The European Commission said on Thursday that it has decided to refer Bulgaria to court over the country's failure to respect limit values of sulphur dioxide in ambient air. "... the latest sulphur dioxide(SO2) data presented show persisting non-compliance with the hourly and/or daily limit values for SO2 in the South-East zone, where the four largest thermal power plants in Bulgaria are located," the EU institution said in a monthly infringement decisions package. The EU sulphur dioxide limits are applicable to Bulgaria as of 2007, when the country joined the EU. This is the second time that Bulgaria faces challenges with meeting EU air quality standards. In 2017, the Court of Justice of the European Union confirmed Bulgaria's failure to comply with the limit values for particulate matter, after the country was referred to court by the Commission. Sulphur dioxide is emitted when fuels containing sulphur - coal and oil - are burned for domestic heating, power generation and motor vehicles. Sulphur dioxide can affect the respiratory system and the functions of the lungs, and causes irritation of the eyes. Sulphuric acid is also the main component of acid rain, which is a cause of deforestation. Particulate matter is mainly present in emissions from industry, traffic and domestic heating, but also results from agricultural emissions.