While only 2% of U.S. homes rely on wood as their primary heating source, residential wood burning accounts for 22% of fine particulate matter in winter air. Researchers estimate 8,600 premature deaths per year are associated with wood-burning fireplaces, furnaces and stoves. This is due to outdoor fine particulate matter (PM2.5) levels.
Throwing another log into a crackling fireplace on a cold winter’s night might seem like a cosy, harmless tradition. However, Northwestern University scientists have found residential wood burning is a major — yet often overlooked — contributor to winter air pollution across the U.S.
This arises because tiny airborne particles are able to penetrate deep into the lungs and enter the bloodstream, where they are linked to increased risks of heart disease, lung disease and even premature death.
Surprisingly, the majority of those most affected live in urban, not rural areas. The health burden also disproportionately falls on people of colour, who burn less wood yet experience higher exposure levels and greater health harms related to wood-smoke pollution. This is likely due to higher baseline mortality rates and a long history of past discriminatory policies.
By reducing indoor wood burning, U.S. citizens could decrease outdoor air pollution, resulting in major health benefits and thousands of saved lives.
“Long-term exposure to fine particulate matter is associated with an increased risk of cardiovascular diseases,” says Northwestern’s Kyan Shlipak, in a research brief. “Studies have shown consistently that this exposure leads to a higher risk of death. Our study suggests that one way to substantially reduce this pollution is to reduce residential wood burning. Using alternative appliances to heat homes instead of burning wood would have a big impact on fine particulate matter in the air.”
Neighbourhood by neighbourhood analysis
For decades, air-quality research and policies have focused on emissions from vehicles, power plants, agriculture, industry and wildfires. However, in the new study, Shlipak, Horton and their collaborators turned to a much less studied and often overlooked source of pollution: wood burning in homes, including emissions from wood-burning furnaces, boilers, fireplaces and stoves.
The researchers first gathered residential wood-burning data from the National Emissions Inventory (NEI), the U.S. Environmental Protection Agency’s comprehensive and detailed account of air pollution sources. The NEI bases its wood-burning emissions estimates on national household surveys, housing data, climate conditions and appliance types.
Following this, the researchers used a high-resolution atmospheric model to simulate how pollution moves through the air. The model accounts for weather, wind, temperature, terrain and atmospheric chemistry to estimate air quality over time.
Wood burning emissions enter the atmosphere, where they are affected by meteorology. Some emissions are considered primary pollutants, such as black carbon, and some interact with the atmosphere and other constituents, and can form additional, secondary species of particulate matter pollution.
To capture precise patterns of these pollutants, the researchers divided the continental U.S. into a grid of 4-kilometer by 4-kilometre squares. For each square, they modelled the amount of pollution generated each hour, how the pollution moves through the air and where it accumulates or disperses over time. Rather than averaging particulate matter across entire cities or counties, the neighbourhood-scale grid enabled the research team to pinpoint hotspots.
The scientists ran the model twice — with residential wood burning emissions and without them — and compared the two simulations. Then, they attributed the difference in pollution levels to wood burning. The results showed that residential wood burning comprises about 22% of PM2.5 pollution in winter, making it one of the single largest sources of fine particle pollution during the U.S.’s coldest months.
Vulnerable populations bear the burden
It was found that particulate matter from wood burning is particularly problematic in cities and suburban communities due to the combined effects of population density, emissions density and atmospheric transport. In many cities, smoke from surrounding suburbs drifts into more densely populated urban cores, which have limited wood-burning emissions. Even cities not typically associated with wood burning, such as those in warmer climates, can experience impacts from wood burning during cold snaps, recreational burning and atmospheric transport.
To determine who is most affected, the researchers combined pollution estimates with U.S. census data and census-tract-level mortality data. The researchers found that although people of color burn less wood, they experienced higher exposure levels and greater harms from wood-burning pollution. In the Chicago metropolitan area, for example, the researchers estimate that Black communities face more than 30% higher adverse health effects from residential wood burning than the citywide average.
The researchers note their study only looks at the outdoor impacts of exposure to wood-burning pollution. Additional impacts from indoor exposure to particulate matter also have public health consequences but were not included in this study.
The study, “Ambient air quality and health impacts of PM2.5 from U.S. residential wood combustion,” has been published in the journal Science Advances.
