The decision follows an advisory issued by the Bihar State Pollution Control Board (BSPCB) that said air quality in Patna was 'very poor' on 20 days and 'severe' on nine days of November 2015.

"Decision to ban plying of 15-year-old diesel vehicles on roads was taken during a review meeting of the environment department chaired by chief minister Nitish Kumar," an official said on Thursday.

The state transport authority had proposed banning of old diesel-run vehicles as far back as October 2013, but the measure could not be implemented then.

Nitish Kumar also instructed the government agencies to draw out a plan to control pollution and told officials to curb burning of plastic and solid waste in the open.

The BSPCB advisory, issued earlier this month, said Patna's air quality index (AQI) in November was 'severe' on nine days and 'very poor' on 20 days.

'Severe' means particulate matter-2.5 (or PM-2.5) is over 400 and can 'affect healthy people and seriously impacts those with existing diseases'. 'Very poor' means PM-2.5 is between 301 and 400 and can cause 'respiratory illness on prolonged exposure'.

Air quality in Patna has deteriorated due to high level of tiny particulate matter, which is bad for health, experts pointed that. Dust, vehicular emissions, burning of waste in the open, brick kilns and construction activities are the factors contributing to an alarming level of air pollution.

A WHO survey, published in May 2014, put Patna behind Delhi in terms of severity of air pollution based on particulate matter in ambient air.

The Central Pollution Control Board (CPCB) has listed Patna as a 'non-attainment city' with PM-10 and PM-2.5 levels persistently exceeding national ambient air quality standards.