Hydrocyanic acid, also known as prussic acid, is found in the pits of fruit such as apricots, peaches, nectarines, bitter almonds and cherries. Car exhaust, and the smoke from wood, tobacco and certain plastics also release amounts of prussic acid.
Hydrocyanic acid is a light blue to colorless liquid that has the smell of bitter almonds. It is toxic to inhale, to get on the skin and to ingest. A concentration of about 300 milligrams per cubic meter in the air is enough to kill a human being, a process that will take between 10 minutes and one hour. It kills because the cyanide ion in the molecule interrupts an enzyme found in the cell's mitochondria.
Prussic acid is used as a rodenticide. It has been used in state-sponsored executions and was used to kill combatants in World War I and concentration camp inmates during World War II. As of 2014, prussic acid is considered a Schedule 3 chemical weapon. Prussic acid poisoning is also a risk for livestock that accidentally ingest sorghum. Though the plants don't normally carry prussic acid, freezing and wilting can release the poison in the plants' cells. Some arthropods, such as millipedes, use prussic acid for defense.