In ordinary conversation, when the term air conditioning is mentioned, people automatically take it to mean cooling room temperature. Technically speaking, though, air conditioning is the process of controlling room temperature through various means that involve ventilation, heating and cooling. In construction, it is referred to as HVAC.

In 1902, The New York Stock Exchange was the very first building to use air conditioning units that separately cooled and heated the air. Later that year, Willis Havilland Carrier installed the very first electrically-run air conditioning unit at a printing plant. The unit not only cooled the air but also controlled the humidity.

In those times, air conditioning was mostly used in manufacturing facilities to improve products by controlling the effects of humidity, thus, increasing employee productivity. It was several decades later, in the 1950s, that homes and automobiles started to be fitted with the air conditioning technology.

An air conditioning unit cools the air through a refrigerant, a substance that alters the temperature of the air. In the early stages of its development, air conditioning units used harmful gases such as ammonia and methyl chloride as refrigerants. They were soon replaced by another type of chemical, the patented substance called Freon.

Freon is safe for humans. However, it is harmful to the environment. When released into the atmosphere, this chlorofluorocarbon gas is believed to be a contributing factor to the thinning of the ozone layer. Most air conditioning units sold today still use this type of gas, particularly the HCFC R-22. It is said that environmentalists will have to wait until 2010 to see the total eradication of HCFC R-22 in air conditioning untis.

Air conditioning units have different types of refrigeration cycles. The first type uses a heat pump that drives out hot air from the room and blows in cooled air though a compressor. The second type pumps in a refrigerant. When the refrigerant evaporates, it takes with it the hot air and leaves cooled air.