Mercury vapor is a type of gas that forms when mercury, a heavy metal, is heated. At room temperature, mercury is a liquid, but when it reaches a certain temperature, it evaporates and creates vapor. This vapor can be harmful if inhaled, as it contains toxic properties that can affect the nervous system. In many applications, such as in fluorescent lamps and mercury vapor lamps, this gas is used to produce light. These lamps work by passing an electric current through the vapor, which emits ultraviolet light that then excites a phosphor coating inside the lamp, producing visible light.