While the ozone layer is recovering over the poles, it is thinning in mid-latitudes from Russia to the Southern Ocean below Australia, a study published last month in the journal Atmospheric Chemistry and Physics found. "Decreases in ozone are less than we saw at the poles before the Montreal Protocol was enacted [in 1987], but UV radiation is more intense in these regions and more people live there," said report co-author Joanna Haigh, from Imperial College London. The weather bureau studied UV radiation in Australia between 1959 and 2009 and found an annual increase of 2 to 6 per cent since the 1990s, above a 1970-80 baseline. The bureau found these changes were related to ozone depletion. Associate Professor Clare Murphy, from the school of chemistry at Wollongong University, said ozone trends were not fully understood. "The largest factor involved in mid-latitude ozone depletion is the nitrogen cycle, which operates by nitrous oxide turning into reactive nitrogen in the stratosphere," Dr Murphy said.

Nitrogen fertiliser is converted into nitrous oxide by soil microbes, creating a stable greenhouse gas that can reach the stratosphere, where the ozone layer protects the earth from most of the sun's UV radiation," she said. "However, once in the stratosphere, nitrous oxide is broken down by high energy radiation from the sun to become reactive nitrogen, which can deplete ozone." Dr Murphy said that last century, concerns about ozone depletion centred on "chlorine chemistry" (CFCs) because of the massive hole over the poles. "Now it’s nitrous oxide, which almost stopped the Concord from flying because they were worried about reactive nitrogen in the stratosphere." The ozone layer above Antarctica, with dark blue gradient indicating depleted ozone levels. Credit:NASA's Earth Observatory Nitrous oxide damage to ozone is ubiquitous, whereas damage from CFCs creates a hole during extreme weather years over the Antarctic, Dr Murphy said. Nitrous oxide was identified as the most damaging substance to the ozone layer in the 21st century by a 2009 study published in Science. That study also suggested one of the best ways to address the problem was to give insurance to Indian farmers.

"In India, particularly, they’re putting in 10 times more nitrogen fertiliser on their crops than they need to because if a crop fails they may starve," Dr Murphy said. "Insurance could pick up the loss." Robin Schofield, director of Melbourne University's environmental science hub, said UV in Australia should be trending downwards because factors such as surface ozone, which is contained in smog, is on the rise and there is evidence of a recovery of stratospheric ozone. The UV Index and skin cancer The UV index relates to the intensity of sunburn-producing UV radiation. Sun protection is recommended when the UV Index is above 3 in clear sky conditions. The higher the number, the more severe. 11+ = Extreme. Avoid sun exposure between 10am and 4pm due to extreme risk of harm.

8-10 = Very High. Unprotected skin and eyes may be damaged and can burn quickly. 6-7 = High. Protection against skin and eye damage is needed. Reduce time in the sun between 10am and 4pm. 3-5 = Moderate. Stay in the shade near midday when the sun is strongest. Moderate risk of harm. 1-2 = Low. There is a low danger from the sun's UV rays for the average person. Note: UV intensity can nearly double with reflection from snow or reflective surfaces such as water, sand and concrete.

Sydney's monthly average maximum UV index Credit:Australian Radiation Protection and Nuclear Safety Agency Heather Walker, Cancer Council Australia's skin cancer committee chair, said UV is the most common cause of skin cancer but the council has not seen any evidence of a trend of more extreme or high UV days. "Queensland is the skin cancer capital of Australia and they get more UV all year round," Ms Walker said. “Skin cancer rates continue to rise but look like they may be stabilising over the next few years in all age groups except for the under 40s." The continued high rate of skin cancer in Australia is partly due to the ageing population, because cancer is a disease of ageing, Ms Walker said. Brisbane average monthly maximum UV index. Credit:Australian Radiation Protection and Nuclear Safety Agency