NEW DELHI - The air pollution level in Indian capital city -- New Delhi and the outskirts of the national capital region (NCR) Thursday continued to be severe for the second straight day, officials said.

According to the Central Pollution Control Board (CPCB), the overall air quality index (AQI) in the city remained in severe category at 428.

"The AQI in Delhi at 9:00 a.m. (local time) today was recorded at 428," data released by CPCB said. "The prominent pollutants are PM 2.5 and PM 10, and air quality is in the severe level category."

As per CPCB, AQI between 0 - 50 is considered "good", between 51 - 100 "satisfactory", between 101 - 200 "moderate", between 201 - 300 "poor", between 301 - 400 "very poor", and between 401 - 500 "severe" and beyond 500 is severe plus emergency.

Health officials say the deterioration in air quality causes respiratory illness on prolonged exposure and people should stop outdoor activity in early morning and after sunset.

"People should also avoid prolonged or heavy exertion and use masks known as N-95 or P-100 respirators while going out to minimize the effect," the experts said.

The System of Air Quality and Weather Forecasting and Research (SAFAR), an institute monitoring air pollution, said an improvement in AQI is expected by evening.

Last month the air quality in the city remained in a hazardous range for several days. The local government even implemented car rationing scheme know as the "odd-even" formula to tackle the rising air pollution in the city.

Delhi tops the chart of the most polluted cities in the world.

Industrial activities and vehicular traffic are significant contributors to air pollution in the city, along with rampant construction. For the past many years, every year ahead of winters the air quality in Delhi becomes bad. Officials in Delhi blame the stubble burning in neighboring states of Punjab and Haryana for the increasing pollution.