Did Delhi’s attempt at a car-free day on Thursday improve its air quality? Probably, but not by as much as the government is claiming, a close reading of the data shows.

On Thursday, cars were asked to remain off the roads from Red Fort to India Gate, over a 6-8 km stretch of the city, from 7 a.m. to 12 noon. Thursday was also a public holiday on account of Dussehra. Using hand-held exposure monitors, two environmental groups, the Centre for Science and Environment (CSE) and the Greenpeace India, separately observed lower levels of air pollution on Thursday morning as compared to Wednesday morning.

The CSE found that the concentration of particulate matter of less than 10 micrometres in diameter (PM 10) was 310 microgramme per cubic metre on average for the five hours for that stretch of road, or 59 per cent less than on Wednesday. The PM 2.5 level, or the concentration of fine particles particularly dangerous for respiratory health, was 265, or 62 per cent less than the previous day.

Even with the reduction, the PM10 levels recorded by the CSE would classify the air quality of the stretch as ‘poor’, according to India’s Air Quality Index, and the PM2.5 levels would qualify as ‘severe’. The Greenpeace monitored PM 2.5 levels over a four-hour period on the same stretch on Wednesday and Thursday and found that the concentration on Thursday was 172 microgramme per cubic metre, as compared to 428 on Wednesday, or an air quality of ‘very poor’ as compared to ‘severe’.

However, the CSE and the Greenpeace’s observations were from exposure monitoring, or the air quality experienced by an individual, while the AQI is with reference to ambient air quality. The AQI values are also for 24-hour averages.

Across the four functioning air quality monitoring stations in the city, the PM 2.5 level for Thursday was half that of Wednesday. But a comparison between Thursday and Wednesday is for a number of reasons unfair. “It will not be fair to compare the difference in particulate matter of today’s 5 hours average with yesterday’s 5 hours average because yesterday was a bad day due to higher winds and mild dust storm during morning hours and the level of PM10 and PM2.5 during these hours were quite high,” Dr. Gufran Beig, project director of the Pune-based Indian Institute of Tropical Meteorology’s System of Air Quality and Weather Forecasting And Research (SAFAR), told The Hindu. At SAFAR’s Lodhi road monitoring station closest to the car-free zone, Thursday morning saw a 14 per cent reduction in PM10 levels compared to the previous six days’ morning average, and a 19.5 per cent reduction in PM2.5 levels, Dr. Beig said. Additionally, Thursday morning’s air quality was better than on Sunday (October 18).

Given that Thursday was a public holiday, traffic volumes were expectedly lower. “We’ve been finding that traffic volumes do not really reduce on weekends, but come down substantially on public holidays,” Anumita Roychowdhury, CSE’s executive director (research and advocacy) and an air pollution expert, told The Hindu.

“We also observed that roads were being cleaned on Wednesday morning which would have raised the particulate matter levels,” Sunil Dahiya, Greenpeace campaigner, told The Hindu. Thursday’s findings simply demonstrated that days when there are fewer cars on the street see an improvement in air quality, the CSE said.