First – all indexes are correct in their own way, except that the devil is in the details. They are all issued by different organizations that use different metrics, methodologies and resources.
Measuring scale
The Air Quality Index (AQI) is meant to measure the safety of the air around you to breathe. All the organizations that report the AQI perform the same task: they measure the density of pollutants (fine particles such as PM2.5 and PM10 and gases such as nitrogen dioxide, ozone and carbon monoxide) in the air at various monitoring stations.
Density (or concentration) is the mass of any material contained in one unit volume. For pollutants, it is usually measured in micrograms per cubic meter. A higher number means more pollutants in the air.
However, one gram of one pollutant may not be as harmful as one gram of another pollutant in the same amount of air. That would be like saying that 10 grams of carbohydrates a day is as good for your body as 10 grams of vitamins!
So the amount of each pollutant in the air (measured in micrograms per cubic meter) is adjusted to a common unitless scale (say 0 to 500 or 0 to 300) that works uniformly for all pollutants.
Purpose? To ensure that the adjusted numbers – the “sub-indices” for each individual pollutant – are comparable. Then, if PM2.5 gets a sub-index of 100 and PM10 gets a sub-index of 50, it roughly means that the PM2.5 in the air is twice as harmful as the PM10 in the air at that moment.
Every organization works differently here. Sub-indexes calculate in their own way. They may have different thresholds for what is mildly unhealthy, what is very unhealthy, and what is truly harmful.
They may even have different category names: one index may classify air quality as poor, good, moderate, severe and very severe, while another may have fewer categories with different definitions, say unhealthy, unhealthy for sensitive groups and hazardous.
Does “dangerous” on one index mean the same as “severe” on another? Not necessarily. It depends on the actual underlying numbers.
The last step is to report the air quality index. This is simple and common for most indices: AQI takes the value of the pollutant with the worst sub-index for a given time and place.
India’s AQI – and its big mistake
The widely used National Air Quality Index (NAQI) given by the Central Pollution Control Board is a 24-hour average, not a real-time measurement. CPCB reports hourly figures for top cities and a comprehensive list of many cities across India every evening on its website.
On another official platform, it also provides hourly sub-indices of pollutant levels and 24-hour AQI for each monitoring station. So if NAQI sounds too good to be true the morning after Diwali, wait a few hours for a reality check.
Moreover, there is a great deficiency in India’s official system. It limits the sub-index of each pollutant to 500, regardless of how large the actual calculated value is. The 24-hour average uses limited values, thus severely underestimating the reality on the worst days.
Imagine a situation where the actual uncapped AQI at a site (after adjusting the concentration levels of all pollutants to a comparable scale) comes out to 1500 in one half of the day and 300 in the other half (usually a cleaner day). CPCB reports the 24-hour average as the average of 500 and 300, i.e. 400.
This does not reflect reality at all: if left uncapped, the average would be 900. This means that the 24-hour average can never exceed 500, even if other indices scream 1000 or stop measuring altogether.
CPCB provides hourly pollutant level density on the portal which sometimes operates erratically. The density data for PM2.5 and PM10 compiled here are more useful than the final AQI because they are raw data and lack any adjustment or limitation.
Worse, as happened this year, hourly data was not available for several monitoring stations during the night between October 20 (Diwali) and October 21 for reasons that are still unclear.
Another commonly used index is the World Air Quality Index Project. It uses a different scale, developed by the US Environmental Protection Agency, which admits that India’s NAQI is more suitable for Asian dust.
Nevertheless, this index is useful because it allows us to compare cities around the world and does not limit the values. Your phone’s weather app and your air purifier may use very different index systems, or they may only report the concentration of a particular pollutant, such as PM2.5.
No index is mutually comparable. Use any index, but compare like-for-like and know the shortcomings of the Indian national index.
