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Where there is Smoke, there are Air Quality Indices

This year’s record-breaking wildfires have made many of us more aware of what’s in the air. What pollutants does the Canadian government consider when reporting air quality, and why are they dangerous?

I woke up from a nap to see a doomsday-esque picture outside the car window. My friends and I were driving back to Montreal during the poorest air quality day of the summer, and the city looked near apocalyptic. The sky was orangey grey, the sun was invisible, and the air smelt strongly of smoke. As someone who lived in Ontario and Quebec most of my life, I’m unaccustomed to bad air quality caused by forest fire smoke. I’ve heard stories about the wildfires in Western Canada but naively thought air quality wasn’t a big concern for folks on this side of the country. Climate change and air pollution are changing this, as is evident in this year’s record-shattering wildfire count, and air quality is something of which more people are becoming aware.

As an air quality newcomer, I had a bunch of questions. Why is an index of 4 considered a moderate health risk in Montreal, but an index of 46 is considered good in Seattle? What do either of these numbers mean, and how are they calculated?

Governments use Air Quality Indices (or AQIs) to quantify air pollution and share it with the public in an easy-to-digest format. Generally, AQIs will measure the concentration of certain common air pollutants and then report the measurement in a single number. Governments have different AQIs because they consider different air pollutants and have different thresholds for what is considered dangerous. For example, the European AQI only considers four air pollutants, but the USA AQI considers five. The USA AQI is always reported on a scale of 0 to 500, but the European AQI can go as high as 1250 depending on the responsible pollutant. Different AQIs for different locations make sense, but it makes it difficult for regular citizens (like me!) to decipher.

What’s in our Canadian Air?

The Canadian government's AQI is called the Air Quality Health Index (AQHI). The AQHI is on a scale of 0 to 10+ and divided into low, moderate, high, and very high health risks. Initially, the 10+ seemed bizarre; why not have the scale end at 11 or 12? But the government chose to end the scale at 10+ because there isn’t much research on the different health impacts of a score of 10 vs 12 vs 17: they are all terrible for our health, so they are all lumped under the category of ‘very high health risk’.

canadian air quality index scale

The Canadian AQHI. Source: Environment Canada.

The Canadian government considers three pollutants: nitrogen dioxide, ground-level ozone, and particulate matter. The final index reported by the Canadian government results from a formula that considers concentrations of each of these pollutants. What makes these three substances so bad, and where do they come from?

Nitrogen dioxide is the most straightforward of the three pollutants. It is a gas made of one nitrogen and two oxygen atoms and is mainly produced by burning fuels like coal, gasoline, and oil. The heat from an engine causes nitrogen and oxygen in the air to combine. Most cars, trucks, and machinery produce nitrogen dioxide, so nitrogen dioxide concentrations can increase around rush hour or near construction sites. On its own, inhaling high concentrations of nitrogen dioxide can irritate our respiratory system and lead to increased asthma attacks. Nitrogen dioxide also reacts with other airborne substances to form acids or nitrates, both harmful to humans and the environment.

Ozone is a bit more complicated. Before researching air quality, I’d heard about the earth’s ozone layer and assumed that was the only place ozone existed. There are two types of ozone: stratospheric ozone (the one I learned about during grade school earth science) and ground-level ozone (the ‘bad’ ozone in AQIs). Both types of ozone are gases made of three oxygen atoms, but stratospheric ozone occurs naturally, while ground-level ozone is created artificially. Stratospheric ozone exists in the earth’s stratosphere or upper atmosphere and is essential because it protects us from the sun’s ultraviolet rays. Ground-level ozone is a secondary pollutant formed by chemical reactions between other primary pollutants. Combine nitrogen oxides (like nitrogen dioxide discussed above) and volatile organic compounds (carbon-containing gases like gasoline fumes), and you get ozone. Heat and sunlight help speed up the reaction between nitrogen oxides and volatile organic compounds, which is why bad air quality and smog are more common on hot, sunny days in the city.

Small Particles have Big Impacts

Compared to the other two pollutants in the index, particulate matter, or PM, isn’t a single molecule or substance. Rather, PM is a ‘mixture of solid particles and liquid droplets found in the air’ (US Environmental Protection Agency). The only requirement for something to be classified as PM is size! Any particle or droplet smaller than 10 micrometres for PM10 or smaller than 2.5 micrometres for PM2.5 is considered particulate matter and classified as a pollutant. That’s tiny. For perspective, a strand of our hair is about 50-70 micrometres in diameter. Examples of PM include soot, dust, pollen, ash, and droplets of liquid or fuel. Combinations of other air pollutants create some PM, and some PM is directly released from roads, construction sites, or forest fires. The smog that’s coated many Canadian cities this summer is a mixture of ground-level ozone and PM emitted by those forest fires.

PM has many negative health impacts, making forest fire smoke hazardous. In general, the smaller the particle, the more dangerous it is. The actual content of the particle – whether it is organic material like soot or tiny pieces of plastic – is less important than its size. Smaller particles can be more deeply inhaled into the lungs, causing more irritation and inflammation. Exposure to PM causes various respiration-related problems, such as worsening asthma, coughing, and irregular heartbeat, and is especially harmful to people with existing health conditions. For this reason, the Canadian AQHI has two different sets of recommendations: one for ‘At-Risk’ populations, or people with heart and breathing problems, and ‘General’ populations.

Climate change and pollution are forcing more people to care about the air, and worsening wildfires are causing many to evacuate their homes on short notice. But with the help of AQIs to summarize information on air pollutants, we can breathe easier even if more of our summer days are smoggy than sunny.

Here are some resources to learn more about the air quality in your area and precautionary measures to take to protect your health:


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