Nevertheless environmental experts see the production of oxides of nitrogen as a major menace in the formation of greenhouse gases and photochemical smog. The interaction of NOx with hydrocarbons from vehicle exhausts and sunlight, can also form low level ozone. In the stratosphere (some 25 km up). Ozone is helpful by absorbing the fiercer part of the ultraviolet radiation of the sun, but at ground level it damages materials and vegetation. It irritates throat, lungs and eyes, and strenuous exercise or work can become painful. Furthermore, the effectiveness of nitrous oxide as a greenhouse gas is magnified by its longer life than carbon dioxide, methane and CFC's.