Eighteen Bulgarian cities have “extremely polluted” air in the past several days, shows data of the Executive Environment Agency, reports Nova TV. The fine dust particles in the air of Sofia, Gorna Oryahovitsa, Pleven, Pernik, Burgas and Dimitrovgrad exceed the norms three-four times. The pollution is caused by traffic, industrial emissions and the use of coal and firewood for heating and is aggravated by the fog, which contains the dust particles close to the ground.