The national capital's air quality on Thursday turned to the 'severe' category, with the overall air quality index (AQI) of Delhi recorded at 407.

According to the Centre-run System of Air Quality and Weather Forecasting (SAFAR), Delhi's overall AQIwas recorded at 9 a.m. at 407, counted under the 'severe' category.

As per SAFAR, the AQI from 0-50 is considered 'safe'. Between 51-100 'satisfactory', between 101-200 is 'moderate' and 201-300 is 'poor', while 301-400 is 'very poor' and 401-500 is in the 'severe' category.

SAFAR has advised people to avoid all physical activity outdoors.

"Give a miss to walk today. If the room has windows, close them. If the air conditioner provides a fresh air intake option, close it. Keep the room clean - don't vacuum. Do wet mopping frequently," it said.

It also advised people to stop any activity level if experience any unusual coughing, chest discomfort, wheezing, breathing difficulty, or fatigue and consult a doctor.

"Masks known as N-95 or P-100 respirators may only help if you go out. Do not rely on dust masks for protection," it added.

The AQI of the National Capital Region was also high with Ghaziabad (470), Noida and Greater Noida (422), Faridabad (398) having a similar experience.

The city had witnessed drastic improvement in the air quality in the last week of November as the AQI was recorded in double digits at the 'satisfactory' category, after being seeing few of the worst air quality days the same month.

The air in the national capital was so toxic after Diwali that the Environmental Pollution (Prevention and Control) Authority (EPCA) had declared a public health emergency in the Delhi-NCR and had advised people, especially children and the aged, to limit their exposure to the environment.