I've just come back of a drugs awareness course and yes, substance abuse can kill instantly.
750 - 1500 young people a year die from substance abuse in the UK apparently.
Aerosols in particular are dangerous. Aerosols are presurised containers and th temperture inside is extremely cold, often beow freezing... the cold of the gas is what attracts some people to this as it acts as an anaestetic but the gas can literally freeze glands and nerves at the back of your throat. By freezing the nerves around your neck and spinal cord, your head literally thinks it has fallen of the rest of the body and can lead to severe shock and death.
Some people get giddy, others get very jumpy and scared, and if you suspect someone has been using, don't jump up at them, approach them cautiously as not to scare them, because many deaths are results of heart attacks that some users experience as a result of a bad trip.
But lastly, why would anyone want to sniff? You get spots around your nose and mouth and your breath smells of chemicals... nasty.