Chemically inert CFCs released near the ground cannot damage the stratospheric ozone layer.

CFCs, or chlorofluorocarbons, once looked unusually safe. They were stable, nonflammable, and barely reacted with other chemicals near Earth’s surface. Those properties made them useful as refrigerants, aerosol propellants, and industrial chemicals.

Their stability turned out to be the problem.

Mario Molina and F. Sherwood Rowland argued in 1974 that CFC molecules could survive in the lower atmosphere for years. That gave them time to mix upward into the stratosphere, the layer of atmosphere that contains most of Earth’s protective ozone.

There, stronger ultraviolet sunlight can break a CFC molecule apart and release chlorine atoms. The chlorine does not simply destroy one ozone molecule and disappear. It can take part in a cycle of reactions that destroys ozone and then frees the chlorine to react again.

Measurements of chlorine chemistry in the stratosphere and the discovery of the Antarctic ozone hole later confirmed that this process could produce depletion on a huge scale.

The older observation was partly correct: CFCs really are unusually unreactive near the ground. The mistake was assuming that chemical stability meant environmental harmlessness everywhere in the atmosphere.