Add delay to knock out gas (N2O) kicking in?

[Dickles]

Right now if it is high enough pressure, N2O knocks someone out instantly. Maybe it did not matter as much when oxygen tanks lasted 40 minutes, but most people don't have their internals on all the time; if you make a small trap room with a can of N2O anyone unprotected that follows you in will pass out instantly. 

I think there should be a couple of seconds before it affects someone, giving them time to turn on internals.