manufactured by DuPont and other companies. These refrigerants were commonly used due to their superior stability and safety properties. Unfortunately, evidence has accumulated that these chlorine bearing refrigerants reach the upper atmosphere when they escape. Once the CFC reaches the stratosphere the UV-radiation is absorbed by the chlorine-carbon bond which causes homolytic cleavage, yielding a chlorine radical. These chlorine atoms act as catalysts in the breakdown of ozone by turning ozone into diatomic oxygen molecules and leaving a chlorine-oxygen radical. This radical in turn steals another oxygen from ozone yielding two diatomic oxygen molecules and regenerating the chlorine radical, which does severe damage to the ozone layer that shields the Earth's surface from the strong UV radiation. The chlorine will remain active as a catalyst until and unless it binds with another free-radical forming a stable molecule and breaking the chain reaction. CFC refrigerants in common but receding usage include R-11 and R-12. Newer and more environmentally-safe refrigerants include HCFCs (R-22, used in most homes today) and HFCs (R-134a, used in most cars) have replaced most CFC use. HCFCs in turn are being phased out under the Montreal Protocol and replaced by hydrofluorocarbons (HFCs), such as R-410A, which lack chlorine.

 
    The external section of a typical single-room air conditioning unit. For ease of installation, these are frequently placed in a window. This one was installed through a hole cut in the wall.
The internal section of the same unit. The front panel swings down to reveal the controls.
A modern Americool window air-conditioner internal section-ravi (06AR25)