On the night of Nov. 20, as temperatures slipped below zero for the first time this year, the only city in the world with a higher concentration of fine particulate matter (PM 2.5) in the air was New Delhi, according to Airly, an organisation based in California that monitors pollution. While a number of European countries besides Poland, such as Germany and Hungary, are burning more polluting brown coal, or lignite, to keep the lights on, experts say it's the use of the fuel at home that will have the biggest impact on health.