It is no joke: laughing gas is now the biggest threat to the Earth's ozone layer, say scientists.
Nitrous oxide, better known as the dental anaesthetic "laughing gas", has replaced CFCs as the most potent destroyer of ozone in the upper atmosphere, a study has shown.
Unlike CFCs (chlorofluorocarbons), once extensively used in refrigerators, emissionsof the gas are not limited by any international agreement.
"The dramatic reduction in CFCs over the past 20 years is an environmental success story," said Dr Akkihebbal Ravishankara, who led the US research.
"But man-made nitrous oxide is now the elephant in the room among ozone-depleting substances."
The ozone layer shields plants, animals and people from harmful ultraviolet radiation.
Thinning of the ozone layer can result in damage to crops and aquatic life, and expose humans to a greater risk of skin cancer. The 1987 Montreal Protocol did not cover nitrous oxide, which is generated by fertilisers, manure, sewage treatment, combustion and industry.