Sulphur dioxide is a yellowish, acrid gas. It is highly water-soluble and has a typical odour of burnt matches. Sulphur dioxide is one of the main components of acid rain. The main source of sulphur dioxide is probably coal-fired power generation, since this often uses low grade coal, which may have a high level of sulphur in it. Sulphur dioxide is also produced during the combustion of hydrogen sulphide, which may be present in biogas from animal sources. Sulphur dioxide levels are regulated in most countries due to its aggressive nature and effect on plant and animal life. Many of the sources of sulphur dioxide are not easily avoided, so the main remedy is filtration to remove sulphur dioxide and other toxic components.