Explained: 350 vs 1,100, CPCB vs IQAir, Who Has Got Delhi's AQI Right

Delhi's sky turned grey and hazy a day after Diwali fireworks lit up the city. The air, toxic, making the national capital the "most polluted major global city". The pollution levels or the Air Quality Index (AQI) soared to severe levels, resulting in difficulty in breathing, itchy eyes and causing discomfort to even healthy people.

IQAir, a Swiss air-quality monitoring company, clocked the city's AQI at a staggering 1,121 on Tuesday. However, India's Central Pollution Control Board (CPCB) reported a much lower AQI of 351, falling under the "very poor" category. The catch is - both the readings are correct but for different reasons.

Also Read: 'Green' Crackers Failed. Delhi Suffers Worst Post-Diwali Air In 5 Years

What Is AQI?

The Air Quality Index (AQI) is a public health tool designed to communicate air pollution levels or the quality of air in simple terms. Eight pollutants namely particulate matter (PM) 10, PM2.5, Ozone (O3), Sulphur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), lead (Pb) and ammonia (NH3) act as major parameters in deriving the AQI of an area, as per the CPCB.

"AQI is a calculated index based on different parameters. It is for common man's understanding of the quality of air," explained Mohan George, Consultant at the Centre for Science and Environment. "AQI has different color-coded ranges simply to tell the citizens that air is good or satisfactory or very poor and what they should be doing," George, also former Additional Director at Delhi Pollution Control Committee, added.

For example, AQI between 401 and 500 or in the severe category is hazardous for even healthy people, and one should avoid going out or exercising in the open.

Also Read: Did Delhi's Green Diwali Work? Air Quality Data From Last Year Shows...

CPCB vs IQAir

Difference In Scale

India's AQI scale is capped at 500. Meaning, AQI beyond 500 will be grouped under the "severe" category, indicating a public health emergency.

The CPCB categorises AQI into six parts: 0-50 is considered 'good', 51-100 'satisfactory', 101-200 'moderate', 201-300 'poor', 301-400 'very poor', and between 401-500 'severe'.

IQAir on the other hand has tighter grouping: 0-50 is considered 'good', 51-100 'moderate', 101-150 'unhealthy for sensitive groups', 151-200 'unhealthy', 201-300 'very unhealthy', and anything above 301 is labelled hazardous.

"When identifying AQI of a city or a station, both CPCB and IQAir, look at the dominating pollutant. For Northern India, it is generally particulate matter, and in winters, you would find PM 2.5, ultra-fine particulate matter to be the key pollutant," said George.

Data Sources

CPCB sources data from government-owned air quality monitoring stations including those operated by CPCB, DPCC, India Meteorological Department (IMD) and Indian Institute of Tropical Meteorology (IITM).

"CPCB relies on reference-grade monitors which provide calibrated data," said Geoge.

IQAir gets data from all the government sources and other sensor-based analysers installed by varied companies and even individuals.

"The US Embassy, for instance, has an air quality monitoring station installed. Similarly, other embassies and individuals have such analysers installed. IQAir takes average of all of these stations," said Sunil Dahiya, Founder and Lead Analyst at Envirocatalysts.

Also Read: 'Supreme Court Chose Right To Burn Crackers Over Right To Live': Amitabh Kant

Calculation

Both CPCB and IQAir follow different methods of calculating AQI. NDTV tried the two calculators and found vast difference in the AQI reading.

Let us assume PM2.5 concentration in an area is 1,100 micrograms per cubic meter. As per the IQAir calculator, AQI of that area will be 2,043. But, CPCB calculator will reflect AQI at 1,054. However, CPCB's final AQI will be under 500.

Whom To Trust?

George suggests looking at labels like "severe" or "hazardous" or the concentration of pollutant and accordingly taking health measures.

For instance, at 11:30 am today, IQAir recorded air quality near Mandir Marg in Delhi at 489 with PM2.5 being the major pollutant. PM2.5 concentration was recorded at 320 micrograms per cubic meter.

At 11 am, CPCB recorded PM2.5 concentration at 335 micrograms per cubic meter.

Therefore, look at concentrations or simply the label "very poor" or "severe".

IQAir or CPCB, the bottom line is that Delhi's air is polluted.