Concerns raised over ‘micro-management’ of air quality data

As smog blankets the national capital, Delhiites continue to complain of burning eyes, scratchy throats and breathing difficulties. The toxic haze has once again brought attention to the accuracy of the city’s official air quality data. Media reports have pointed out glaring inconsistencies and questionable practices near several monitoring stations, raising doubts about the credibility of pollution readings.

Last month, an online survey of 14,978 residents revealed that 87 per cent have little or no trust in the AQI data released by the government. While 62 per cent said they had no trust and 25 per cent reported low trust, only 13 per cent expressed average or higher trust, reflecting a credibility gap in official air quality monitoring.

Amid these concerns, allegations that the authorities might be “micro-managing” pollution readings by spraying water near monitoring stations have sparked debate about whether such actions could distort air quality data.

Delhi’s Environment Minister Manjinder Singh Sirsa, however, defended the practice. “How would we suppress the dust? It’s natural- through sprinkling. It is being done throughout the city, including around stations. We are successfully able to control pollution,” he said. But experts warn that such measures, if done excessively near monitors, can be misleading.

How Delhi’s AQI is measured?

Dr Mohan P George, Consultant for Clean Air and Sustainable Mobility at the Centre for Science and Environment (CSE) and former Additional Director at the Delhi Pollution Control Committee, explains that India’s AQI is based on the concentration of pollutants such as PM2.5, PM10, nitrogen dioxide (NO2), sulfur dioxide (SO2), carbon monoxide (CO), ozone (O3), and lead.

“At least three parameters are required to calculate AQI,” he said. “Usually, PM2.5, PM10, and NO2 are used. The data from continuous analysers is converted into an index value ranging from 0 to 500 - from ‘good’ to ‘severe,’ with corresponding colour codes and public advisories.” The Central Pollution Control Board (CPCB) operates Delhi’s Continuous Ambient Air Quality Monitoring Stations (CAAQMS), which automatically transmit real-time data every few minutes. There are currently 39 such stations across the city.

Can data be tampered with?

Experts say direct tampering with CAAQMS instruments is nearly impossible, but readings can be influenced indirectly.

“The CAAQMS instrument itself cannot be tampered with,” said Dr Kumar. “But sprinkling water around stations to suppress dust can temporarily reduce PM10 levels by 10–20 per cent for a few hours. Once the surface dries, dust re-suspends, so the effect is short-lived.” Dr George, however, said such interventions “serve no scientific purpose”. “There’s no concrete study linking water sprinkling to alter AQI readings, but it certainly increases humidity and is more cosmetic than corrective,” he said.

Is outdated data responsible?

A recent Council on Energy, Environment and Water (CEEW) study found that while Delhi’s Air Quality Early Warning System (AQEWS) successfully predicted over 80 per cent of poor-air days last winter, it still relies on emission inventories from 2016 for NCR districts and 2018 for Delhi.

“These outdated inventories make it difficult to reflect current pollution sources,” the report noted.

Dr Kumar explained, “Emission factors must be updated regularly across all sectors. While Delhi now has cleaner BS-VI vehicles, the number of vehicles has grown. Outdated data doesn’t capture this, leading to underestimation of emissions from transport and industry.”

Is Delhi’s AQI data reliable?

Experts agree that when properly calibrated, Delhi’s reference stations produce credible data, but consistency is key.

“If everything is properly maintained, the AQI is reliable,” said Dr George. “However, the raw concentration data, not the index, is more scientifically useful. AQI simplifies complex data for public understanding.” Dr Kumar added that even high-end monitoring stations need regular calibration. “Even Rs 1-crore CAAQMS unit must be recalibrated periodically. Sensors are sensitive to humidity and temperature, so readings can vary with weather conditions,” he said.

Why individual devices show higher AQI?

Social media is filled with Delhi residents sharing screenshots from home air purifiers or pocket sensors showing far higher AQI levels than official data.

“There’s no such thing as an ‘AQI reader’,” said Dr George. “Most devices are particle counters that estimate mass concentration and convert it into AQI using different formulas. They’re affected by humidity, temperature, and air movement.”

Dr Kumar noted that while high-quality sensors can approximate real values within 10–20 per cent, cheap ones can vary widely. “If every monitor across the city shows ‘hazardous’ levels, we must accept that the city’s air is severely polluted. The question isn’t whether the number is 350 or 400, it’s what we’re doing about it,” he said.