Fossil fuels are carbon or hydrocarbon fuels, derived from what was living material, and found underground or beneath the sea. The most common forms are coal, oil and natural gas. They take millions of years to form. Their energy is only released upon burning, when the carbon and hydrogen within them combine with the oxygen in air to form carbon dioxide (CO₂ ), or carbon monoxide (CO) and water (H₂O). Other elements within the fuels (such as sulfur or nitrogen) are also released into the air after combining with oxygen, causing further pollution with sulfur dioxide and nitrogen oxide gases. In the case of coal, ash particles are also a problem.