Overview of Hong Kong's Air Quality Health Index

Posted on June 3rd 2015
Share: aqicn.org/faq/2015-06-03/overview-of-hong-kongs-air-quality-health-index

Note: This the fith article of series on 'Worldwide Air Quality Scales'.

It is already a long time since the Hong Kong EPA has updated its Air Quality Index standard from the traditional Air Pollution Index to the so called Air Quality Health Index (AQHI), but we never got a change until now to have a deeper look at it, which we will do in this article.

The Hong Kong EPA has actually done an excellent work at explaining how the AQHI is defined, and the information one needs to understand the AQHI is available from their website. The most obvious simplification is the change of the traditional 0-500 index into a new simplifed 0-10+ range, sub-divded in five health risk categories, as shown below:

1 2 3 4 5 6 7 8 9 10 10+

But there is actually much more than a range simplification: the new AQHI is also introducing the very clever concept of Added Health Risk (%AR), which was originally defined, in 2007, by a team of four partners from South Africa, and first used in Canada.

Unlike the traditional AQI for which the AQI is defined as the maximum of the Individual AQIs, the AQH is defining the overall Augmented Risk (AR) as the sum of the Individual pollutants Augmented risk (IAR). The idea is, for instance, that if both ozone (O3) and particule matter (PM2.5) levels are high, then the risk for health is likely twice as big as if only particule matter level was be high. In the case of the traditional AQI scale, the AQI does not taken into account the multiple pollutant situation, i.e. the AQI is only defined as the maximum level from all pollutants.
AQHI Augmented Risk = IAR ( NO2 ) + IAR ( SO2) + IAR ( O3 ) + max( IAR ( PM2.5 ), IAR ( PM10 ) )

Traditional AQI = max( IAQI ( PM2.5 ), IAQI ( PM10 ), IAQI ( O3 ), IAQI ( NO2 ), IAQI ( SO2 ) )
The Individual pollutants Augmented risk (IAR) are based on epidemiological studies using Hong Kong's health statistics and air pollution data. They work by defining a correlation cooeficient between pollutant level and relative risks, which are measured by hospital admissions. But because graphs are quite often much easier to understand, the IAR breakpoints are plotted on the graph below:

Few interesting points are worth noticing: First this is yet another evidence of epidimiologic correlation between PM10 and PM2.5 - meaning that for countries where only PM10 is available, it it possible to deduct a PM2.5 like AQI - like we explained in this article.

Second, the highest augmented risk for equivalent concentration is the Ozone. Because the way the epidemiologicial study is done, it most likely reflects the risk due to a prolonged exposure. In other words, if compared, then it should be done with the 8 hours Ozone standard rather than the hourly standard. That can actually be a problem since we have recently updated our system to the use hourly instant cast Ozone AQI as we do believe that was is important for the citizens as a health decision is the actual concentrations, and not the one from 8 hours ago.

Last, in case one pollutant is not available, then the calculation is not possible anymore. For this reason, the AQHI is using a 3 hours rolling average, so that, when a pollutant is missing, it can be estimated as the average as the previous 2 hours. Moreover, if there is more than one hour of missing pollutnat data, then the AQHI is not reported.

So, now that the concept behind the AQHI is understood, the last step is to have a practipal comparisons of the reported values using the traditional AQI scale and the new AQHI scale, which is done in the example below using the past 15 days data from Shanghai Jing'an station (静安监测站).

For the sake of a consistent visual comparison, we introduce an intermediate color for the moderate (3-6) levels, as following:
10+ . The visual comparison result speaks for itself, with AQHI reporting higher levels (
) than traditional AQI (
) when several pollutants are high at the same time (most of the time Ozone and PM2.5), and AQHI reporting low
when only one pollutant is high (happens when only PM2.5 is moderate
, but all other pollutants are low
). That means that no scale is better than any other one, but each scale is having their own strong points are reporting specific pollution event.

HK AQHI Standard

US AQI Standard

US Individual AQI


As the conclusion, what is the most important is to remember as that each scale has its own specificity, and the most important is to keep the diversity in the scales: We are starting to believe that having only one unique scale might not be the right solution, and we are now working on a global solution which will allow users the select the scale which best fits their needs.

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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