I happen to think the idea of purchasing chem suits, gas masks, etc., is pretty absurd for most people. Who's going to lug these things around all the time and get into them in sufficient time to make a difference? And you're exactly right, many of the conditions that justify wearing a chem suit are undetectable without highly specialized equipment.
Also, I'm told that some non-trivial (perhaps large) percentage of this gear being sold on the web is junk. The next time someone wants to sell you a "Genuine Israeli Gasmask," you might simply do better to save your $$ and learn how to hold your breath.
I think at some point you have be realistic about what threats are plausible and manageable and which are really beyond the limits of reasonable preparation. I realize some people here will disagree with me, but that's the fun of public discussion. <img src="/images/graemlins/smile.gif" alt="" />