In the broadest sense air conditioning can refer to any form of cooling, heating, ventilation that modifies the condition of air, mostly used for thermal comfort.

An air conditioner is an appliance, system designed to extract heat from a room using a refrigeration cycle. The most common uses of modern air conditioners are for comfort cooling in buildings and in vehicles.

Humidity control

Refrigeration air conditioning equipment usually reduces the humidity of the air processed by the system. The relatively cold evaporator coil condenses the water vapor from the processed air, sending the water to a drain or onboard tank which may need to be emptied every so many hours depending on the heat level.

Since humans perspire to provide natural cooling by the evaporation of perspiration from the skin, drier air improves the comfort provided. The comfort air conditioner is designed to create between 40% and 60% of relative humidity in the occupied space.

Some air conditioning units dry the air without cooling it, and are better classified as dehumidifiers. They work like a normal air conditioner, except that a heat exchanger is placed between the intake and exhaust. In combination with convection fans they achieve a similar level of comfort as an air cooler in humid tropical climates, but only consume about ⅓ of the electricity. They are also preferred by some people who find the draft created by air coolers discomforting.

Health implications

A poorly maintained air-conditioning system can occasionally promote the growth and spread of microorganisms, such as Legionella pneumophila, the infectious spore responsible for Legionnaires' disease, or thermophilic actinomycetes Conversely, air conditioning, including filtration, humidification, cooling, disinfection, 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.