By Anna Wright
One of the biggest benefits of trees is their ability to improve air quality. Urban forests are often called a city’s lungs because they produce oxygen, store carbon, and reduce particulate matter pollution. However, some trees are better at it than others!
Urban trees improve local air quality by reducing particulate matter — tiny particles of pollutants, chemicals, and materials like brake dust that float through the air. When inhaled, such particles can damage a person’s lungs and exacerbate existing health conditions. Particulate matter exposure has been linked in numerous studies to asthma, lung diseases, and cardiovascular diseases. In San Francisco, vehicles are one of the biggest sources.
For neighborhoods near freeways and other major roadways, trees provide an essential service by reducing particulate matter exposure in two important ways. First, their leaves disperse large clouds of particulate matter, which dilutes it and reduces the amount inhaled by humans. Second, they trap the particles; particulate matter sticks to tree leaves until it’s eventually washed away by rain.
Trees with large leaf surface areas and large canopies are most effective at dispersing, diluting, and trapping particulate matter concentration. The denser the foliage, the better! Waxy, rough, or hairy leaf surfaces trap particles better than glossy surfaces.
Due to weather patterns, San Francisco’s particulate matter levels are much higher in winter. Evergreen trees, which retain their leaves in winter, reduce particulate matter exposure during this most critical time.
Five trees commonly planted in our urban forest meet the criteria laid out above.* If you want a tree that works hard to improve air quality, consider the options in the slides below when choosing a species.
*Most of these species have not been studied for particulate matter capture. This list was created using the leaf and bark characteristics identified in particulate matter studies.
Zhang WK, Wang B, Niu X. Study on the Adsorption Capacities for Airborne Particulates of Landscape Plants in Different Polluted Regions in Beijing (China). Int J Environ Res Public Health. 2015;12(8):9623-9638. Published 2015 Aug 14. doi:10.3390/ijerph120809623, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4555302/