Drug dogs used as terrorist prevention dogs?

I was just wondering, if we had dog that can detect such agents as nerve gas. If the dog were to sniff a particular potent agent, wouldn’t it kill the dog? (Maybe that would be the tip off). But really, a lot of these chemical and biological weapons are nasty. How would you train a dog to detect these without killing them? And do we have such dogs? Maybe they have figured a way to do this. Any thoughts would be appreciated. Thanks.