Refrigerants are critical to a great number of common appliances and industrial uses. From air conditioning systems and food storage to the vehicles we use daily, every industrialized society the world over would be in retrograde without them. But like most matters tied to climate action, energy efficiency, and sufficient living, we are still developing a clearer understanding of what comprises the various refrigerants we use, both past and present, as well as their environmental impacts.

Common Applications for Refrigerants

Different classes of refrigerants are used for different appliances. (For a detailed breakdown of the various class systems, this is a useful resource.)

Refrigerants are a “working fluid” that convert thermal energy into mechanical energy, and vice versa. Refrigerants go through repeated phase changes (liquid to gas then back again) to achieve this effect. We need them for refrigeration systems that keep perishable foods fresh, whether in our home refrigerator/freezer units or stored in supermarkets, warehouses, food processing facilities, and the trucks and trains that transport fruit, vegetables, meat, and dairy products long distances. We need them for the air-conditioning systems and/or heat pumps used to cool our residences, commercial buildings, cars, and various public transit vehicles. We need them to store pharmaceuticals, to keep large data centers from overheating, and to ensure electricity transmission lines remain operable in virtually all weather conditions. They are ubiquitous and indispensable.

Their physical make-up, however, has not always been constant. For the better part of the last four decades, industrial refrigerants have become a regulated (loosely at times) chemical group owing to their toxicity and global warming potential (GWP). And while the inherent phase changes required for refrigerants to be effective are relatively stable, this wasn’t always the case.

The first efficient air conditioners of the modern era (ones that didn’t consume ice), which came onto the market at the turn of the 20^th century, used combustible gases like propane, ammonia, sulfur dioxide, and methyl chloride, all of which present safety concerns. By the early 1930s, American mechanical and chemical engineer Thomas Midgley, Jr. helped synthesize the first commercially viable synthetic refrigerants, chlorofluorocarbons (CFCs), also known by their trademark name, Freon. Other synthetic, non-toxic models followed, including hydrochlorofluorocarbons (HCFCs). Decades would pass before evidence emerged – amidst the environmental justice movement of the 1970s and 1980s – that CFCs and HCFCs actively contribute to depletion of the ozone layer. This led to the passing of the Montreal Protocol (full name: Montreal Protocol on Substances that Deplete the Ozone Layer), adopted in 1989, an international treaty designed to phase out CFCs and gradually phase out HCFCs from home and commercial use. Their use remained legal and loosely regulated in the ensuing years, however, especially in the case of HCFCs in home air-conditioning units because they posed lower Ozone Depletion Potential (ODP) than CFCs.

Volatility, Safety, and a Regulated Industry

While the Montreal Protocol did a lot to raise awareness of the issue of synthetic refrigerants and their environmental impacts, it also paved the way for a new class of refrigerants to assume greater share of the market. Hydrofluorocarbons (HFCs) are organic compounds that were manufactured to replace CFCs and older HCFCs. (They are also commonly used in some foam insulations, aerosols propellants, and other uses.) While these compounds present little to no ODP risk, they are potent greenhouse gases with a global warming potential many thousands time greater than CO2, and an atmospheric shelf life of several decades.

As the twentieth century came to a close, the mechanical and chemical engineering communities had to reckon with the fact that HFCs were not the panacea many were hoping for. Steps to mitigate their impacts, however, were slow to arrive, even as atmospheric concentrations of HFCs have continued to increase. But arrive they did.

Hydrofluorocarbons were included among the list of seven greenhouse gases detailed in the Kyoto Protocol (1997), one of the earliest and most pivotal international treaties of its kind that acknowledged anthropogenic climate change and the contributive role of greenhouse gases. In 2006, the European Union began actively encouraging the adoption of natural refrigerant alternatives (although it should be mentioned, few viable options existed at that time.) In 2016, the United Nations Framework Convention on Climate Change, in the wake of the passage of the Paris Agreement, labeled HFC’s “climate’s low-hanging fruit” and something that should be eliminated for the sake of the planet. And in 2019, the Kigali Amendment to the Montreal Protocol was passed to impose a legally binding measure to reduce global consumption of HFCs, as well as provide financial support to developing nations. In short, the widespread use of HFCs was being examined and scrutinized with greater frequency during the opening decades of the twenty-first century, thus opening new avenues for cleaner alternative refrigerants that pose significantly fewer risks to global health.

Transitioning to a Cleaner Energy Economy

As industrialized nations continue to sign new treaties and codify efforts to decarbonize various economic sectors (e.g., energy, transportation, manufacturing, construction, et al.), the unilateral phasing out of HFCs are now central to such efforts. The European Union and the United States have assumed positions of leadership in this regard, most notably with the EU’s F-gas Regulation, which went into effect in 2020 and prohibits the use of HFCs with a high GWP in all professional refrigeration equipment.

Of course, this all begs the multi-part question: what mechanisms will be used in place of HFCs while ensuring we can stay cool and our food stays fresh, all while preventing the planet from warming further?

The biggest potential game-changer in this arena are Hydrofluoroolefins (HFOs). Unlike other greenhouse gases that are toxic and linger in the atmosphere, HFOs benefit from a chemical bond that allows its gases to break down in a matter of days, not years. This translates to an ODP of zero and a GWP of nearly zero. According to Honeywell Advanced Materials, which introduced the first line of HFO refrigerants in 2012, the increasing use of HFOs in everything from cars and trucks to supermarkets and commercial refrigeration units has “prevented more than 200 million metric tons of greenhouse gas emissions. That’s equivalent to the emissions from more than 42 million cars [or] the emissions produced by powering 19 million homes.”

Indeed, the fulfillment of a clean energy economy will rely heavily on the complete elimination of all freon agents – which extend to HCFCs and HFCs – that continue to pose a risk to the health of the planet and its inhabitants. For the time being, most of the world’s nations continue to phase them out with each passing year.