Well, to get a duster high, you purposefully deprive your brain of the oxygen needed to keep you functioning properly, without setting off it's 'BREATHE' panic reflex that it gets when too much CO2 builds up in your blood. This is done by, of course, using Duster to replace all the oxygen, without building up CO2 because the chemicals in the Duster remain in your lungs/blood, thusly preventing your brains 'Too much CO2!' breathing panicky reflex. Now, without enough oxygen and without too much CO2, you get to experience the so-called 'pleasant' feeling of your brain suffocating without panicking like you're supposed to. Now, to get that into your lungs, you breathe the air into your lungs that comes out of the top when you pull the trigger, but do you REALLY want to do that?
