The last week or so has seen dismal air quality in the national capital. The media does its job of reporting, but often, they report conflicting numbers.
And as always, after creating a confusion through frenzied reporting, the media steps in to “clarify” the confusion that they created in the first place.
A variation of this magnitude is alarming, so what’s really happening? All of the reports linked above have attributed the difference between the AQI readings to two things:
- Central Pollution Control Board (CPCB) caps the AQI (perhaps arbitrarily) at 500, whereas IQAir has no such cap on AQI.
- CPCB results are from pollution analysers whereas IQAir relies on sensor data that may be more susceptible to errors (this doesn’t explain the magnitude differences, though)
The first point above indicates that AQI is one common scale, on which CPCB imposes a cut-off at 500, like a small ruler being limited to 15cm. Private agencies like the Swiss IQAir (the media loves to stress on the Swiss bit) allow the scale to continue uninhibited without an arbitrary cut off. Like measuring with a 50m measuring tape instead of the small ruler. This is where the misinformation begins. The AQI values from the two sources are not numbers on the same scale at all.
What the media reports as the contributory causes of the magnitude difference is neither entirely convincing nor correct. So what is the real cause?
Let us understand the CPCB’s AQI calculation methodology first. They use a simple system that is explained on their website – there are 8 pollutants in the air that are continuously monitored, and as long as data for 3 of them are available (including PM2.5 or PM10), the AQI can be reported as “The worst sub-index (among the pollutants) for that location.” The sub-indices for each pollutant is calculated “using its 24-hourly average concentration value (8-hourly in case of CO and O3) and health breakpoint concentration range.”
To put it simply, as long as PM2.5 or PM10 readings are available along with at least two other pollutants, the raw data is averaged and that concentration value is used to calculate the sub-index for a particular pollutant. The highest numerical sub-index value is the AQI. The sub-index calculation is the process of normalising the different scales for different pollutants so that they can be compared on the same scale.
CPCB’s methodology uses a term “health breakpoint” and it is important to understand this term before moving forward. A health breakpoint is a range for a certain pollutant with a known health effect. Therefore, different pollutants may have different numeric values but may produce an equally detrimental health effect and are therefore assigned the same level of danger. For example, 91-120 micrograms of PM2.5 per cubic metre of ambient air puts it in the same category – Poor air quality – as 251-350 micrograms of PM10 per cubic metre of ambient air. For the same level, 181-280 micrograms of nitrogen dioxide per cubic metre of ambient air, and 10-17 milligrams of carbon monoxide per cubic metre of ambient air are used for the breakpoint calculations.
IQAir, which is another AQI provider, gives AQI readings as the the US Environment Protection Agency (US EPA) formula. They call it AQI+.
US EPA calculates AQI through a similar normalisation formula, but the health breakpoint values are different from what the Indian agency uses.
For example, for the same AQI as in the example above, 201-300 that US EPA designates as “Very Unhealthy”, breakpoints for PM2.5 are 125.5-225.4 micrograms per cubic metre, for PM10 is 355-424 micrograms per cubic metre, for 15.5-30.4 ppm of carbon monoxide. Note that while CPCB range for PM2.5 is 91-120, EPA range is 125.5-225.4. The ranges for other pollutants too is similarly wider as per the EPA standards when compared with the Indian standards.
The normalisation formula is:
Since US EPA uses a bigger range for the breakpoints, the AQI value ends up being numerically larger in magnitude than CPCB’s AQI.
For example, for a PM2.5 concentration of 31 micrograms per cubic metre, CPCB AQI is 52 (assuming two other pollutants are low, for the sake of calculation). For the same PM 2.5 value, EPA AQI is 92. As the pollutant concentration increases, the breakpoint ranges continue to change the resultant AQI values.
As I write this post (November 24 at 2pm), IQAir shows an AQI+ of 171 with the main pollutant being PM2.5 at a level of 84 micrograms per cubic metre. Yes, the air quality is better today.
As per CPCB AQI, given PM2.5 84, NO2 24.70, ozone 76 micrograms per cubic metre, the AQI would be 84.
AQI 84 versus AQI+ 171 for the same pollution level.
The little + makes all the difference. The devil, as they say, is indeed in the details!
As for the capping at 500, the qualitative descriptors are capped at 500 by both CPCB and EPA. The reason is that anything above 500 is equally hazardous and no further breakout is necessary because the measures to combat those levels of pollution would have to be the most stringent, and would not vary whether the AQI is 700 or 1000. However, the AQI can still quantitatively go above 500.
The AQI ranges for both Indian standards and US EPA standards are given below, for reference.
Indian AQI range & probable impacts:
0-50: This range defines air quality as good as it shows minimal or no impact on health.
51-100: This is a satisfactory air quality range and it can show effects such as breathing difficulty in sensitive groups.
101-200: The range shows moderate air quality with impacts such as breathing discomfort for children and elderly people, and people already suffering from lung disorders and heart disease.
201-300: AQI falling in this range communicates that the air quality is poor and shows health effects on people when exposed for the long term. People already suffering from heart diseases can experience discomfort from short exposure.
301-400: This range shows very poor air quality and causes respiratory illness for a longer duration of exposure.
401-500: This is the severe range of AQI causing health impacts to normal and diseased people. It also causes severe health impacts on sensitive groups.
US-EPA AQI range & probable impacts:
0-50: The range shows that the air quality is good and it poses no health threat.
51-100: This range is moderate and the quality is acceptable. Some people may experience discomfort.
101-150: The air quality in this range is unhealthy for sensitive groups. They experience breathing discomfort.
151-200: The range shows unhealthy air quality and people start to experience effects such as breathing difficulty.
201-300: Air quality is very unhealthy in this range and health warnings may be issued for emergency conditions. All people are likely to be affected.
301-500: This is the hazardous category of air quality and serious health impacts such as breathing discomfort, suffocation, airway irritation, etc. may be experienced by all.
(Source: Prana Air)