The air quality in Delhi improved to “poor” on Monday after remaining in the “very poor” category over the weekend, PTI reported, citing data from the Central Pollution Control Board. The overall Air Quality Index of the city was recorded at 251 in the evening even as authorities said the quality would deteriorate over the coming few days.

However, the Centre-run System of Air Quality Forecasting and Research reported an overall AQI of 308, falling in the “very poor” category. While Dwarka recorded an AQI of 361, Narela recorded 300. In most parts of Delhi, the AQI was over 200.

The neighbouring cities of Faridabad, Gurugram, Noida and Greater Noida recorded “poor” air quality, while Ghaziabad experienced “very poor” air quality.

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”.

The PM2.5 level in Delhi was recorded at 134 on Monday. Also called particulate matter, the PM2.5 are particles in the air that have a diameter of less than 2.5 micrometres. These particles can travel deep into the lungs and affect lung function. The PM10 level (presence of particles in the air with a diameter of less than 10 micrometres) was recorded at 280.

An unidentified official of the pollution control board said factors like vehicular pollution, construction activities, and wind blowing in from the crop stubble-burning areas in neighbouring states led to the worsening air quality. Satellite images released by the United States’ National Aeronautics and Space Administration on Sunday showed several spot fires burning in Haryana and Punjab.

Congress President Rahul Gandhi on Sunday called for collective steps to reduce pollution levels and described the deteriorating air quality in the Capital as a “serious problem”. “Air pollution in Delhi is a serious problem. It is our responsibility to take possible steps to reduce the pollution around us,” he wrote in a Facebook post. “Without the support of countrymen, no government can provide relief from air pollution.”