Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the slow release of a gas that was trapped, frozen, absorbed or adsorbed in some material. It can include sublimation and evaporation which are phase transitions of a substance into a gas, as well as desorption, seepage from cracks or internal volumes and gaseous products of slow chemical reactions. Boiling is generally thought of as a separate phenomenon from outgassing because it occurs much more rapidly.
Outgassing is a challenge to creating and maintaining clean high-vacuum environments. NASA maintains a list of low-outgassing materials to be used for spacecraft, as outgassing products can condense onto optical elements, thermal radiators, or solar cells and obscure them. Materials not normally considered absorbent can release enough light-weight molecules to interfere with industrial or scientific vacuum processes. Even metals and glasses can release gases from cracks or impurities, but moisture, sealants, lubricants, and adhesives are the most common sources. The rate of outgassing increases at higher temperatures because the vapour pressure and rate of chemical reaction increases. For most solid materials, the method of manufacture and preparation can reduce the level of outgassing significantly. Cleaning surfaces or baking individual components or the entire assembly before use can drive off volatiles.
Outgassing is the source of many tenous atmospheres of terrestrial planets or moons. Many materials are volatile relative to the extreme vacuum of space, such as around the Earth's Moon, and may evaporate or even boil at ambient temperature. Materials on the lunar surface have completely outgassed and been ripped away by solar winds long ago, but volatile materials may remain at depth. Once released, gases almost always are less dense than the surrounding rocks and sand and seep toward the surface. The lunar atmosphere probably originates from outgassing of warm material below the surface. At the Earth's tectonic divergent boundaries where new crust is being created, helium and carbon dioxide are some of the volatiles being outgassed from mantle magma.
Outgassing can be significant if it collects in a closed environment where air is stagnant or recirculated. This is, for example, the origin of new car smell. Even a nearly odourless material such as wood may build up a strong smell if kept in a closed box for months. There is some concern that softeners and solvents that are released from many industrial products, especially plastics, may be harmful to human health. For example, the European Commission has banned the use of DEHP (di-2-ethyl hexyl phthalate) in PVC manufacturing due to such concerns. Some types of RTV sealants outgas the poison cyanide for weeks after application. These outgassing poisons are of great concern in the design of submarines and space stations.
