The carbon dioxide laser (CO2 laser) was one of the earliest gas lasers to be developed. The active laser medium (laser gain/amplification medium) is a gas discharge which is air- or water-cooled, depending on the power being applied. Melasma (also known as chloasma faciei): or the mask of pregnancy when present in pregnant women) is a tan or dark skin discoloration. Melasma is thought to be caused by sun exposure, genetic predisposition, hormone changes, and skin irritation.