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Unfortunately, chlorine- and fluorine-bearing refrigerants reach the upper atmosphere when they escape. In the [[stratosphere]], substances like [[Chlorofluorocarbon|CFCs]] and [[HCFC]]s break up due to [[UV]] radiation, releasing their chlorine free-radicals. These chlorine free-radicals act as [[catalyst]]s in the breakdown of ozone through chain reactions. One CFC molecule can cause thousands of ozone molecules to break down. This causes severe damage to the [[ozone layer]] that shields the Earth's surface from the Sun's strong UV radiation and has been shown to lead to increased rates of skin cancer. The chlorine will remain active as a catalyst until and unless it binds with another particle, forming a stable molecule. CFC refrigerants in common but receding usage include [[Trichlorofluoromethane|R-11]] and [[Dichlorodifluoromethane|R-12]].
Newer refrigerants with reduced [[ozone depletion]] effect such as [[HCFC]]s ([[chlorodifluoromethane|R-22]], used in most homes today) and [[hydrofluorocarbon|HFC]]s ([[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), that do not contain [[chlorine]] atoms. Small example of common HFCs in current use: [[R-410A]] (which is itself a blend of other HFCs: [[Difluoromethane|R-32]] and [[R-125]]) ; designed to be a drop-in replacement for [[
More benign refrigerants are currently the subject of research, such as [[supercritical fluid|supercritical]] [[carbon dioxide]], known as [[R-744]].<ref>[http://www.r744.com/knowledge/faq r744.com – Everything R744], The Natural Refrigerant R744 (CO)<sub>2</sub>, 2006–2012</ref> These have similar efficiencies{{Citation needed|date=September 2009}} compared to existing CFC- and HFC-based compounds, and have many orders of magnitude lower global warming potential. General industry and governing body push are toward more GWP-friendly refrigerants. In industrial settings [[ammonia]], as well gasses like [[ethylene]], [[propane]], [[iso-butane]] and other [[hydrocarbons]] are commonly used (and have own R-x customary numbers), depending on required temperatures and pressures. Many of these gases are unfortunately flammable, explosive, or toxic; making their use restricted (i.e. well-controlled environment by qualified personnel, or a very small amount of refrigerant used). [[Hydrofluoroolefin|HFO]]s which can be considered to be HFC with some carbon-carbon bonds being double bounds, do show promise of lowering GWP very low to be of no further concern. In the meantime, various blends of existing refrigerants are used to achieve the required properties and efficiency, at a reasonable cost and lower GWP.
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