Some compounds of fluorine (such as sodium fluoride, stannous fluoride and sodium MFP) are added to toothpastes to prevent dental cavities. These are now regularly added to water as well. Most general anesthetics are derived from compounds of fluorine. Fluorine-18 emits positrons and has a relatively large half-life. This makes it ideal for use in positron emission topography. Anti-reflective coatings contain compounds of fluorine. Fluorine can be used for plasma etching, flat panel display and MEMS (microelectromechanical) fabrication. HF (hydrofluoric acid) is used to etch glass, usually in light bulbs. Fluorine is used in one step of the production of halons such as freon. Fluorine is used to obtain pure uranium from uranium hexafluoride. Compounds of fluorine are used in refrigeration and air-conditioning systems. Another compound of fluorine is used in the electrolysis of aluminum. This process allows pure aluminum to be obtained! A who family of broad-spectrum antibiotics (these work on a number of different bacteria) contain fluorine. Much commercially produced fluorine is used to make sulfur hexafluoride. This compound is used as a dielectric medium in the electrical industry.