Trees Please Hamilton

Green Solutions to Air Pollution

The Effects of Trees on Air Quality

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Air quality and the presence of trees in urban environments are not isolated issues.

Trees can help to improve the air quality through trapping metallic particles, such as the harmful particulate matter (PM) blowing around from vehicle and industrial pollution.

What is Particulate Matter (PM)?

Particulate matter is the general term used to describe a mixture of solid particles and liquid droplets in the air, and includes aerosols, dust, ash, smoke, fumes, and pollen. Particulate matter is commonly characterized according to size: PM10, which is particulate matter smaller than 10 microns (0.01 millimetres) across, and PM2.5, which is smaller than 2.5 microns (0.0025 mm) across.

PM2.5 Size

Why is PM2.5 so important? 

Fine particulate matter of this size or smaller can more easily penetrate the resipiratory system. This can lead to breathing difficulties, particularly for high-risk individuals such as people with asthma, cardiovascular or lung disease, children, and elderly. In addition, PM2.5 is a carcinogen, and an established cause of lung cancer. There is no known level of exposure to PM2.5. An estimated 560 cancer cases in Ontario are attributed to PM2.5 exposure in the outdoor air.

Human Effects of PM.png

How does PM2.5 end up in the air?

PM2.5 is commonly emitted through fuel combustion and the burning of organic matter, but can also form through airborne chemical reactions. The Air Quality in Ontario 2014 report identified motor vehicle traffic, industrial sources, and residential fireplaces and stoves as the key contributers to residential PM2.5 air pollution. Other sources include smelters, power plants, forest fires, and agricultural burning.

Hwy 401.jpg

Highway 401, the busiest highway in North America

Who monitors PM2.5?

Within Ontario, there are 40 monitoring stations run by the Ministry of the Environment and Climate Change. Most stations collect PM2.5, ozone, nitrogen dioxide concentrations and provide live air quality reports and daily forecasts. Three stations are located in Hamilton at Beasley Park, Sackville Hill Memorial Park, and near the Highway 403 entrance off of Main St. W. in Westdale.

Hamilton West Air Monitoring Site

Hamilton West Air Monitoring Site

How does Hamilton stack up?

Not well. Although PM2.5 levels in Ontario have been declining since 2005, Hamilton’s Downtown (Beasley Park) station recorded the highest average annual concentration in the province in 2014 at 10.8 μg/m3  (micrograms per cubic metre). This is higher than the reference level of 10 μg/m3 established by Canadian the Canadian Ambient Air Quality Standards and the World Health Organization’s Air Quality Guidelines.

Hamilton Factories

Hamilton’s Industrial North End

However – the official air quality stations do not tell the whole story!

Cancer Centre Ontario’s 2016 report confirms what EH (Environment Hamilton) has believed for a long time: “The current air quality monitoring system does not provide sufficient data to reflect variations in concentration within an urban area”. More monitoring is necessary to determine neighbourhoods and other areas where people may be exposed to higher PM2.5 concentrations. Areas potentially at a higher risk include those nearby major highways and throughfares as well as near the industrial core at the north end of the city.

In fact, both of our tree inventory sites are in high-risk air quality zones. 

Our Crown Point location at St. Christopher’s Park is in the McAnulty Boulevard area – an area identified as a hot spot for PM2.5 – it has a calculated increased mortality of 14.3% due to air pollutants, significantly higher than the Hamilton-wide average of 11.5%.


Our McQuesten location is nearby or inside the neighbourhood with the lowest average age of a person experiencing a cardiovascular emergency – 57 years old, a staggering 22 years sooner than the average in other Hamilton neighbourhoods. This could be at least partly due to PM2.5 exposure, which is linked to long-term cardiovascular issues.

How can trees improve the air that we breathe?

Some trees (such as the silver birch shown below) are able to capture metallic particles flying through the air with the hairy surfaces of their leaves.


A study done at Lancaster University determined that trees can reduce residential PM2.5 exposure by over 50%.

The scientists started by measuring how much air pollution go into a certain number of houses in Lancaster using dust monitoring devices and by swiping surfaces and then analyzing what was collected with magnetic remanence, a technique that provides information on concentrations of iron-bearing particles.

Then the team placed a screen of 30 young silver birch trees in wooden planters in front of four of the houses, including one of the control houses, for 13 days. Wipes from all eight houses showed that ones with the tree screens had 52 to 65% lower concentrations of metallic particles. A comparison of all of the dust monitoring data from the two original control houses confirmed that drop, showing a 50% reduction in PM1, PM2.5, and PM10 in the house with the trees in front.

Overall, it is clear that the presence of trees in urban environments can considerably reduce inhalation of harmful PM2.5, and it is for this reason that we monitor air quality alongside the tree inventory!


Cancer Care Ontario’s 2016 Report:

Ministry of Environment and Climate Change’s Air Quality Site:


Clean Air Hamilton’s 2011 Report:

Article on Tree Capture and University Study:



Author: treespleasehamilton

A project of the Hamilton Naturalists' Club and Environment Hamilton. Funding by Ontario Trillium Foundation.

One thought on “The Effects of Trees on Air Quality

  1. Great information. Lucky me I came across your website by chance (stumbleupon). I’ve book-marked it for later!


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