Ground Ozone Air Quality Index update with Instant Cast

Posted on May 18th 2015

(Attribution: Houston Clean Air Network )

The ground Ozone Air Quality Index calculation has just been updated to follow the Instant Cast concept, i.e. to report the pollution right now instead of the pollution from the previous hours. For more information about the Instant Cast, please refer to this article.

The calculation for the Ozone Index is now using the hourly reading, instead of the previous 8 hours average, but still applying the same 8 hours AQI breakpoints formula for AQI below 100. The previous procedure for calculating the 8-hour average concentrations is not used any more for Instant reporting. Above AQI 100, the normal 1 hour Ozone breakpoints are used (while before, the AQI above 100 was defined as the maximum of the 1 hour and 8 hours readings).

The graph on the right is summarizing the breakpoints in use. The green line is the one corresponding the "Ozone Instant Cast" breakpoints in use on the World Air Quality Index project

Index Values
Color coding
AQI Category1-hour
8-hours breakpoints
proposed new
standard from US EPA
0 - 50
Good-0 - 600 - 50
51 - 100
Moderate-60 - 7550 - 65
101 – 150
for Sensitive Groups
125 - 16575 – 9565 - 85
151 – 200
Unhealthy165 - 20595 – 11585 - 105
201 – 300
Very Unhealthy205 - 405115 – 375105 - 200
301 – 500
Hazardous405 - 505--

The US EPA EPA is proposing to strengthen the National Ambient Air Quality Standards (NAAQS) for ground-level ozone, based on extensive scientific evidence about ozone's effects (more information). This new standard (plotted in grey in the above graph) is only for 8 hours average breakpoints, and since it is not yet approved - it is not used for the "Ozone Instant Cast" breakpoints.

For those interested in understanding how this scale compares with the scale used by other countries, the first important point to pay attention to is that the the concentration unit used for the scales can vary: For the US EPA, the Ozone concentrations are expressed in ppm (particles per million) while for many other countries, including India and China, it is expressed in µg/m3;

Therefore, the first step for comparing the scales is to use a primary concentration unit. In this case, ppm is kept as the primary unit. The conversion from µg/m3; is somewhat straightforward and consists in multiplying the number of particles by the molecular mass of the ozone times the density of air.

The density of air is dependent on the ambient temperature and atmospheric pressure, but, in order to simplify, a common assumption of a 25 degrees Celsius ambient temperature and 1 atmospheric pressure is used. Under those conditions, the following formula is used for the conversion:
Ozone Concentration (ppb) = 2.1414 x Ozone Concentration (µg/m3)
The scales can now be compared on the same ppm scale, and the two graphs below shows the respective scales for 8 hour and 1 hour. So, once again, as it has been seen many times before, the scales used outside of US are actually much more safe than the one in US!

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

    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

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    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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