Sulfate particles are also produced naturally from gases emitted by marine organisms. Industry and motor vehicles add significantly to the concentration of particles in the atmosphere - high concentrations are found in big cities and industrial areas. These particles remain airborne for long periods, lowering visibility. The brown haze which is often seen over large cities in autumn, winter and spring is due mainly to particles. Particles in the air can cause breathing difficulties and worsen respiratory diseases.
Some particles contain cancer-producing materials.
It is thought to exacerbate many respiratory diseases, including asthma. Sulfur dioxide is produced whenever sulfur-containing compounds are burnt. Its commonest source in Australia is power-stations burning coal containing slight sulfur impurities. The Australian Foundation for Science and the Science and Technology Awareness Program of the Commonwealth Department of Industry, Science and Resources are also supporters of Nova .