Aug. 28 (Bloomberg) -- Nitrous oxide, the laughing gas used by dentists to pacify anxious patients, is putting stress on the environment and is now the biggest man-made destroyer of the ozone layer.

N2O, a gas produced by using fertilizers and burning fuels, accounts for about 40 percent of ozone-destruction after the use of gases such as chlorofluorocarbons, or CFCs, was banned because of their harmful effects, said A.R. Ravishankara, a U.S. National Oceanic and Atmospheric Administration chemist.

Laughing-gas emissions are rising by about 0.25 percent a year and may be responsible for up to 80 percent of human-caused ozone depletion by 2050, said the co-author of a study in the journal Science. Because N2O is also a greenhouse gas, reducing its output will result in a “win-win for both ozone and climate,” his team wrote.

“Nitrous oxide right now is more than twice as large as the next largest ozone-depleting gas,” Ravishankara said in an Aug. 25 telephone interview from Boulder, Colorado. “Nitrous oxide is also a greenhouse gas so if you were to reduce emissions because of the ozone layer issue, you’d also be helping the climate issue.”

The ozone layer, which protects humans from carcinogenic ultraviolet radiation, was depleted by man-made compounds such as halons and CFCs, whose use in aerosol cans and refrigerators was banned by the 1987 Montreal Protocol, an international treaty that didn’t regulate against N2O.

While overall emissions of ozone-depleting gases have gone down, meaning less damage will be done to Earth’s protective layer going forward, N2O emissions haven’t declined so now account for a bigger proportion of the total

Ozone-Depletion ‘Potential’

The researchers calculated the so-called ozone-depletion potential of N2O and found that while proportionally it is only one-sixtieth as damaging as a gas called CFC-11, because so much more nitrous oxide is emitted, it now accounts for more than twice as much ozone destruction.

While the sum total of gases regulated by the Montreal Protocol is still more damaging to the atmosphere than nitrous- oxide emissions, “things will change with time with CFCs declining and N2O growing,” Nobel Prize-winner Paul Crutzen, professor of atmospheric chemistry at the Max-Planck Institute for Chemistry in Mainz, Germany, said in an e-mail.

Crutzen, who wasn’t part of today’s study, shared the 1995 Nobel Prize for Chemistry for his research into ozone.

The ozone layer is found mainly in the stratosphere about 25 kilometers (16 miles) above Earth. Ozone depletion above the poles is caused by reactions of ozone, or O3 molecules, with chlorine or nitrogen oxides, triggered by a combination of low temperatures and sunlight.

Antarctica, Arctic

Because temperatures are colder, Antarctica experiences more depletion than the Arctic and a “hole” forms annually. The ozone hole typically reaches a maximum extent in September or October and last year covered 27 million square kilometres, 2 million square kilometers more than in 2007, according to the World Meteorological Organization in Geneva.

While the making of many ozone-depleting gases has been phased out, they remain in the atmosphere and also leak out from old appliances such as refrigerators. That means it may take 50 more years for the ozone layer to fully recover, United Nations Secretary-General Ban Ki-moon said in September.

N2O is also produced by natural processes, when bacteria convert compounds containing nitrogen in rivers and forests into the gas, Ravishankara said. Anthropogenic or man-made sources include sewage treatment, fuel combustion and fertilizer.

“It should be regulated but this will be a slow and difficult process because N2O is a biological by-product from application of nitrogen fertilizers in agriculture,” Crutzen said. “It is possible to reduce N2O emissions by better procedures of nitrogen-fertilizer application. Now only 30 percent of the fertilizer is taken up by the plants.”

To contact the reporter on this story: Alex Morales in London at amorales2@bloomberg.net.

Last Updated: August 27, 2009 22:05 EDT