Fossil fuels cause pollution when burned for use and when spilled during transport or mining. Burning fossil fuels is the largest producer of pollution.
Most fossil fuels are burned to turn into energy, and the gasses released into the air by burning, in turn causes air and water pollution. Gases released by the burning and combustion of fossil fuels include carbon monoxide, nitrogen oxides, sulfur oxides and hydrocarbons. In the air, these gases becomes a carcinogen, which can be inhaled and can also mix with falling rain to form acid rain.
Fossil fuels also become pollutants when spilled during transport. While this is a rare occurrence compared to airborne particulate matter being released when converting fossil fuels to energy, it still has a severe, negative environmental impact when it happens. Spilled fossil fuels, and the brine used in drilling for them, can leach toxins into ground water and soil.
Similar to spills during transportation, mining and drilling accidents are rare, but there have historically been occasions when fossil fuels have leaked into the environment. The diminished water quality near leaks has reportedly caused sickness, and can even cause the death of any people or animals directly exposed to the leaked fossil fuel.