Its called a non-lethal agent for a reason, its not lethal. It makes you cough a bit and cry a little. You are not forced to inhale tear gas, you'll get a little bit on your face but if you do your drills correctly, you will inhale no gas (thats the point). Its not disturbing when you get tear gassed, you cry and your nose runs, those who ate too much at lunch might throw up (if they have a weak stomach and gag reflex).
Then again, you are not forced to do anything. If you are scared of getting tear gassed when you could potentially be exposed to real lethal agents during a real attack at some point in your career...well, we are a voluntary organisation. No one forced anyone to sign up.
Anyone remember that kid who made the big hoopla a few months ago about hearing protection etc when he was joining the artillery? I'm getting flashbacks.