One of the six type of harm (see TRACEM) that can be encountered at a terrorist incident Asphyxiants interfere with oxygen flow during normal breathing There are two types of asphyxiants; simple and chemical
a state of asphyxia or suffocation Condition caused by insufficient intake of oxygen Extrinsic causes include choking, toxic gases, exhaust gas (principally carbon monoxide), electric shock, drugs, anesthesia, trauma, crushing injuries of the chest, Compression of the chest, injury of the respiratory nerves or center, diminished environmental oxygenation, and drowning
Releasing any gas in a confined or unventilated area can lower the concentration of oxygen to a level that is immediately dangerous to life or health (see the first link under Further Info below) Be very careful entering a truck or room where solid or gaseous CO2 is stored Likewise, some businesses and laboratories store Dry Ice in large top-loading ice chests; be very careful sticking your head down to retrieve a block from the bottom