Introduction

Trees have often been called the lungs of the world for their respiratory-like ability to release breathable oxygen into the air. But they could equally be considered the liver of the world, as they also filter atmospheric toxins and make the air cleaner and healthier to breathe.

Urban areas typically feature worse air quality than their rural counterparts, underscoring the value of city trees and their power to mitigate the effects of airborne pollutants. In addition to their many environmental benefits, trees both directly and indirectly improve air quality — particularly around high-pollution areas, like highways. Let’s examine the ways in which urban forests help clean our air.

The Hazards of Polluted Air and the Cleaning Power of Trees

According to Vibrant Cities Lab, urban air pollution causes 200,000 premature deaths per year in the United States. This sad reality reflects the many health hazards posed by breathing polluted air: stroke, heart disease, cancer, pneumonia, diabetes, cognitive impartment, and neurological diseases, among others.

And the problem is far more rampant than you might expect: The World Health Organization states that 99% of the earth’s population in 2019 lived in a location that failed to meet their air quality guidelines. This is a startling fact which highlights the importance of mitigation solutions — including urban forestry.

Trees have the ability to directly absorb and sequester pollution, like carbon; they likewise can temporarily “catch” pollutants in the air. The National Park Services states that “trees absorb gaseous molecules in the air. Tiny pores on the tree leaf surfaces called stomata take in air that includes toxic pollutants. This means that pollutants like SO2, NO2, CO, and ozone are permanently converted inside the leaf.” Trees can also “remove particulate matter by ‘catching’ them temporarily. With rain or precipitation, the particulates can be dissolved in the stormwater runoff or transferred to the soil.”

While no mitigation device solves air pollution, the scale and value of city trees in this regard is dramatic. The Conservation Law Foundation estimates that trees absorb 45 million tons of carbon every year. Even a single tree offers air-quality benefits: The Tennessee Environmental Council notes that one mature tree every year can remove 400 pounds of carbon dioxide, provide $1,250 in air pollution controls, and generate $638 worth of oxygen.

There are also indirect benefits to healthy urban trees. Large canopies lower air temperature, which alters pollution concentration; likewise, the shading provided by trees reduces human reliance on energy-consuming (and pollution-emitting) air conditioning.

It’s also important, however, to think about the placement of these trees — which often provide the most air-quality value along busy streets or highways, as both visual buffers and biofilters for air pollution.

Fighting Roadway Pollution

One of the leading producers of air pollution across the world is car travel, which emits carbon dioxide into the atmosphere. Of the 200,000 premature deaths in the United States attributed to polluted air, more than 50,000 of them are from vehicle emissions.

Highways and major streets are, unsurprisingly, areas with higher concentrations of vehicle-emission pollution. Research by the Journal of Exposure Science & Environmental Epidemiology found that PM10 pollutant particles were 20-60% higher within 150 meters of highways. Higher pollution concentration means higher health hazards: a study by The Lancet suggests that living near highways raised the risk of respiratory and cardiovascular disease by 20-30%.

Urban forests are a crucial weapon in the fight against car pollution. Strategic placement of these tree plantings is also an influential component of their air-quality performance. Trees along highways serve as a powerful pollution intervention tool, capturing as much pollution as possible at its source and reducing horizontal spread.

Overall, U.S. urban trees offset emissions from 10 million cars (according to CLF). And trees directly adjacent to major roadways, according to a Georgia State University study, reduced soot by up to 37% and reduced ultrafine particles up to 7%.

Researchers have found that “vegetative barriers can act in two ways against pollution, such as car exhaust: directly blocking it, and also absorbing it. It’s possible that up to 50% of particulate matter could be reduced after the trees grow to maturity.”

Conclusion

In addition to all the environmental advantages of city trees, pollution mitigation is essential to keeping our urban air healthy. Positioning these trees near high-pollution zones — like, as discussed, highways and major thoroughfares — is where they provide the most value in this regard.