Air Quality Scale in Quebec and Montreal

Posted on July 16th 2015

Flag of Quebec.svg
The Fleurdelisé, flag of Quebec.
(attribution: wikipedia)

The Air Quality data for Montreal and Quebec has been available for a while, but we recently got few questions about the AQI scale used in this part of Canada. One of them is from Marie A., who asked:

I would like to know why the Air Quality Index on your web site is different from the AQI on the Montreal's website? Do you recalculate anything to meet the US EPA AQI standards? If so, how? With what data?

For example, date July 14th 2:00pm, most of the indexes on your website range from 53 to 65 (one index is 37) and it is supposed to be realtime AQI. On the website of Montreal city, date July 14th 2:00pm, most of indexes range from 16 to 37 and it is realtime AQI as well.

So, can you explain me the difference and what the standards details you are using?
This is indeed a very good question. and we will explain in this article the different scales in use, how the scales compare to each other, and how the convertion can be done.


As we explained in one of our previous articles, all the AQI figures published on the World Air Quality Index project are currently [1] using the US EPA standard. Neither Montreal nor Quebec are expection to this rule, so this confirms that the values displayed on the page city/montreal are using the US EPA standard.

This is the reason for the difference between the World Air Quality Index project readings and the Montreal city website ( The Montreal city EPA (Environment Protection Agency) is using its own proper AQI scale, which is clearly described on the same website. To make it slighly more "complex", Quebec is actually using another AQI scale (specified at this link) compared to Montreal (tabarnac!). And of course, Canada is using yet another scale, called AQHI for Air Quality Health Index - but that's something we already wrote about in another article.

The summary of Montreal and Quebec AQI breakpoints are listed in this table:

Pollutant Quebec Montreal
Averaging Period Reference Value Averaging Period Reference Value
Fine Particles (PM2.5) 3 hours 35µg/m3 3 hours 35µg/m3
Ozone (O3) 1 hour 82 ppb 1 hour 160 µg/m3
Nitrogen dioxide (NO2) 1 hour 213 ppb 1 hour 400 µg/m3
Sulfur dioxide (SO2) 4 minutes mobile 213 ppb 10 minutes mobile 500 µg/m3
Carbon monoxide (CO) 1 hour 30 ppm 1 hour 35 µg/m3


In order to convert the values to the US EPA standard, the raw concentrations expressed in milligrams or ppb/ppm (particle count per billion/million) are required. Unfortunately, neither Montreal nor Quebec county is providing those raw concentrations. Instead they are only providing the Air Quality readings already converted to their own respective AQI scales. But this actually not a problem, since the re-convertion to concentration can be done using this simple linear formula:
Raw concentration (pollutant) = Reference value (pollutant) * AQI }(pollutant) / 50
For instance, if an PM2.5 AQI value of 18 is publised on Montreal city website, then the raw concentration in milligrams is 18*35/50 = 12.6 mg/m3. Note that for Ozone, CO, SO2 and NO2 the unit is different for Quebec and Montreal (ppb for Quebec - US standard, and mg for Montreal EU standard), but that is just an additional convertion (partly explained in this article).

Note that the PM2.5 AQI is based on 3-hours average, while on the World Air Quality Index project, the hourly PM2.5 Instant Cast AQI reporting is used. Since it is not possible to deduct the hourly data from the 3 hours average, this actually means that the PM2.5 AQI reported on the World Air Quality Index project for Montreal and Quebec is not exactly an "Instant Cast" reporting.


So, last, here is the visual comparison of 3 AQI scales (US EPA, Quebec and Montreal) for PM2.5 and Ozone (note that for Ozone, the ppm unit is used - so the convertion from mg to ppm is done for Montreal).

One interresting point to notice is that both Quebec and Montreal are using the AQI value of 25 as the breakpoint between Good (green) and Aceptable (yellow), while for the US EPA, this breakpoint is set at 50. So, all in all, when comparing apple-to-apple with the right convertion and breakpoints in mind, the reported AQI range is very similar between the US EPA standard and Quebec / Montreal standards.

[1] We are now working on an option which will allow users to select any AQI scale they want.


Note: This article is part of a series on Worlwide Air Quality scales.

For more information about specific countries or continent, please refer to thoses articles:
Thailand and Malysia
Hong Kong / Canada (Air Quality Health Index)
South America
Quebec and Montreal
For information about the 24 hours averaging used or Ozone and Particulate Matter (PM2.5), please refer to those two articles: Ground Ozone Index - PM2.5 Instant Cast

Click here to see all the FAQ entries
  • Nitrogen Dioxyde (NO2) in our atmosphere
  • Ozone AQI Scale update
  • Kriging Interpolation

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    À propos de la qualité et de la mesure de la pollution atmosphérique:

    A propos des niveaux de qualité de l'air

    IQANiveau de pollution de l'air Impact sur la santé
    0 - 50BonLa qualité de l'air est jugée satisfaisante, et la pollution de l'air pose peu ou pas de risque.
    51 -100ModéréAir quality is acceptable; however, for some pollutants there may be a moderate health concern for a very small number of people who are unusually sensitive to air pollution.
    101-150Mauvais pour les groupes sensiblesLa qualité de l'air est acceptable; Cependant, pour certains polluants, il peut y avoir un problème de santé modérée pour un très petit nombre de personnes qui sont particulièrement sensibles à la pollution de l'air.
    151-200MauvaisTout le monde peut commencer à ressentir des effets sur la santé; les membres des groupes sensibles peuvent ressentir des effets de santé plus graves.
    201-300Très mauvaisAvertissements de santé de conditions d'urgence. Toute la population est plus susceptible d'être affecté.
    300+DangereuxAlerte de santé: tout le monde peut ressentir des effets de santé plus graves.

    Pour en savoir plus sur la qualité de l'air, consultez le sujet Qualité de l'air sur wikipedia ou le guide AirNow de la qualité de l'air et de votre santé .

    Pour des conseils de santé très utiles de Docteur Richard Saint Cyr, consultez le blog

    Usage Notice: All the Air Quality data are unvalidated at the time of publication, and due to quality assurance these data may be amended, without notice, at any time. The World Air Quality Index project has exercised all reasonable skill and care in compiling the contents of this information and under no circumstances will the World Air Quality Index project team or its agents be liable in contract, tort or otherwise for any loss, injury or damage arising directly or indirectly from the supply of this data.


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