Fossil fuels such as coal and oil can contain a significant amount of sulfur. When fossil fuels are burned, about 95 percent or more of the sulfur is generally converted to sulfur dioxide (). Such conversion happens under normal conditions of temperature and of oxygen present in the flue gas. However, there are circumstances under which such reaction may not occur.
 can further oxidize into sulfur trioxide () when excess oxygen is present and gas temperatures are sufficiently high. At about 800 °C, formation of  is favored. Another way that  can be formed is through catalysis by metals in the fuel. Such reaction is particularly true for heavy fuel oil, where a significant amount of vanadium is present. In whatever way  is formed, it does not behave like  in that it forms a liquid aerosol known as sulfuric acid () mist that is very difficult to remove. Generally, about 1% of the sulfur dioxide will be converted to . Sulfuric acid mist is often the cause of the blue haze that often appears as the flue gas plume dissipates. Increasingly, this problem is being addressed by the use of wet electrostatic precipitators.