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

  • Read or publish comments (they rely on Disqus)

    Sobre a medição da aualidade do ar e poluição

    Sobre os níveis de qualidade do ar

    -Valores do Índice de Qualidade do Ar (AQI)Níveis de preocupação de saúde
    0 - 50Boa0-50: Boa - A qualidade do ar é considerada satisfatória, a poluição do ar representa pouco ou nenhum risco
    51 -100Moderado50-100: Moderado - A qualidade do ar é aceitável; No entanto, para alguns poluentes pode haver um problema de saúde moderada para um número muito pequeno de pessoas que são mais sensíveis à poluição do ar.
    101-150Não Saudável para Grupos SensíveisMembros de grupos sensíveis podem ter efeitos na a saúde. O público em geral não é susceptível de ser afetado.
    151-200Não saudável150-200: Insalubre - Toda a população pode começar a sentir os efeitos na saúde; membros de grupos sensíveis podem apresentar efeitos mais sérios de saúde.
    201-300Muito Prejudical à Saúde200-300: Muito Insalubre - As advertências de saúde de situações de emergência. Toda a população é mais susceptível de ser afectada.
    300+Perigoso300+: Perigoso - alerta de saúde: todos podem experimentar efeitos mais graves para a saúde

    Para saber mais sobre Qualidade do Ar e Poluição, verifique o wikipedia Qualidade do Ar tópico ou o guia AIRNow a Qualidade do Ar e sua saúde.

    Para informações sobre saúde muito úteis em Pequim, procure Doutor Richard Saint Cyr MD, consulte blogue.

    Aviso de uso: Todos os dados da Qualidade do Ar não são validados no momento da publicação e, devido à garantia de qualidade, esses dados podem ser alterados, sem aviso prévio, a qualquer momento. O projeto Índice de Qualidade do Ar Mundial exerceu todas as habilidades e cuidados razoáveis na compilação do conteúdo desta informação e sob nenhuma circunstância o A equipe do projeto World Air Quality Index ou seus agentes podem ser responsabilizados em contrato, responsabilidade civil ou de outra forma por qualquer perda, lesão ou dano decorrente direta ou indiretamente do fornecimento desses dados.


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