WARSAW • The Polish capital offered free public transport on Monday to encourage residents to leave their cars at home and stop contributing to the world's worst smog.

The cities of Krakow and Kielce also announced similar incentives amid stifling pollution, reported Radio Poland.

Warsaw readings for a pollutant known as small particulate matter exceeded Polish environmental norms fivefold on Sunday, forcing the government to advise the city's nearly two million residents to stay indoors, reported Bloomberg.

According to monitoring site AirVisual, Warsaw was the most polluted major city in the world on Monday. Poland's systemic smog problem intensifies during cold spells, when in addition to its coal-based power industry, millions of households are heated by furnaces that are all too often fed by burning the cheapest available materials.

Prime Minister Beata Szydlo, herself a coal miner's daughter, supports the use of the dirty fossil fuel because it ensures energy security and provides jobs.

The government is working on ways to reduce the smog, she said on her Twitter account on Monday, without giving details.

"The free transport initiative has a great educational value, but I doubt it will have any lasting impact," Mr Piotr Siergiej, a spokesman for Polish Smog Alarm, a not-for-profit organisation, said by phone. "Household furnaces are responsible for 53 per cent of the emissions in Poland and I really don't understand why we still don't have any norms for them. It's as if we weren't really fighting the air pollution."

AirVisual listed Warsaw's air quality index at 231 on Monday, meaning worse smog than in the Indian city of Kolkata (224), Dhaka in Bangladesh (220) and Beijing (197).

Poland is the European Union's biggest per capita polluter, with more than 80 per cent of its electricity generation coming from coal.

According to the World Health Organisation, Poland is home to 33 of 50 of the EU's most polluted cities.