We have seen that the stratosphere can be polluted in two ways. Direct injection of material occurs naturally when explosive eruptions inject material into the stratosphere. Aircraft and rockets fly directly into or through the stratosphere, adding to this injection. The second way in which the stratosphere can be affected is by the release of long lived inert gases into the troposphere. These eventually reach the stratosphere, where they are broken down by ultraviolet radiation releasing their pollutants. In this way, nitrous oxide provides NOx, chlorofluorocarbon provide chlorine, halons and methyl bromide provide bomine and methane supplies HOx and water to the stratosphere. These materials have an impact on the production and maintenance of ozone levels in the stratosphere.