental standards can cause more outdoor air pollution than a truck.
For example, researchers found that — for the same energy output — wood burners that met the European Union’s 2022 EcoDesign standard produced 750 times as much PM2.5 as a heavy goods vehicle.
The scientists also warn of a spike in indoor particulate pollution whenever someone opens the stove door to add more fuel.
Wood pellet boilers avoid this problem because they use a closed, automatic system to feed the pellets into a furnace.
They are also highly efficient and have low particulate emissions. In addition, their fuel is sustainable because it is made from waste wood.
On the downside, wood pellet boilers are expensive, they require frequent maintenance and a large storage space for the pellets.
In addition to the EPA recommendations for minimizing emissions from wood stoves (see above), Dr. James of the Asthma and Allergy Foundation of America has some further advice:
“[M]ake sure the area where the wood-burning stove is used is properly equipped […] This includes a quality air filtration device, maintaining smoke alarms, installing a carbon monoxide detector, keeping a fire extinguisher handy and in proper working condition, and keeping anything flammable away from the stove.”
The good news is that a hi-tech alternative to burning fuel of any kind is now available. Known as a heat pump, it produces no direct pollution or greenhouse gas emissions.
A heat pump captures and concentrates the heat outside a house — either from the open air or underground — and brings it indoors.
Like a refrigerator or air-conditioning unit, it uses electricity to alternately vaporize and condense a refrigerant.
The refrigerant absorbs heat from the atmosphere or ground as it vaporizes, then releases the heat indoors as it condenses into a liquid.
The pump uses less electricity than an ordinary electric heater, such as a fan heater, for the same amount of heat output.
Installation costs are a drawback for most heat pumps, though. However, newer, “high-temperature” designs have the potential to reduce these costs.
Conventional, “low-temperature” heat pumps operate most efficiently at around 35–45°C, compared with temperatures of 60–80°C in a gas or oil-fired central heating system.
Installing one of these pumps, therefore, entails extra home insulation and an overhaul of the central heating system to increase the surface area of radiators, or install underfloor heating.
Newer, high-temperature pumps, however, can deliver temperatures of 60–80°C. So, in theory, they can be incorporated into an existing central heating system.
These pumps, such as the ones made by the Swedish company Vattenfall, raise temperatures by using carbon dioxide or propane rather than a conventional refrigerant. They also use a large indoor water tank to store or “buffer” the heat.
“Basically the heat pump uses CO2 rather than a synthetic product like a lot of traditional heat pumps,” explained Emily Faull, a spokesperson for Vattenfall.
“There is also a buffer (a giant water tank) which supplies hot water to the house as well as heating,” she added.
However, it is worth noting that high-temperature heat pumps use more electricity than the conventional type, which increases their running costs.
Another clean alternative to biomass and fossil fuels is hydrogen, which produces no greenhouse gases or particulates when it burns.
Research and early trials are underway to investigate the safety of hydrogen as an alternative fuel source for heating homes.
In the meantime, one of the fastest-growing technologies is “district heating,” which involves piping heat from factories, waste incinerators, or underground (“geothermal” heat) into homes.
This works well in areas of high population density, such as cities, but less well in rural areas.
Much research and investment lie ahead, but technologies like these offer a win-win for tackling climate change and improving public health.
The World Health Organization (WHO) estimates that indoor air pollution currently causes 3.8 million premature deaths every year.
So while it may not have the aesthetic appeal of a wood stove or an open fire, low-carbon heating has the potential to save millions of lives worldwide in the coming decades.