Activated carbon works via a process called adsorption, whereby pollutant molecules in the fluid to be treated are trapped inside the pore structure of the carbon substrate. Carbon filtering is commonly used for water purification, in air purifiers and industrial gas processing, for example the removal of siloxanes and hydrogen sulphide from biogas. It is also used in a number of other applications, including respirator masks, the purification of sugar cane and in the recovery of precious metals, especially gold. It is also used in cigarette filters