Natural refrigerants are refrigerants that exist in nature rather than being engineered. Three cover almost every cold chain application: ammonia (NH3, refrigerant number R717), carbon dioxide (CO2, R744), and hydrocarbons such as propane and isobutane (R290 and R600a). Each moves heat the same way any refrigerant does, through compression, condensation, expansion and evaporation. What sets the three apart from the hydrofluorocarbons, or HFCs, that dominate most retail and transport refrigeration today is that none was invented in a lab to solve a refrigeration problem. Ammonia and CO2 both ran industrial plants before HFCs existed; hydrocarbons have cooled small appliances for just as long.
Each also carries a low or negligible Global Warming Potential, the standard measure of how much a given mass of gas warms the atmosphere compared with the same mass of CO2. HFCs sit far higher on that scale, gram for gram, the main reason all three are back in wide use rather than a footnote from before synthetic refrigerants took over.
The phase-down behind the switch
HFCs arrived as the ozone-safe replacement for CFCs and HCFCs and became the default across supermarkets, cold stores and transport refrigeration for a generation. The problem surfaced later: gram for gram, many HFCs trap far more heat than CO2 does. The Kigali Amendment to the Montreal Protocol commits signatory countries to cutting the volume of HFCs they produce and import, stepped down in stages rather than banned outright. The EU's F-Gas Regulation runs a parallel quota system, with its own schedule and bans on specific high-GWP refrigerants in new equipment.
The industry's response splits two ways: newer synthetic HFOs built with a low Global Warming Potential from the start, or a return to the three natural options that never had the problem. Both routes work. Natural refrigerants tend to win in new-build plant, where the machine room is designed around ammonia or CO2 from day one; HFOs stay common where an existing site can only accept a drop-in replacement rather than a full redesign.
Ammonia's efficiency and its toxicity
Ammonia carries heat exceptionally well for its molecular weight, which is why refrigeration engineers never fully abandoned it even while HFCs dominated everywhere else. A well-designed ammonia plant runs on less compressor power than an equivalent HFC system holding the same load, and the refrigerant itself costs little. The same molecule is toxic at concentrations far below where it becomes flammable, with a sharp smell that gives a leak away long before a person takes real harm, a safety feature rather than a flaw.
That toxicity is why ammonia systems keep their compressors, pipework and pressure vessels in a dedicated machine room, built to a standard such as IIAR guidance or EN 378, with forced ventilation, gas detection and restricted access rather than open exposure to staff. It is why ammonia belongs in large-scale cold storage warehousing and cold rooms and cold stores with a trained crew on site, and almost never in a retail cabinet on a shop floor. The charge in an industrial plant is large enough that a release matters, the point of isolating it in a machine room rather than occupied space.
CO2 above its critical point
Carbon dioxide behaves differently from ammonia and hydrocarbons because its critical point, the pressure and temperature above which it stops behaving as a distinct liquid and gas, sits close to conditions an ordinary system passes through. Below that point, CO2 runs a conventional cycle. Above it, in transcritical operation, the high side of the loop carries no condenser in the usual sense: a gas cooler brings the CO2 down in temperature without it condensing, at pressure several times higher than an equivalent ammonia or HFC system holds.
That high pressure means CO2 equipment needs pipework, valves and vessels rated for the load, raising the engineering bar above a low-pressure HFC retrofit. CO2 itself is non-toxic, non-flammable, and releases next to none of the warming a leak of HFC or ammonia would. In hot climates, running fully transcritical loses efficiency, so many CO2 systems pair it with a subcritical booster stage to hold performance through summer.
Hydrocarbons and the flammability limit
Propane and isobutane cool efficiently and carry no toxicity at the concentrations a leak would produce, but both are flammable, classified A3 under the ASHRAE refrigerant safety standard, the same class as other hydrocarbon fuels. That flammability is managed almost entirely through charge size: keep the total charge in a sealed system below a defined threshold, and a full release cannot build a flammable concentration large enough to ignite in a normal room.
That charge limit is why hydrocarbons show up almost exclusively in small, factory-sealed systems: domestic refrigerators, vending machines, and retail display cabinets, welded shut at the factory and never opened in the field. Scale a hydrocarbon system up to a walk-in cold room or warehouse plant and the charge needed crosses the same threshold that keeps small units safe, so hydrocarbons stop being a serious option well before ammonia or CO2 do.
Large plant, small cabinet
The three end up sorted by scale more than by preference. Large-volume plant, the kind running a distribution centre, a blast freezing tunnel or a multi-chamber cold store, favours ammonia or CO2 because both handle a big load efficiently and can be engineered around a dedicated machine room with trained staff. Small, sealed equipment favours hydrocarbons because the charge stays under the flammability threshold by design, with no machine room to build at all.
Refrigeration engineers, IIAR-trained technicians, and standards bodies including ASHRAE and CEN sit behind the codes that make each option workable at its scale. What none of the three has solved is mobile equipment: a vehicle or shipping container vibrates constantly, gets serviced in the field by generalist technicians rather than refrigeration specialists, and cannot isolate a machine room the way a fixed building can. That is one reason synthetic refrigerants remain common in refrigerated road transport and reefer shipping even as cold stores move to natural options.