Nitrous oxide, commonly known as laughing gas, is now the dominant ozone-depleting substance emitted by humans - and is likely to remain so throughout the century, a new study suggests.

Researchers suggest use of the compound - which is produced by the breakdown of nitrogen in fertilisers and sewage treatment plants - should be reduced to avoid thinning the protective ozone layer that blankets the Earth.

The ozone layer shields Earth from the sun’s ultraviolet rays, which increase the risk of cancer and threaten crops and aquatic life.

Human-produced chemicals called chlorofluorocarbons (CFCs) made headlines in the 1980s when it became clear they were eating a hole in the ozone layer above Earth’s polar regions. An international treaty called the Montreal Protocol regulated production of CFCs and certain other ozone-depleting gases in 1987, and they were phased out completely by 1996.

Since then, Earth’s ozone - both the polar hole and the atmospheric layer around the whole planet - has been on the mend. But the emission of nitrous oxide, which is not regulated by the Montreal Protocol, could reverse those gains - and could even make the situation worse.

“Right now, nitrous oxide is the most important ozone-depleting gas that is emitted,” says A. R. Ravishankara of the US National Oceanic and Atmospheric Administration, lead author of the new research. “It will continue to be so unless something is done.”

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By Lisa Grossman, New Scientist