Explainer: What to make of the variations in air quality readings

The skies of Delhi-NCR and parts of Punjab and Haryana were engulfed in a thick haze on the morning of October 21, a day after Diwali. While the government-run Central Pollution Control Board (CPCB) and private air pollution monitoring agencies were on the same page in categorising the air quality as “polluted”, the readings of a major pollutant — particulate matter 2.5 (PM2.5) — by the different agencies varied, sparking a debate.

Of the 39 monitoring stations in Delhi, 35 had hit the ‘red zone’, which according to the CPCB could lead to respiratory illness. The CPCB data showed that the overall level of PM2.5 in Delhi was 488 micrograms per cubic meter on October 22, several times the exposure limit advised by the World Health Organisation (WHO). Its permissible limit is 15 micrograms per cubic meter in 24 hours.

However, scientists stress that irrespective of the calculations, the crux of the matter is that Delhi’s air pollution is severe.

Different categorisations

IQAir, a Swiss air-quality monitoring company, read Delhi’s Air Quality Index (AQI) at 1,121 on Tuesday (October 21) and CPCB reported a much lower AQI of 351. Delhi’s air quality on Diwali this year was worse than last year (328 in 2024), and higher than the previous two years — 218 in 2023 and 312 in 2022.

In Punjab, the CPCB readings showed Ludhiana with the highest AQI at 271, followed by Amritsar at 224 and Jalandhar at 247.

The CPCB derives the AQI from eight pollutants — particulate matter (PM) 10, PM2.5, ozone (O3), sulphur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), lead (Pb) and ammonia (NH3).

It 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’.

On the other hand, IQAir categorises air quality in a watertight compartment. Here, 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.

Making sense of it

Sunil Dahiya, founder and lead analyst, Envirocatalysts, an environmental organisation, says that both CPCB and IQAir use the same baseline of PM2.5 concentration in micrograms per cubic meter (µg/m³) to calculate AQI, but use different cutoff points and formulas to calculate the AQI number.

“While IQAir, which uses the US EPA formula, assumes that air pollution leads to huge health impacts at lower concentrations, the Indian National Air Quality Index assumes higher thresholds. Ideally, all media, researchers and policymakers should use the concentrations rather than AQI numbers, but for the general public, AQI is a good indicator, though it doesn’t make much difference which one they should rely on once it crosses the good threshold, because anything above that is detrimental,” he points out.

On whether awareness regarding AQI has increased, Dahiya says, “Over the past years, we have seen increasing awareness and understanding of air quality and AQI in the general public, but the understanding hasn’t yet resulted in them changing their activities or even taking safeguard measures and demanding clean air to the level it should be.”

Timing and sensors

Dr Gufran Beig, Chair Professor at the National Institute of Advanced Studies (NIAS), says that the farm fire incidents in northern India “have not been much this year”, or else the problem would have been worse.

“As far as weather conditions are concerned, Diwali this year was celebrated at a time when winter has not completely set in. Had it been celebrated in November, conditions would have been severe. Weather conditions in North India are still not that favourable for pollutants to accumulate,” he adds.

According to Beig, several private agencies use low-cost sensors which are vulnerable to weather and give high readings.

“Many private agencies follow the algorithm of the WHO. They don’t cap the reading of AQI at 500. CPCB caps its reading at 500 AQI, which is equivalent to 450 micrograms per cubic meter. The concept of AQI only comes when we have a 24-hour average of the PM2.5 concentration. The PM2.5 concentration needs to be collected for 24 hours at any station and then a daily average value is taken out to convert it into AQI. But CPCB calculates AQI at each hour and caps it at 450 micrograms per cubic meter. This means that even when concentrations exceed 450 micrograms per cubic meter, the AQI cannot go higher,” Beig explains.