In some countries, particularly in the northern hemisphere, coal and oil contain significant amounts of sulfur. Unless special steps are taken to remove sulfur dioxide, it is released into the atmosphere. Power stations and industrial plants, which are often sited close to cities, can produce large quantities of the gas. As well as affecting human health, sulfur dioxide can be harmful to plants, turning leaves yellow and drying, bleaching, and even killing, foliage. In the atmosphere, sulfur dioxide can form acidic particles, or react with cloud droplets, contributing to acid rain.
Some atmospheric particles are from natural sources.
These include wind-blown dust, pollen, sea salt, and material from volcanic eruptions. Lead compounds, which are emitted by motor vehicles fuelled with leaded petrol, are cumulative poisons. They slowly build up in the body. Urban haze Urban haze is mainly due to fine particles, which cause scattering or absorption of light. Haze is typically brown and limits visibility.