New Delhi, Nov 21 (IANS): The overall air quality of the national capital on Thursday slipped into the higher end of the 'very poor' category and may deteriorate further due to the extremely low wind speed, after being in 'poor' for days.

According to the Centre-run System of Air Quality and Weather Forecasting (SAFAR), the overall AQI was recorded at 340.

"The overall Delhi AQI is in the higher end of very poor category as further deterioration is predicted and may touch severe in some parts of Delhi by November 22," SAFAR said.

It said the main factors working towards the deterioration of Delhi Air quality is the extremely low wind speed and strong near-surface inversion during the night time.

"Hence the ventilation is minimum and leading to accumulation of pollutants near the surface. The conditions are likely to improve by November 23 afternoon since a strengthening of surface winds are forecast owing to the passing of Western disturbance," it said.

It also said that though the fire counts are relatively high at 1,011 (as per SAFAR multi-satellite product) as compared to last week, the stubble level winds are easterly and not favourable for plume intrusion.

SAFAR advised sensitive people to avoid all physical activity outdoors. "Also, if asthematics, keep relief medicine handy," it said.

For the general public, it advised stopping the outdoor activity in the early morning and after sunset.

"Avoid prolonged or heavy exertion. Go for a short walk instead of a jog and take more breaks. Stop any activity if you experience any unusual coughing, chest discomfort, wheezing, breathing difficulty, or fatigue," it added.