Residents of Delhi and adjoining areas were granted no respite on Thursday as the air quality stayed at "very poor" levels.
According to the Centre-run System of Air Quality and Weather Forecasting And Research (SAFAR), the Air Quality Index of the capital in the morning was recorded at 312, falling in the 'very poor' category.
The AQI between the range of 51 to 100 is considered as satisfactory, 101-200 is moderate, 201-300 falls under the category of poor. While 300-400 is considered as 'very poor', levels between 401-500 fall under the 'hazardous category'.
According to SAFAR, "the overall air quality of Delhi is in the lower end of the very poor category, with PM2.5 as lead pollutant. This is the first time in the season that it entered in this zone firmly and likely to remain the same for next two days with a marginally increased magnitude."
"Everyone should reduce heavy exertion. People with heart or lung disease, older adults, and children should avoid longer or heavy exertion," a SAFAR advisory read.
The Delhi government has attributed the dip in air quality to rampant stubble burning in neighbouring states, a regular feature in October-November.
The period between October 15 and November 15 is considered critical as maximum number of stubble burning incidents take place in this period in Punjab and adjoining states and is one of the main reasons for alarming spike in pollution in Delhi-NCR.
A NASA satellite imagery late Wednesday evening showed a large number of fire spots due to stubble burning in Haryana, Punjab and western Uttar Pradesh.
However, according to SAFAR, Thursday's worsened air quality was due to changing local weather conditions rather than external intrusion (stubble burning).
"The stubble burning activity in Haryana, Panjab, and nearby border regions has shown an increasing trend over the last 48 hrs; additionally, few new fires are observed over western Uttarpradesh. However, the wind directions are not favourable for the direct plume transport to Delhi and SAFAR model estimates the biomass contribution as just 5 per cent," it said.
SAFAR warned that further deterioration in Delhi's air quality may start from the fourth week of October. It said stagnant weather conditions coupled with firecrackers or stubble burning are the likely triggers.
"In a landlocked city like Delhi, it may lead to rapid accumulation and may trigger high pollution events. However, if local emissions are controlled, it will be a good check to observe and avoid air quality crises", it said.