How we measure air quality and what the numbers mean

The wildfire smoke hovering over parts of eastern Canada and the U.S. this week has led many to start poring over charts and figures to get a sense of the air quality in their area.

But what exactly do these indexes measure, and how should they be interpreted?

Different countries have different ways of measuring air quality, including Canada, said Dan Westervelt, an associate research professor at Columbia Climate School's Lamont-Doherty Earth Observatory.

There is some variability between the indices used by countries, but he says they generally measure similar elements in the air.

Here's a quick primer.

Canada uses what's called the Air Quality Health Index (AQHI), a measurement designed to make it easier for the public to understand the level of risk involved.

There are 286 measuring sites in a total of 203 communities across the country, according to the federal government.

The index measures health effects associated with air pollution on a simple scale of one to 10+, with 10 and anything above being the worst. The AQHI takes into account the following pollutants: ground-level ozone, particulate matter and nitrogen dioxide. All three are found in wildfire smoke.

The index was designed to inform the public about the "health risk from air pollution on any given day," according to Health Canada.

Anything between seven and 10 is considered "high risk." At that point, Environment Canada advises the public to consider reducing strenuous outdoor activities. 

A measurement above 10 is considered "very high risk." 

In those circumstances, Environment Canada advises the general population to reduce or reschedule strenuous outdoor activities and recommends that at-risk populations, such as young children, seniors and those with chronic conditions, avoid strenuous activities altogether.

You can find the level of air quality in your area here.

The U.S. uses a different scale of measurement, with a much higher range. The Air Quality Index (AQI) is based on air quality standards and is measured from 0 to 500. The higher the value, the greater the level of air pollution.