Airveda always strives to provide the most accurate and reliable data to citizens. Based on new information available from govt. sites we are updating our AQI calculation so that Airveda app will now show AQI levels above 500 for both Indian and US AQI. We believe this presents a clearer picture of overall air quality instead of capping at 500 which we were previously doing.
What is AQI?
The Air Quality Index is a tool for effective communication of air quality status to people in terms, which are easy to understand. It transforms complex air quality data of various pollutants into a single number (index value), nomenclature and colour.
Understanding Indian AQI
Indian AQI defines six AQI categories, namely Good, Satisfactory, Moderately polluted, Poor, Very Poor, and Severe. Each of these categories is decided based on ambient concentration values of air pollutants and their likely health impacts (known as health breakpoints). AQ sub-index and health breakpoints are evolved for eight pollutants (PM10, PM2.5, NO2, SO2, CO, O3, NH3, and Pb) for which short-term (upto 24-hours) National Ambient Air Quality Standards are prescribed.
Based on the measured ambient concentrations of a pollutant, sub-index is calculated, which is a linear function of concentration (e.g. the sub-index for PM2.5 will be 51 at concentration 31 µg/m3 , 100 at concentration 60 µg/m3 , and 75 at concentration of 45 µg/m3 ). The worst sub-index (or maximum of all parameters) determines the overall AQI. AQI categories and health breakpoints for the eight pollutants are as follows:
What are we changing?
Based on the table above, while it is clear that PM2.5 of 250ug/m3 corresponds to AQI 401 it is not clear what is the PM2.5 level corresponding to AQI 500. Given this lack of clarity we were using 999ug/m3 for PM2.5 and 1999 ug/m3 for PM10 as the values corresponding to AQI 500. However more recently CPCB has released a calculator on their website which gives us additional information about this.
You can download the calculator here: Calculator
According to this calculator PM2.5 of 350ug/m3 corresponds to AQI of 500 and it increases from there. Similarly for PM10, 510 ug/m3 corresponds to AQI 500. Based on this new information, we have updated our AQI numbers. Where earlier you would only see AQI numbers upto 500, now they may go significantly higher. We believe this makes better sense as AQI should reflect more clearly the difference between PM2.5 levels of 250ug/m3 vs 750ug/m3. Previously 250 ug/m3 was 401 while 750ug/m3 was 467, now 750ug/m3 will be 785.
What about US AQI?
Similarly US AQI on their website calculator says "Values above 500 are considered Beyond the AQI." when you try to convert PM levels at say 750ug/m3. So for now we were capping US AQI at 500. However we noticed that the AirNow API returns AQI numbers higher than 500 given air quality numbers in India. Keeping in line with this new information we are also updating our US AQI values to also show AQI numbers greater than 500.
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