Refrigerants are specialized chemical compounds that facilitate the cooling process by cycling between liquid and gas states within systems like air conditioners and refrigerators. Many refrigerants belong to halogenated hydrocarbons, which contain halogen atoms like fluorine, bromine, or chlorine. The presence of chlorine atoms became a major environmental concern because they damage the Earth’s protective stratospheric ozone layer. Historically, the two main classes of refrigerants containing chlorine are Chlorofluorocarbons (CFCs) and Hydrochlorofluorocarbons (HCFCs).
Chlorofluorocarbons (CFCs)
Chlorofluorocarbons (CFCs) represent the first generation of synthetic refrigerants, widely adopted for their stability, non-flammability, and low toxicity. CFC molecules are composed solely of chlorine, fluorine, and carbon atoms, lacking hydrogen. This composition makes them extremely stable in the lower atmosphere, allowing them to persist and drift upward into the stratosphere.
Examples include R-12 (Dichlorodifluoromethane) and R-11 (Trichlorofluoromethane), used extensively in early air conditioning, refrigeration, and as aerosol propellants. The stability of CFCs became a hazard once they reached the upper atmosphere. Because of their high capacity to destroy ozone, CFCs have an Ozone Depletion Potential (ODP) near 1.0, with R-11 serving as the benchmark ODP value. The manufacture of these chemicals was almost entirely phased out in developed nations by 1996 under international agreements.
Hydrochlorofluorocarbons (HCFCs)
Following the phase-out of CFCs, Hydrochlorofluorocarbons (HCFCs) became the second generation of chlorine-containing refrigerants used as temporary replacements. HCFCs differ structurally from CFCs because they include at least one hydrogen atom bonded to the carbon chain. This addition of hydrogen makes the HCFC molecule less stable in the lower atmosphere, allowing it to be broken down before reaching the stratosphere.
A common example is R-22 (Chlorodifluoromethane), which was extensively used in residential and commercial refrigeration systems for decades. Although HCFCs still contain chlorine and contribute to ozone depletion, their ODP is significantly lower than CFCs, typically ranging from 0.005 to 0.2. R-22 has an ODP of 0.055, making it about twenty times less damaging than R-11. This reduced, but still present, environmental risk led to their scheduled phase-out, with the production and import of R-22 in the United States ending in 2020.
How Chlorine Damages the Ozone Layer
Chlorine-containing refrigerants cause environmental damage when they decompose in the stratosphere, 10 to 50 kilometers above the Earth’s surface. In this layer, stable CFC and HCFC molecules are exposed to intense ultraviolet (UV) radiation. This high-energy UV light breaks the carbon-chlorine bond within the molecule, releasing a free chlorine atom.
Once liberated, this single chlorine atom initiates a powerful catalytic reaction cycle that destroys ozone (\(\text{O}_3\)) molecules. The free chlorine atom reacts with ozone to form chlorine monoxide (\(\text{ClO}\)) and an oxygen molecule (\(\text{O}_2\)). The resulting chlorine monoxide then reacts with a free oxygen atom (O) to release the chlorine atom and form another oxygen molecule.
This process is catalytic because the chlorine atom is regenerated at the end of the two-step cycle, allowing it to immediately attack and destroy another ozone molecule. A single chlorine atom can repeat this cycle thousands of times, destroying a large number of ozone molecules before the chlorine is sequestered into a stable compound. The Ozone Depletion Potential (ODP) quantifies a substance’s relative ability to cause this destruction compared to CFC-11.
The Shift to Chlorine-Free Refrigerants
The recognition of chlorine’s destructive role accelerated the development of a third generation of refrigerants that completely exclude the halogen. Hydrofluorocarbons (HFCs) were the immediate replacements for HCFCs and CFCs, containing hydrogen, fluorine, and carbon, but no chlorine. Because HFCs contain no chlorine, their ODP is zero, meaning they pose no direct threat to the ozone layer.
While HFCs solved the ozone depletion problem, many compounds like R-134a were found to be potent greenhouse gases, contributing to global warming. This necessitated a transition to a fourth generation of ultra-low Global Warming Potential (GWP) refrigerants. These include Hydrofluoroolefins (HFOs), which rapidly break down in the lower atmosphere, and natural refrigerants like carbon dioxide (\(\text{R-744}\)), propane (\(\text{R-290}\)), and ammonia (\(\text{R-717}\)). All these modern alternatives are chemically designed to be completely free of chlorine, eliminating the potential for stratospheric ozone damage.