Thick black smoke curling out of smokestacks, horrible–tasting chemicals in your drinking water, pesticides in your food –– these are examples of pollution. Pollution is any contamination of the environment which causes harm to the environment or the inhabitants of the environment. There are many kinds of pollution, and there are many pollutants. Some obvious kinds of pollution are pollution of the air, soil, and water. Some less obvious, or less salient, kinds of pollution are radioactive, noise, light pollution, and green-house gasses. Air pollution can be caused by particles, liquids, or gases that make the air harmful to breathe. There are two main types of air pollution: primary and secondary. Primary pollutants enter the air directly, like smoke from factories and car exhaust. Secondary pollutants are chemicals that mix together to pollute the air, like mixtures of emissions, or waste output, from vehicles and factory smoke that change to form more dangerous pollutants in the air and sunlight. Soil pollution can be caused by pesticides, leakage from chemical tanks, oil spills, and other chemicals which get into the soil by dumping or accidental contamination. Soil pollution can also cause water pollution when underground water becomes contaminated by coming into contact with the polluted soil. Water pollution can be caused by waste products, sewage, oil spills, and litter in streams, rivers, lakes, and oceans. Some scientists believe that water pollution is the largest cause of death and disease in the world, causing about 14,000 deaths in the world each day. Radioactive pollution can be caused by leaks or spills of radioactive materials.