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Air recirculation is when a circulation device moves air in a closed space around more than once. Such devices can serve either personal or commercial purposes. They are used and adapted to a range of circumstance, from homes and cars to industrial buildings and laboratory clean rooms. Air recirculation keeps out, or limits the amount of, outside air coming into a space in order to avoid pollutants, unwanted odors and to aid in air conditioning.
Air recirculation systems recycle air already contained within a space and control the amount of new air entering that space; in many cases, they also pass old air through a filtration system. Simple home fans might be considered a form of this type of circulation although they do not provide filtration and only circulate air within a very limited space.
Small fans are mainly used to provide an isolated artificial breeze, whereas air recirculation systems are designed to maintain a fixed air flow and indoor air quality throughout a vehicle or building. Recirculating air can also help control temperatures since air conditioning systems can be more effective when re-warming or re-cooling air that has already passed through the system a few times. Buildings in colder climates use the recirculation of air to keep out cold air and to contribute to thermal conditioning.
If not well designed and maintained, air recirculation systems might aggravate respiratory conditions and lead to the spread and concentration of air contaminants. In enclosed areas — most quickly in small ones such as in the passenger compartment of a car — dangerous carbon monoxide levels can develop if fresh air is kept completely blocked from entry for an extended period of time. To deal with this potentially serious problem, many car manufacturers add carbon dioxide detectors as safe-guards to their air conditioning and recirculation systems. People can elect to install carbon dioxide detectors in homes as well and, in some U.S. states and other jurisdictions, installation has become obligatory.
There are certain circumstances in which specially tailored systems are needed for unique purposes. In the case of buildings in which industrial processes are carried out, appropriate ventilation is necessary, and air recirculation systems are designed to regularly bring in needed amounts of fresh air and to expel air that has been contaminated. In a laboratory clean room, which is a room maintained with as few contaminants as possible for research or manufacturing purposes, a particularly powerful airflow is required to ensure sanitation.