Individuals selected prior to the initiation of the gas chambers typically experienced what?

The main component in a gas chamber is tear gas. it can give irritating effects on the person's mucous membrane.

getting inside a gas chamber is part of the army training. they get trained to wear and clear their gas mask. but some usually forget to clear it when they enter the gas chamber, they get nervous and forget the instructions.   they are suggested not to have a full stomach. they also have initial lectures on what a gas chamber looks like. 

I'll go with that