Within the next 35 years, the majority of the world’s population will live in cities. Already, densely packed urban centers pose challenges to preserving residents’ quality of life and health and the city’s air quality. Fine particulate matter (PM2.5) is chief among the concerns. In fact, as city populations rise, the mortality rate from diseases caused by PM will also spike. So research team at The Nature Conservancy, a nonprofit focused on finding solutions to large environmental problems, set out to figure out how well urban vegetation might help offset air pollution.
The team focused on 245 cities, which currently house some 910 million people or one-quarter of the global population. They collected PM2.5 levels and information on the density of trees, focusing primarily on street trees. They found that trees, through a process called dry deposition, already provide some level of PM2.5 relief for more than 63 million people in their study group. Trees have the greatest impact to areas within 30 meters (roughly 100 feet) of where they’re planted, but can have some effect up to 300 meters (1,000 feet) away. Most notably, homes on a tree-lined street experience a 50-percent drop in indoor PM concentrations.
If done strategically — that is in high-population areas and with a selection of species known to trap particles well, such as red cedar, red maple, and elm trees — a further 115 million people could experience some reduction in PM exposure. Tree-planting, especially in densely packed neighborhoods, is part of an effective strategy to reduce particulate-based air pollution both indoors and outdoors.