Singapore PSI and PM2.5 AQI: Why is there a difference between the two readings?

Posted on June 25th 2013
Share: aqicn.org/faq/2013-06-25/singapore-psi-and-pm25-aqi-why-is-there-a-difference-between-the-two-readings

Please note that this article was orginally written in June 2013.
In April 2014, the Singapore NEA has updated the PSI calculation to also include PM2.5. http://www.nea.gov.sg/anti-pollution-radiation-protection/air-pollution-control/psi

With the recent Southeast Asia Smog from Indonesia affecting Singapore and now Malaysia, we got many questions about why there is a difference between the data which can be read from Singapore NEA website (nea.gov.sg) and the World Air Quality Index project Singapore webpage.

For instance, here is what can be read today in NEA website:

For some historical reason, Singapore is using the PSI (Pollutant Standard Index) to evaluate the Air Quality. In the image above, number 1 corresponds to the PM10 value that is used for the PSI evalation. The value 67(/59) can be read as 67 μg/m3, corresponding to a PSI of 59. The PSI is evaluated as the maximum of the individual PSI for each of the pollutants: PM10, SO2, NO2, O3 (Ozone) and CO2.

PSI(Singapore-North) = max( PSIPM10-based, ... PSIO3-based )= max (59, ..., 77) = 77

This said, the interesting thing is that, in this same table, the PM2.5 data is also provided in the last column (see 3). This data is provided in μg/m3 only (see 2), and there is no conversion to something like the PSI (i.e. a conversion from the PM2.5 mass to a Pollution or Quality Index). This conversion however exists, and is defined by the US Environmental Protection Agency. The easiest way to do the conversion is to use the online calculator, available at airnow.gov:


If you select the PM2.5 (1), then enter the mass concentration of 48 (2), can click on Calculate, you will obtain the AQI of 132 (3). So, based on the PM2.5 AQI conversion, the PSI that is used for Singapore could be extended (let's call it PSI++) to also take into account the PM2.5 information. In which case, the PSI++ would be the maximum of the regular PSI (based on PM10 only) and the PM2.5 AQI:

PSI++ = max( PSI, AQIPM.25 ) = max( 77, 132 ) = 132

This PSI++, that is commonly referred as AQI (or Air Quality Index), is what is being used on the the World Air Quality Index project, for all the cities (provided PM2.5 is available for the city). And this explains why the values are different between the NEA website and the World Air Quality Index project.

Note: With the PSI update from April 2014, the only remaining difference is that the World Air Quality Index project is using a 1-hour PM2.5 reading (following the instantcast) while Singpore PSI is based on the 3 hours average.

Moreover, when doing the convertion, make sure you use the 1-hour reading for the PM2.5 concentration rather than the 24-hours averaged value, as shown on the below image:


http://www.haze.gov.sg/haze-updates/pollutant-concentrations/type/PM25-1Hr


If you want to know more about PM10 vs PM2.5, and especially why PM10 is still used, please check the faq entry about why is PM2.5 often higher than PM10? Is PM10 still a relevant measure?


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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
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India
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China
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Hong Kong / Canada (Air Quality Health Index)
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South America
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Australia
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Quebec and Montreal
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Singapore
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Poland
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Indonesia
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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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    대기 품질 및 환경 오염 측정에 관하여 :

    대기 품질 지수 단계에 대하여

    AQI지수구분구간의미
    0 - 50좋음대기오염 관련 질환자군에서도 영향이 유발되지 않을 수준
    51 -100보통환자군에게 만성 노출시 경미한 영향이 유발될 수 있는 수준
    101-150민감군영향환자군 및 민감군에게 유해한 영향이 유발될 수 있는 수준
    151-200나쁨환자군 및 민감군(어린이, 노약자 등)에게 유해한 영향 유발, 일반인도 건강상 불쾌감을 경험할 수 있는 수준
    201-300매우나쁨환자군 및 민감군에게 급성 노출시 심각한 영향 유발, 일반인도 약한 영향이 유발될 수 있는 수준
    300+위험환자군 및 민감군에게 응급 조치가 발생되거나, 일반인에게 유해한 영향이 유발될 수 있는 수준
    (Reference: see airkorea.or.kr)

    대기 품질과 오염에 대해 더 많은 것을 알아보려면 위키피디아의 공기품질 문서(영어)을 보거나 공기 품질과 건강에 대한 AirNow 가이드(영어)를 참조해보세요.

    베이징 의사 Richard Saint Cyr MD의 건강 관련 팁을 보려면 www.myhealthbeijing.com 에 블로그를 확인하세요.


    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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    Temperature unit:
    Celcius