Environment: Sofia, most polluted capital of Europe Bulgaria's capital Sofia, a city of about 1.3 million people, is the European capital with the most polluted air, data by the European Environment Agency shows. The air pollution, blamed in part on old cars and the use of firewood for heating, can cause respiratory problems and even lung cancer. Now country risks paying huge fines to the European Commission for failing to reduce the levels of pollution. A natural sound version of an AFPTV report.