A space blanket is a blanket designed to be used in emergency situations to reduce heat loss from a person's body. The blanket consists of a thin sheet of plastic material (often PET film) that is coated with a metallic reflecting agent, usually gold or silver in color, which reflects about 80% of the wearer's radiated body heat back to him or her. In the US, space blankets are made by vacuum depositing a very precise amount of pure aluminum vapor onto a very thin, durable film substrate. Space blankets are included in many emergency, first aid, and survival kits because they are usually waterproof and windproof. That, along with the light weight of space blankets, has made them popular among outdoor enthusiasts and emergency workers. Space blankets are also used by marathoners (Heatsheets® by AFM Inc.), given to them at the end of the race. The first space blanket was designed in 1964 for the US space program. Light weight is the main advantage of space blankets.
Because space blankets are generally poor at preventing heat loss due to heat conduction from a person to his surroundings, there is controversy regarding the usefulness of space blankets. For mountaineering purposes a bivouac sack is much more effective.