Sulfur dioxide, nitrogen oxides, particulate matter, fluorides, carbon dioxide, ozone. What do all of these hard-to-pronounce things have in common? They are all making their way into your body when you breathe. That’s right, these air pollutants are everywhere, even when you can’t see them. In cities, there’s a mouthful in every breath.
There are two types of air pollutants: primary and secondary. Primary pollutants are toxic as soon as they are released into the air and typically have a source that can be pinpointed. The biggest threats in this category in cities are particulate matter (PM), sulfur dioxide (SO2) and fluorides. Secondary pollutants, on the other hand, form in the air from interactions whose components might not have been toxic on their own. The major secondary pollutant we find in cities is ozone (O3).
When we talk about ozone as an air pollutant, we’re referring to ground-level ozone (which we don’t like) as opposed to stratospheric ozone (which we do like) that creates a layer in the atmosphere protecting us from UV rays. Ground-level ozone is common in areas with dense populations and traffic because ozone forms when hydrocarbons and nitrogen oxides (NOx) from industry and automobile emissions interact with sunlight.
Particulate matter consists of microscopic particles from car exhaust, road dust, industry and other emissions. It is usually measured in two categories according to size: PM10, the larger kinds, and PM2.5, the smaller and more dangerous. The smaller the particle, the deeper into your lungs it can travel, and once it’s down there, it stays there. This leads to respiratory illnesses like asthma and lung cancer — outdoor PM causes 3.2 million deaths every year worldwide. SO2 and fluorides are produced by fossil fuel combustion, which of course there’s a lot of in cities.
With the known health consequences of respiratory illness, cardiovascular disease, and lung cancer, it is clear that we should avoid exposure to these toxic air pollutants, but we don’t always have a choice. People in cities are especially vulnerable, since they have such frequent exposure to high concentrations of them. According to the World Health Organization, concentrations of PM exceed safe levels on the streets of more than 600 U.S. cities. Thankfully, city trees and greenery offer the beginnings of a solution to urban air pollution.
A study in London linked the annual removal of 90.4 tons of PM10 by urban trees to a decrease in 2 deaths and 2 hospitalizations per year. And according to a study in the U.S., the amount of PM2.5 removed annually by trees in 10 cities across the country in 2010 ranged from 4.7 tons in Syracuse to 64.5 tons in Atlanta. In the same cities, estimates of the annual monetary value of human health effects associated with PM2.5 removal, such as hospital admissions, respiratory symptoms and related deaths, ranged from $1.1 million in Syracuse to $60.1 million in New York City. That’s right: trees save lives and money.
Strategic placement of grass, ivy and other plants in cities can reduce the street level concentrations of NO2 and PM by 40 and 60 percent, respectively. There are multiple ways trees help to make urban air cleaner by filtering out pollutants:
Temperature Reduction
Lowering temperatures reduces the movement of harmful ambient particles and prevents more pollutants from evaporating into the air. Trees create a great cooling effect by shading homes and streets, breaking up urban heat islands, and releasing water vapor into the air through their leaves. More tree crowns mean less dark surfaces like parking lots and paved streets being exposed to sunlight and emitting heat. Tree canopy cover in Los Angeles has decreased over the last 50 years, and a corresponding 6°F increase has been measured. Depending on the tree placement, trees can cool a city by up to 10°F, reducing the concentration of PM and other air pollutants with each degree.
Removal of Pollutants
The first way trees remove air pollution is by particle interception: trapping pollution particles on their leaves and bark. Once the particle has been removed from the air, it is usually washed off the tree by rain or falls onto the ground with leaves and twigs. Studies have shown that in one urban park, tree cover removed 48 pounds of PM, 9 pounds of NO2, 6 pounds of SO2 and 100 pounds of carbon — daily. Silver birch trees in particular have been studied for their particle interception abilities: They have been found to reduce concentrations of PM by more than 50 percent.
A more complex way that trees filter the air is through gas uptake by leaf stomata. The stomata are tiny pores on tree leaves, and they absorb air to collect CO2 in order to perform photosynthesis. During that uptake of air, they also absorb gaseous pollutants in the air. Once inside the leaf, the gas diffuses throughout the leaf’s pores. It is then absorbed by films of water inside the leaf where it will either form acids or react with inner-leaf surfaces to become less toxic. It is estimated that one tree can absorb almost 10 pounds of polluted air through its leaf stomata every year.
Energy Effects on Buildings
Now that we understand trees’ chemical abilities, we can factor in their physical ones. Trees shade buildings in the summer and block winds in the winter, so it makes sense that they reduce building energy use for both heating and cooling purposes. Minimizing energy needs lowers the amount of fuel combustion necessary and therefore reduces the amount of pollution from power plants entering the air in the first place.
Once seen as an aesthetic window dressing, trees have never seemed as important in cities as they do now that we know their full potential. We can’t decide to stop breathing when we walk down a city street, but we can decide to plant and maintain healthy trees and hedges in cities to support the cause for greener cities. The air we breathe is a little bit cleaner thanks to each and every tree. Let’s keep it that way.