The other severely polluted cities were Muzaffarpur, Haldia, Lucknow, Agra and Varanasi.

According to national standards, the air quality index (AQI) above 300 is considered 'very poor' and can cause respiratory illness on prolonged exposure. The major pollutant remained PM2.5. The 24-hour concentration of PM2.5 was recorded at 230 g/m³. Both HSPCB and environmental activists blamed the construction work being carried out on the national highway for increasing pollution levels.

"We have noticed a sudden spurt in the level of air pollution in the city. Although the level depends on various factors, including wind speed, humidity and wind direction, construction of the underpass at Rajiv Chowk could also be a factor. Besides, vehicular emissions generally remains high on Monday. All these factors lead to increase in accumulation of particulate matter in the lower atmosphere," said an official of HSPCB.

On Monday, the maximum temperature in Gurugram went up to 25 degrees Celsius and the minimum temperature remained at 7.1 degrees Celsius. "I don't think, weather is now responsible for the increase in pollution, as days are relatively clearer," said a Met official.

Blaming the authorities, a city-based environmentalist, Vivek Kamboj, said, "There is a need to make sure that construction norms are followed while contructing roads, flyovers and underpasses. However, authorities have not been following the norms. Authorities should stop construction work if they can't follow the norms as air quality is more important than any development."