Sulfur dioxide is an acrid yellowish gas with the typical "burnt match" odor. It is produced by combustion of sulfur, typically found in coal or biogas. The main source is still coal-firing, mostly from power stations and similar large-scale combustion systems. There is very little way to remove the sulfur from coal, and the only real solution is to filter out the sulfur dioxide after combustion. Sulfur dioxide is one of the main contributions to acid rain and smog formation, and has long been a target for legislation in most countries. Sulfur dioxide is highly water soluble, which makes measurement slightly difficult. It is best measured using an appropriate heated hose and dryer, otherwise there will be a loss of approximately 20 % of the sulfur dioxide due to absorption.