The term air conditioning refers to the cooling and dehumidification of indoor air for thermal comfort. In a broader sense, the term can refer to any form of cooling, heating, ventilation or disinfection that modifies the condition of air. An air conditioner (AC or A/C in North American English, aircon in British and Australian English) is an appliance, system, or mechanism designed to stabilise the air temperature and humidity within an area (used for cooling as well as heating depending on the air properties at a given time), typically using a refrigeration cycle but sometimes using evaporation, most commonly for comfort cooling in buildings and motor vehicles. The concept of air conditioning is known to have been applied in Ancient Rome, where aqueduct water was circulated through the walls of certain houses to cool them. Similar techniques in medieval Persia involved the use of cisterns and wind towers to cool buildings during the hot season. Modern air conditioning emerged from advances in chemistry during the 19th century, and the first large-scale electrical air conditioning was invented and used in 1902 by Willis Haviland Carrier. A poorly maintained air-conditioning system can occasionally promote the growth and spread of microorganisms, such as Legionella pneumophila, the infectious agent responsible for Legionnaires' disease, or thermophilic actinomycetes, but as long as the air conditioner is kept clean these health hazards can be avoided. Conversely, air conditioning, including filtration, humidification, cooling, disinfection, etc., can be used to provide a clean, safe, hypoallergenic atmosphere in hospital operating rooms and other environments where an appropriate atmosphere is critical to patient safety and well-being. Air conditioning can have a positive effect on sufferers of allergies and asthma. In serious heat waves, air conditioning can save the lives of the elderly. Some local authorities even set up public cooling centers for the benefit of those without air conditioning at home. Poorly operating air conditioning systems can generate sound levels that contribute to hearing loss, if exposures are endured over a long term. These levels are similar to the exposure of living near a busy highway or airport for a considerable length of time. Properly functioning air conditioners are much quieter. A portable air conditioner or portable A/C is an air conditioner on wheels that can be easily transported inside a home or office. They are currently available with capacities of about 6,000 to 60,000 BTU/h (1,800 to 18,000 watts output) and with and without electric resistance heaters. Portable true air conditioners come in two forms, split and hose. Evaporative coolers, sometimes called conditioners, are also portable.
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