<Li> Waste deposition in landfills, which generate methane . Methane is highly flammable and may form explosive mixtures with air . Methane is also an asphyxiant and may displace oxygen in an enclosed space . Asphyxia or suffocation may result if the oxygen concentration is reduced to below 19.5% by displacement . </Li> <Li> Military resources, such as nuclear weapons, toxic gases, germ warfare and rocketry </Li> <P> Natural sources: </P> <Ul> <Li> Dust from natural sources, usually large areas of land with little or no vegetation </Li> <Li> Methane, emitted by the digestion of food by animals, for example cattle </Li> <Li> Radon gas from radioactive decay within the Earth's crust . Radon is a colorless, odorless, naturally occurring, radioactive noble gas that is formed from the decay of radium . It is considered to be a health hazard . Radon gas from natural sources can accumulate in buildings, especially in confined areas such as the basement and it is the second most frequent cause of lung cancer, after cigarette smoking . </Li> <Li> Smoke and carbon monoxide from wildfires </Li> <Li> Vegetation, in some regions, emits environmentally significant amounts of Volatile organic compounds (VOCs) on warmer days . These VOCs react with primary anthropogenic pollutants--specifically, NO, SO, and anthropogenic organic carbon compounds--to produce a seasonal haze of secondary pollutants . Black gum, poplar, oak and willow are some examples of vegetation that can produce abundant VOCs . The VOC production from these species result in ozone levels up to eight times higher than the low - impact tree species . </Li> <Li> Volcanic activity, which produces sulfur, chlorine, and ash particulates </Li> </Ul>

Where does the majority of air pollution come from