To help reduce global warming, nearly 200 nations, including the U.S., amended the Montreal Protocol to phase down by mid-century the hydrofluorocarbon (HFC) refrigerants used in most air-conditioning systems.

Researchers at the U.S. National Institute of Standards and Technology (NIST) conducted a comprehensive, four-year search for the best single-component, low-global warming potential (GWP) replacement refrigerants. GWP is defined as the warming potential of one kilogram of a gas relative to one kilogram of carbon dioxide.

The study screened a database of more than 60 million chemicals, estimating the properties based solely on their molecular structure. Possible replacement fluids were sought for small AC systems typical for homes and small businesses.

A final list of 27 fluids identified as candidates from a performance viewpoint are at least slightly flammable, which is not allowed under U.S. safety codes for most end uses. Several fluids among the list of refrigerants are highly flammable, including propane, the fuel for outdoor grills.

In other words, the NIST study found no ideal refrigerant was identified that combined low GWP with required performance and safety features.