The ozone layer is a region in Earth's stratosphere that contains high concentrations of a bluish gas called ozone. Although ozone constitutes only about one-millionth of the ＿＿（46）＿＿ gases, it absorbs most of the sun's ultraviolet radiation. Without the ozone layer, this radiation would destroy all life on the surface of the planet. Ultraviolet radiation creates and perpetuates ozone. Because an ozone molecule is struck by an ultraviolet ray, it falls apart, ＿＿（47）＿＿ free oxygen. An oxygen atom can combine with another free oxygen to ＿＿（48）＿＿ more ozone, and this cycle absorbs most UV radiation. Some manufactured chemicals interfere with this cycle, thus reducing the amount of ozone in the stratosphere. Among the worst offenders are chlorofluorocarbons (CFCs), usually found in refrigerants and aerosol sprays and now generally ＿＿（49）＿＿ due to their harmful effects. Falling ozone levels have caused a thinning of the ozone layer above Antarctica, ＿＿（50）＿＿ the ozone hole. The Antarctic ozone hole has increased dramatically in size over the past two decades. A smaller hole over the Arctic is now developing.