The national capital's air quality was in the very poor category on Tuesday morning, the first time this season, with calm winds and low temperatures allowing accumulation of pollutants.
According to the Ministry of Earth Sciences' Air Quality Early Warning System for Delhi, an increase in farm fires in Punjab, Haryana and neighbouring regions of Pakistan is also going to impact the air quality in Delhi-NCR.
The city recorded an air quality index (AQI) of 304 at 9:30 am, which falls in the very poor category. The 24-hour average AQI was 261 on Monday, the worst since February. It was 216 on Sunday and 221 on Saturday.
Wazirpur (AQI 380), Vivek Vihar (AQI 355) and Jahangirpuri (AQI 349) recorded the highest pollution levels.
An AQI between 0 and 50 is considered 'good', 51 and 100 'satisfactory', 101 and 200 'moderate', 201 and 300 'poor', 301 and 400 'very poor', and 401 and 500 'severe'.
A senior scientist at the Delhi Pollution Control Committee said the dip in the air quality can be attributed to low wind speed and temperatures which allowed accumulation of pollutants.
Stubble burning has also increased in neighbouring states. Also, the ventilation index is low, he said.
Ventilation index is the speed at which pollutants can get dispersed. A ventilation index lower than 6000 sqm/second, with average wind speed less than 10 kmph, is unfavourable for dispersion of pollutants.
PM10 levels in Delhi-NCR stood at 300 microgram per cubic meter ( g/m3) at 9 am — the highest this season so far, according to CPCB data. PM10 levels below 100 g/m3 are considered safe in India.
PM10 is particulate matter with a diameter of 10 micrometers and is inhalable into the lungs. These particles include dust, pollen and mold spores.
The levels of PM2.5 finer particles which can even enter the bloodstream were 129 g/m3. PM2.5 levels up to 60 g/m3 are considered safe.
NASA's satellite imagery showed a large cluster of fires near Amritsar and Firozpur in Punjab and Patiala, Ambala and Kaithal in Haryana.
According to the System of Air Quality and Weather Forecasting and Research, farm fires contributed 3 percent particulate matter to Delhi's PM2.5 concentration on Monday.
It is likely to be negligible for the next two days due to a change in the wind direction from northwesterly to southeasterly, the government agency said.
On Tuesday morning, the maximum wind speed was 4 kilometers per hour, according to the India Meteorological Department.