Trees keep our air fresh and clean so that we can breathe easier and safer.  Urban trees help improve air quality because they remove carbon dioxide and other greenhouse gases from the atmosphere in a process called photosynthesis and return oxygen back into the atmosphere.

Greenhouse gases are gases that trap heat within them and prohibit the heat from being released into space.  This phenomenon is known as the “greenhouse effect.” Greenhouse gases, primarily carbon dioxide, are absorbed by trees and turned into carbohydrates that the plants then use to fuel their growth.

Trees are known as “carbon sinks” because of their ability to remove carbon dioxide from the atmosphere.  One acre of new forest has the ability to remove or sequester approximately 2.5 tons of carbon dioxide from the atmosphere on average.  Young trees are capable of absorbing carbon dioxide at a rate of 13 pounds per tree each year.

When a tree is ten years old, they are able to absorb an estimated 48 pounds of carbon dioxide every year.  Trees then release enough oxygen back into the atmosphere to provide fresh air for two human beings.

According to the Worldwatch Institute in their paper “Reforesting the Earth,” for every ton of new trees that grow up, about 1.5 tons of carbon dioxide is removed from the air, and 1.07 tons of life-giving oxygen is produced.

If every family in America planted one tree, the amount of carbon dioxide in the atmosphere would decline by one billion pounds every year.  By planting 100 million trees, you could reduce an estimated 18 million tons of carbon dioxide from the atmosphere per year and save Americans over $4 billion in utility bills.

The shade that trees provide reduces the need for air conditioning in homes and office buildings.  Burning fossil fuels to supply the electricity for air conditioning releases greenhouse gases into the atmosphere.  Trees are capable of reducing the need for air conditioning by up to 30 percent which as a result reduces the amount of fossil fuel burned to produce electricity.

According to the United States Forest Service, trees offset approximately 25 percent of human-caused carbon dioxide emissions from 1952 to 1992.  A tree is capable of generating almost $32,000 worth of oxygen and providing $62,000 worth of air pollution control during a 50-year lifespan. That same tree also recycles over $37,000 worth of water and controls over $30,000 worth of soil erosion.

Trees remove other air pollutants besides carbon dioxide such as sulfur dioxide, ozone, nitrogen oxides, and particulates.  The stomata on the surface of tree leaves absorb these gaseous pollutants with normal air components.

Particulates are small particles emitted in the smoke from burning fossil fuels like diesel.  Particulates enter our lungs and cause respiratory problems. Some studies suggest that at the current rate, particulate pollution could claim an estimated 6.2 million lives every year by 2050.  Trees can reduce up to 60 percent of street-level particulates in urban settings.

One study from The Nature Conservancy (TNC) examined the use of trees in 245 cities around the world.  The study reported that the average reduction of particulate matter near just one tree was between 7 and 24 percent.  The cooling effect of the tree was up to 2°C or 3.6°F.

Doctor Rob McDonald of the TNC told the BBC that, “All the cities we looked at, if all the people in them spent an extra US $4 a year on planting trees, you could save between 11,000 and 36,000 lives each year.  This is mostly as a result of having cleaner air.”

In just one day, studies have shown that one urban park tree covering was capable of removing 6 pounds of sulfur dioxide, 0.5 pounds of carbon monoxide, 9 pounds of nitrogen dioxide, 48 pounds of particulates, and 100 pounds of carbon from the air.

The United Nations World Health Organization (WHO) says that the air quality in many cities is not monitored, so it is challenging to get accurate understandings of the global impact of air pollution.  However, in 2014 the WHO reported that about 90 percent of the global population living in cities was exposed to particulate matter pollution that exceeded the WHO air quality guidelines.

“In this urban century when there are going to be an extra two billion people in cities, smart cities should be thinking about how nature and trees can be part of the solution to keep air healthy,” said Doctor McDonald.

These studies demonstrate that public health is closely related to the amount of greenery in the spaces that we live in.  It is vital that urban trees become integral parts of our cityscapes in order to keep our air clean and our buildings cool.  The more people learn about the benefits of urban tree planting, the more we can work together to make our cities greener.