Pyrophoric flares work on the principle of ejecting a special pyrophoric material out of an airtight cartridge, usually using a gas generator, e.g. a small pyrotechnic charge or pressurized gas. The material then self-ignites in contact with air. The materials can be solid, e.g. iron platelets coated with ultrafine aluminium, or liquid, often organometallic compounds; e.g. alkyl aluminium compounds, e.g. triethylaluminium. Pyrophoric flares may have reduced effectiveness at high altitudes, due to lower air temperature and lower availability of oxygen; however oxygen can be co-ejected with the pyrophoric fuel.