Pollution is the introduction of contaminants into the natural environment that can harm ecosystems, organisms, and humans. There are several types of pollution including air, water, thermal, soil, and radioactive pollution. Air pollution occurs when air contains harmful gases, dust, or fumes, and can be caused by both natural sources like volcanoes and forest fires, as well as man-made sources such as the burning of fossil fuels. Water pollution poses major health risks and is caused by sources like waste water, industrial waste, and agricultural runoff. Soil pollution results from contaminants and pollutants building up in soil from dumping and indirect air pollution effects. Radioactive pollution introduces nuclear materials into the air, water and land from