Ammonia (R-717) and carbon dioxide (R-744) tie for the lowest global warming potential of any refrigerant, each with a GWP of just 1. That means they trap no more heat than the same weight of CO₂ itself, which is the baseline the scale is measured against. Propane (R-290) comes in close behind at 3.3. All three are natural substances, and they dramatically outperform the synthetic refrigerants that have dominated air conditioning and refrigeration for decades.
How the GWP Scale Works
Global warming potential measures how much heat a greenhouse gas traps over 100 years compared to the same mass of carbon dioxide. CO₂ is set at 1 by definition. The higher the number, the worse the climate impact. Older refrigerants like R-410A, widely used in home air conditioners, have a GWP around 2,088. R-404A, common in commercial refrigeration, sits at roughly 3,922. Even a small leak of these substances releases the warming equivalent of several tons of CO₂.
The Three Lowest-GWP Refrigerants
Ammonia (R-717): GWP of 1
Ammonia has been used as a refrigerant since the 1800s and remains the workhorse of industrial cold storage, food processing plants, and ice rinks. It’s thermodynamically efficient, meaning systems that use it tend to consume less electricity for the same cooling output. The catch is toxicity: ammonia is harmful to breathe at relatively low concentrations, so it’s classified as a higher-toxicity refrigerant. That’s why you’ll find it in large industrial facilities with trained operators and safety systems, not in your home air conditioner.
Carbon Dioxide (R-744): GWP of 1
CO₂ works as a refrigerant but operates at much higher pressures than conventional options, which means equipment must be built to withstand that stress. It has gained significant traction in supermarket refrigeration systems, particularly in Europe, and in heat pump water heaters. CO₂ is non-flammable and non-toxic at the concentrations involved in refrigeration, making it safer to use in occupied buildings than ammonia. The trade-off is that the high-pressure equipment costs more upfront and requires specialized technicians.
Propane (R-290): GWP of 3.3
Propane is increasingly popular in small self-contained systems like household refrigerators, portable air conditioners, and commercial display cases. It performs well thermodynamically. Research from the U.S. Department of Energy shows propane’s cooling efficiency is competitive with, and in some system designs slightly better than, R-410A. The limitation is flammability: propane carries an A3 safety classification from ASHRAE, meaning it’s highly flammable. Building codes typically restrict the refrigerant charge size in occupied spaces, which limits propane to smaller systems where only a modest amount of the gas is needed.
How Synthetic Low-GWP Refrigerants Compare
A newer class of synthetic refrigerants called hydrofluoroolefins (HFOs) was designed to break down quickly in the atmosphere, giving them low GWP values. R-1234yf, now standard in most new car air conditioning systems, has a GWP of less than 1 by some assessments. R-1234ze, used in commercial chillers, falls in a similar range. On paper, these look like ideal replacements.
However, HFOs come with an environmental concern that natural refrigerants don’t share. When HFOs break down in the atmosphere, many of them produce trifluoroacetic acid, or TFA, a persistent chemical that accumulates in water and soil. A 2024 study published in Environmental Science & Technology found that HFO-1234yf alone was responsible for an estimated 6,900 tons of TFA emissions per year in the EU as of 2020, with projections reaching 47,650 tons per year by 2050 if adoption continues to grow. TFA resists further breakdown, so it builds up indefinitely in the environment. The long-term ecological consequences of that accumulation aren’t fully understood, but the sheer scale of projected emissions has drawn concern from environmental scientists.
U.S. Regulations Are Forcing the Shift
The AIM Act, passed in 2020, gave the EPA authority to phase down high-GWP refrigerants. The rules are already taking effect. As of January 1, 2025, new residential and light commercial air conditioning and heat pump systems in the U.S. cannot use refrigerants with a GWP of 700 or higher. That effectively eliminates R-410A from new installations.
Commercial refrigeration faces even stricter limits on a staggered timeline. By 2027, new supermarket systems cannot use refrigerants with a GWP above 1,400. By 2032, large retail refrigeration systems (those holding 200 pounds or more of refrigerant) will need to use options with a GWP below 150. That threshold is low enough to push many commercial operators toward natural refrigerants like CO₂ or propane, since few synthetic options can meet that bar without the TFA concern.
Why the “Best” Refrigerant Depends on the Application
If the only criterion were climate impact, ammonia and CO₂ would win everywhere. But refrigerant selection always involves balancing GWP against safety, efficiency, cost, and the physical constraints of the equipment. A home refrigerator can safely use a small charge of propane. A supermarket with hundreds of feet of piping might choose CO₂ because a leak won’t poison anyone. An industrial cold storage warehouse might choose ammonia because it delivers the best efficiency at scale and the facility can support the required safety infrastructure.
Most new residential air conditioners in the U.S. are transitioning to R-32 or R-454B, synthetic blends with GWP values between roughly 675 and 750. These aren’t as low as the natural refrigerants, but they work in existing equipment designs with relatively minor modifications, they’re only mildly flammable, and they meet the new 700 GWP threshold. For homeowners, this is the change you’ll encounter when replacing an air conditioner or heat pump in the coming years.
The overall direction is clear: the industry is moving toward lower GWP values across every application. Natural refrigerants with a GWP of 1 to 3.3 represent the floor, and regulations are steadily pushing systems closer to that floor, especially in commercial refrigeration where the largest volumes of refrigerant are used.