A level up to 100 is considered "good" or "satisfactory", an index value between 101 and 200 is "moderate", between 201 and 300 is "poor", 301 to 400 is categorised "very poor", and a higher value is "severe".
Expecting a highly polluted Diwali, a pollution task force headed by the CPCB on Friday issued advisories to a Supreme Court-appointed environment body, suggesting closure of industries, construction and intensified traffic regulation between November 1 and 10. Punjabi Bagh recorded the highest pollution level in the national capital at 434.
The task force also advised public to avoid strenuous outdoor exercises to minimise their exposure to the toxic air, and cut down the use of private cars during the first 10 days of November. In the NCR, Ghaziabad and Gurugram already recorded "severe" pollution levels, while Noida and Greater Noida breathed "very poor" air.
Twenty-nine monitoring stations located in different parts of the city recorded very poor air quality while four stations recorded severe air quality.
The PM2.5 level (presence of particles in the air with a diameter of less than 2.5 micrometres) was recorded at 179.More news: U.S. mail bombs: FBI focuses on Florida in suspicious packages investigation
Environmentalists said that the dip in air quality was not just due to localised factors such as construction dust, vehicular pollution, but also regional factors such as stubble burning in Punjab and Haryana. "The weather conditions include wind direction, temperature etc".
The air quality in Delhi had shown signs of improvement on Monday as it moved from "very poor" to "poor" category; however, it fell to "very poor" levels again on Wednesday even as firefighting continued at the Bhalswa landfill site. The residential bio-fuel emission declined significantly in Delhi at 64% since 2010, the report said.
"The prevailing meteorological conditions are not very favourable for dispersal of pollutants for the next two days due to very low ventilation index and low wind speed", the IITM warned.
But there is no escape from air pollution.
The Central Pollution Control Board and SAFAR on Saturday issued separate advisories.
SAFAR asked people with heart or lung disease, besides older adults and children to avoid prolonged or heavy exertion.