Mint Explainer | Some AQI trackers show Delhi at 1,500. The official index says 500—here’s why
India's official index severely downplays the data on the days of the worst air by capping the figure at 500, howsoever big it may be.
Air quality indices on your phone’s weather app and the one reported by the government do not seem to agree with each other, and those obsessed with the data have too many contradicting numbers to go by. By some measures popular on social media, parts of Delhi crossed the index value of 1,000 at times—even 1,500—at their worst. Look at the official government scale, and it will never cross 500. Why is this so, and what does this mean for us?
First things first—all indices are correct in their own way, except that the devil lies in the details. They are all released by different organizations, which use different scales, methodologies and sources.
The measurement scales
The air quality index (AQI) is supposed to measure the safety of the air around you for breathing. All 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 different monitoring stations.
Density (or concentration) is the mass of any material contained in one unit of volume. For pollutants, it is typically measured in micrograms per cubic metre. A higher figure means more of a pollutant 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 10 grams of carbohydrates in a day have the same utility for your body as 10 grams of vitamins!
So, the quantity of each pollutant in the air (measured in micrograms per cubic metre) is adjusted to a common unit-less scale (say, 0 to 500, or 0 to 300) that works for all pollutants uniformly.
The purpose? To ensure that the adjusted numbers—"sub-indices" for each individual pollutant—are comparable. After this, if PM2.5 gets a sub-index of 100, and PM10 gets a sub-index of 50, that roughly means that the PM2.5 in the air is doubly as harmful as the PM10 in the air at that point of time.
This is where each organization works differently. They calculate the sub-indices in their own ways. They may have different thresholds for what’s mildly unhealthy, what’s very unhealthy, and what’s really harmful.
They may even have different names for categories: one index may classify air quality as poor, good, moderate, severe and very severe, while another may have fewer categories with different definitions, let’s say, unhealthy, unhealthy for sensitive groups, and hazardous.
Does “hazardous" of one index mean the same as “severe" of 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 across most indices: the AQI takes the value of the pollutant with the worst sub-index for that time and location.
India’s AQI—and its big flaw
The widely used National Air Quality Index (NAQI) given by the Central Pollution Control Board is a 24-hour average, and not a real-time measure. The CPCB reports data on its website for the top cities every hour and a comprehensive list of many towns and cities across India every evening.
On another official platform, it also provides hourly pollutant-level sub-indices and the 24-hour AQI for each monitoring station. So, if the NAQI on the morning after Diwali sounds too good to be true, wait for a few hours for a reality check.
Plus, there's a big drawback in India’s official system. It caps the sub-index of each pollutant at 500, no matter how big the actual calculated value is. The 24-hour average uses capped values, thus severely downplaying the reality on the worst days.
Imagine a situation where the actual uncapped AQI at a location (after adjusting the concentration levels of all pollutants to a comparable scale) comes out to be 1,500 in one half of the day, and 300 in the second half (usually the daytime is cleaner). The CPCB will report the 24-hour average as the average of 500 (instead of 1,500) and 300, i.e. 400.
That doesn’t reflect the reality at all: if left uncapped, the average would have been 900. This means the 24-hour average can never exceed 500, even when other indices cry 1,000 or stop measuring altogether.
The CPCB does provide hourly pollutant-level densities on a portal that sometimes works erratically. The density data for PM2.5 and PM10 compiled from here is more useful than the final AQI, because it is raw data and is devoid of any adjustment or capping.
Worse, as it happened this year, hourly data was not available for several monitoring stations on the night between 20 October (Diwali) and 21 October for reasons that aren’t clear yet.
Another index that is commonly used is run by 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 better suited for Asian dust.
Yet, this index is useful because it allows us to compare cities across the world and does not cap 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 like PM2.5.
No index is comparable with each other. Use any index, but compare like for like, and know the flaws with India’s national index.
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