There are six Air Quality Index (AQI) categories on a scale from 0-500 - Good, Satisfactory, Moderate, Poor, Very Poor and Severe.
The air pollution level in Indian capital city New Delhi and outskirts of the national capital region (NCR) remained in poor category on Wednesday, officials said. Authorities have warned of a spike in the pollution levels after Diwali on Wednesday even if "partially toxic crackers" are burnt compared to a year ago.
The deterioration of air quality comes ahead of Diwali when a further spike in pollution is expected to hit the city.
D Saha, former additional director and head of air quality management division at the CPCB had earlier said that Delhi was likely to have a clean Diwali.
People residing in the national capital breathed easy on Wednesday after the air quality showed slight improvement due to increased wind speed and requisite control measures undertaken by the government.
The AQI on Tuesday morning was recorded at 400, just a point below "severe".
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Coarser and highly unsafe PM2.5 or particles with diameter less than 2.5 mm continue to dominate the pollution factor in Delhi, and are still at least four times higher than the national standards and nine times the worldwide standards. The ventilation coefficient is expected within 7000-8000 m2/s. Winds coming from north westerly direction bring influence of biomass burning pollutants in Delhi-NCR.
Delhi's air quality is expected to deteriorate to "severe plus emergency" category after Diwali, the Centre-run System of Air Quality Forecasting and Research (SAFAR) said.
"Even if 50 per cent of the total load of toxic firecrackers as compared to Diwali-2017 is added, the prevailing weather conditions will aggravate the high smoke level and make air quality to persist in severe range for at least two days on November 8 and November 9", SAFAR said in a report.
The pollution authority could also consider inducing artificial rain after Diwali to wash away hazardous pollutants.
Shamika Ravi, a members of the Economic Advisory Council to the Prime Minister, tweeted today: 'Haven't heard a single firecracker in Delhi, yet the air purifiers are blazing red - showing risky levels of pollution.