Tree Science

How Trees Clean the Air We Breathe: What Science Shows

Air pollution is a major public-health concern, contributing to asthma, heart disease, lung cancer, and premature death worldwide. While technology plays a role in reducing emissions, one of the most effective and natural solutions grows outside our windows: trees. Decades of research confirm that trees significantly improve air quality by removing pollutants, cooling the atmosphere, and reducing the formation of harmful gases.

Trees Are Natural Air Filters

Trees physically capture airborne pollutants on their leaves, needles, bark, and branches. Particles such as dust, soot, smoke, pollen, and tire and brake-wear residue cling to tree surfaces and are later washed to the ground by rain or fall with leaf litter.

A U.S. Forest Service study found that trees remove over 17 million tons of air pollutants annually in the United States, including particle pollution and gases linked to respiratory and cardiovascular disease (Nowak & Greenfield, 2018). This particle removal contributes to measurable reductions in illness and healthcare costs.

Trees Absorb Harmful Air Pollutants

Through tiny leaf pores called stomata, trees absorb gaseous pollutants from the atmosphere. These pollutants include:

• Nitrogen dioxide (NO₂) – A traffic-related pollutant linked to asthma and reduced lung function

• Ozone (O₃) – A major respiratory irritant that triggers asthma attacks

• Sulfur dioxide (SO₂) – Associated with bronchitis and lung inflammation

• Carbon monoxide (CO) – Can reduce oxygen delivery in the body

Once absorbed, trees convert or store these compounds, reducing their concentration in the air (Nowak et al., 2014).

Trees Produce Oxygen and Store Carbon

During photosynthesis, trees use sunlight to convert carbon dioxide (CO₂) and water into oxygen and glucose. This not only removes CO₂—a greenhouse gas—from the atmosphere but also supplies the oxygen needed to sustain life.

A mature tree can provide enough oxygen annually for two to four people, depending on species and size (American Forestry Association, 2021).

Trees Reduce Heat and Slow Pollution Formation

Heat increases the formation of ground-level ozone, a pollutant that irritates lungs, worsens asthma, and increases hospitalization risks. Tree canopy cools urban areas through shading and evapotranspiration—the release of water vapor from leaves—which can lower air temperatures by 2–9°F (The Nature Conservancy, 2016).

Cooler air means less ozone formation and fewer heat-related pollutants, providing dual health benefits, especially in cities.

Trees Reduce Pollution Exposure Near Roads

Traffic pollution is a major source of harmful particulate matter (PM2.5), nitrogen oxides, and toxic compounds. Research shows that planting trees near roads and highways can significantly reduce pollutant concentrations.

Vegetation barriers can reduce particulate pollution by up to 52%, depending on species and canopy density (Baldauf, 2017). This is especially important for protecting children, seniors, and individuals with asthma or COPD.

Why Cleaner Air Improves Health

Improved air quality translates to better public health. Reduced exposure to particle pollution and harmful gases is associated with:

• Better lung function and fewer asthma attacks

• Lower rates of chronic respiratory disease

• Reduced cardiovascular strain

• Fewer premature deaths attributable to pollution

Communities with higher tree canopy coverage experience lower rates of asthma and respiratory distress, especially among children and older adults (Mueller et al., 2022).

What Communities Can Do

To maximize health benefits, communities can:

• Protect mature trees, which provide the greatest air-quality benefits

• Increase tree canopy in urban “hot spots,” school zones, and near busy roads

• Select species with high leaf surface area and pollution-tolerant traits

Urban forestry is one of the most cost-effective public-health strategies available and yields long-term benefits for both environmental and human health.

Conclusion

Trees are not merely decorative—they are a scientifically proven public-health tool. By filtering pollutants, absorbing harmful gases, cooling urban areas, and improving overall air quality, trees protect the air we breathe and the health of our communities. Investing in tree preservation and canopy expansion is an investment in cleaner air, healthier lungs, and longer lives.

References (APA 7th Edition)

American Forestry Association. (2021). The benefits of trees: Urban forests and public health. American Forests. https://www.americanforests.org

Baldauf, R. (2017). Roadside vegetation design characteristics that can improve local, near-road air quality. Multidisciplinary Digital Publishing Institute, 4(3), 16. https://doi.org/10.3390/atmos4030016

Mueller, W., et al. (2022). Exposure to urban greenspace and pathways to respiratory health: A global review. Science of the Total Environment, 806, 150577. https://doi.org/10.1016/j.scitotenv.2021.150577

Nowak, D. J., & Greenfield, E. J. (2018). U.S. urban forest statistics, values, and projections. Journal of Forestry, 116(2), 164–177. https://doi.org/10.1093/jofore/fvx004

Nowak, D. J., Hirabayashi, S., Bodine, A., & Hoehn, R. (2014). Modeled PM2.5 removal by trees in ten U.S. cities and associated health effects. Environmental Pollution, 193, 119–129. https://doi.org/10.1016/j.envpol.2014.05.035

The Nature Conservancy. (2016). Planting healthy air: A global analysis of the role of urban trees in addressing particulate matter pollution and extreme heat. The Nature Conservancy. https://www.nature.org