LAUGHING gas has replaced CFCs as the biggest threat to the Earths ozone layer, say scientists. Nitrous oxide, better known as the dental anaesthetic 'laughing gas, is now the most potent destroyer of ozone in the upper atmosphere, a study has shown. Unlike CFCs (chlorofluorocarbons), once extensively used in refrigerators, emissions of the gas are not limited by any international agreement. 'The dramatic reduction in CFCs over the last 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 the harmful effects of ultraviolet radiation from the Sun. Thinning of the ozone layer can result in damage to crops and aquatic life, and expose humans to a greater risk of skin cancer. Under the 1987 Montreal Protocol action was taken to phase out ozone-destroying man-made chemicals such as CFCs. But the treaty did not cover nitrous oxide, which is generated by fertilisers in the soil, livestock manure, sewage treatment, combustion and industry. Human activities account for about one third of global nitrous oxide emissions. The gas is also produced naturally when bacteria in the soil and oceans break down nitrogen-containing compounds. Dr Ravishankaras team from the US National Oceanic and Atmospheric Administration (NOAA) calculated the 'ozone depletion potential (ODP) of nitrous oxide and found that it was similar to many CFCs. Telegraph Writing in the journal Science, the researchers warned that if nothing was done the gas would remain the biggest ozone-depleter throughout the rest of this century. Curbing nitrous oxide emissions would enhance the ozone layers recovery. Since nitrous oxide was also a greenhouse gas, this would represent a 'win-win for both the ozone layer and the Earths climate, said the scientists. Telegraph