Throwing another log into a crackling fireplace on a cold winter’s night might seem like a cozy, harmless tradition. But Northwestern University scientists have found residential wood burning is a major—yet often overlooked—contributor to winter air pollution across the United States.

Although only 2 percent of US homes rely on wood as their primary heating source, residential wood burning accounts for more than one-fifth of Americans’ wintertime exposure to outdoor fine particulate matter (PM2.5), the new study found. 

These tiny airborne particles can 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. Among their findings, the scientists calculated that pollution from residential wood burning is associated with about 8,600 premature deaths per year. 

Surprisingly, the majority of those most affected live in urban, not rural areas. The health burden also disproportionately falls on people of color, 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, Americans could decrease outdoor air pollution, resulting in major health benefits and thousands of saved lives.

The study was published Jan. 23 in the journal Science Advances.

“Long-term exposure to fine particulate matter is associated with an increased risk of cardiovascular diseases,” said Northwestern’s Kyan Shlipak, who led the study. “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.”

“We frequently hear about the negative health impacts of wildfire smoke, but do not often consider the consequences of burning wood for heat in our homes,” said Northwestern’s Daniel Horton, the study’s senior author. “Since only a small number of homes rely on wood burning for heat, facilitating a home-heating appliance transition to cleaner burning or non-burning heat sources could lead to outsized improvements in air quality.”  

Horton is an associate professor of Earth, environmental, and planetary sciences at Northwestern’s Weinberg College of Arts and Sciences, where he directs the Climate Change Research Group (CCRG). Shlipak is an undergraduate in mechanical engineering at Northwestern Engineering and a member of the CCRG.

Neighborhood by neighborhood 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 team first gathered residential wood-burning data from the National Emissions Inventory (NEI), the US 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.

Then, the Northwestern team 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,” Horton said. “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, Shlipak and Horton divided the continental US into a grid of 4-kilometer by 4-kilometer 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 neighborhood-scale grid enabled the research team to pinpoint hotspots. 

The team 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 percent of PM2.5 pollution in winter, making it one of the single largest sources of fine particle pollution during the country’s coldest months.

Vulnerable populations bear the burden

Shlipak and Horton 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.

We estimate that long-term exposure to emissions from wintertime wood burning is associated with approximately 8,600 deaths per year, and this estimate does not account for particulate matter exposures in other seasons. Kyan Shlipak

“Our results suggest that the impacts of residential wood burning are primarily an urban and suburban phenomenon,” Shlipak said. “This finding underscores the public health relevance of this pollution. We estimate that long-term exposure to emissions from wintertime wood burning is associated with approximately 8,600 deaths per year, and this estimate does not account for particulate matter exposures in other seasons.”

To determine who is most affected, the researchers combined pollution estimates with US 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 percent higher adverse health effects from residential wood burning than the citywide average.

“While a lot of emissions from residential wood burning come from the suburbs, pollutants emitted into the air don’t typically stay put,” Horton said. “When this pollution is transported over densely populated cities, more people are exposed. Because people of color tend to be more susceptible to environmental stressors due to the long tail of past discriminatory policies, we estimate larger negative health outcomes for people of color.” 

“People of color face both higher baseline mortality rates and higher rates of exposure to pollution from wood burning,” Shlipak said. “However, people of color are correlated with lower emissions rates, indicating that a large fraction of this pollution is transported to these communities, rather than emitted by them.”

Shlipak and Horton 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.