A comparison of worldwide Air Quality Scales - Part 1

Posted on March 20th 2015
Share: aqicn.org/faq/2015-03-20/a-comparison-of-worldwide-air-quality-scales-part-1

World Health Organization

Very often we receive inquiries from users asking about the numbers published on the World Air Quality Index project, and wondering about the difference with the values expressed in other websites. And that's actually normal, since the numbers, also called Air Quality Indexes (AQI), can be very confusing for beginners. The last message we received is the following one:
We have been checking air quality index for the Tianjin, China since last year, however we found the data has such a big difference between your website and another official (air.tjemc.org.cn) one which is stated on yours as well. I am wondering which one can provide more accurate data, as it will affect the decision for outdoor or indoor. Please help me with this issue. Thank you!
We have been writing quite a lot about this issue before, for instance from a comparative PM2.5 data for shanghai, singapore PSI and PM2.5 AQI and more recently about the nowcast. But, because we are still getting many questions, we are now writing this article to explain the difference of the AQI scales used all over the world. The first part of this article will focus on China, and the second part will extend the comparison to scales used in Japan, South Korea, India, Thailand, Malaysia, Europe, Mexico, etc.


Back to the initial question about the Air Quality Index in Tianjin, China, the two pictures below are snaphots of Tianjin's EPA website air.tjemc.org.cn. The picture on the left shows the PM2.5 concentration, and the one on the right the converted AQI.

1 hour 2 and 24-hours 3 average PM2.5 readings 1 in µg/m3 AQI values 2 and dominent pollutant 3


Starting from the picture on the left, the values highlighted as 2 and 3 are acutally not even converted to any AQI scale, but just plain raw readings expressed in micrograms per cubic meter (µg/m3). One might think that providing raw values is odd, but this is actually much more frequent than one would expect. Many Environmental Protection Bureau (EPB) all over the world are just providing raw unconverted values.

At the World Air Quality Index project, only values converted to the US EPA scale are provided. In order to convert the raw values to the US EPA scale, you can just use the airnow calculator (snapshot on the right), or our own version of the same calculator:

Using the calculator is simple: To convert, for instance, PM2.5 readings, select the PM2.5 pollutant 1, then enter the mass concentration 2, and finally click on Calculate to obtain the corresponding AQI figure 3.

So, when checking Air Quality data provided on other websites, always carefully check the unit for the data, and make sure to convert it to the AQI scale if it is expressed as raw µg/m3. Note that on the World Air Quality Index project, only AQI based on the 1-hour reading (and not the 24-hours average) are provided. If you want to know the explanation, check this post about the nowcast.


Then, for the picture on the right (AQI readings), it gets slightly more complex since there are different kind of AQI scales all over the world. The World Health Organisation (W.H.O.) is offering air quality guidelines based on reviews of the accumulated scientific evidences. But almost each country is having its own scale, and this is also the case for China, which is using the scale known as HJ 633-2012. The one in use on the World Air Quality Index project is the US EPA scale. But, since graphics are better than many words, here are the two scales for the PM2.5 pollutant.

The most important is to notice that for AQI of 200 and above, the two scales are having the same breakpoints. So, next time, when checking Air Quality data provided on other websites, always carefully check the unit for the data, and make sure to double check which AQI scale is in use.


In the second part of this article, we will be writing about other scales used in the world, and there will be some very interesting surprise, like for instance the AQI scale used for Ulanbaataar and its 100-250 Low pollution range...


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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    О качестве воздуха и измерениях загрязнения:

    О уровнях качества воздуха

    -Индекс качества воздуха (ИКВ = AQI) ЗначенияУровни концерна здравоохранения
    0 - 50хорошоКачество воздуха считается удовлетворительным, и загрязнение воздуха представляется незначительным в пределах нормы.
    51 -100удовлетворительноеКачество воздуха является приемлемым; однако некоторые загрязнители могут представлять опасность для людей, являющихся особо чувствительным к загрязнению воздуха.
    101-150Нездоровый для чувствительных группМожет оказывать эффект на особо чувствительную группу лиц. На среднего представителя не оказывает видимого воздействия.
    151-200нездоровыйКаждый может начать испытывать последствия для своего здоровья; особо чувствительные люди могут испытывать более серьезные последствия.
    201-300Очень НездоровыйОпасность для здоровья от чрезвычайных условий. Это отразится, вероятно, на всем населении.
    300+опасныйОпасность для здоровья: каждый человек может испытывать более серьезные последствия для здоровья

    To know more about Air Quality and Pollution, check the wikipedia Air Quality topic or the airnow guide to Air Quality and Your Health.

    For very useful health advices of Beijing Doctor Richard Saint Cyr MD, check www.myhealthbeijing.com 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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