If I were organizing a test like this and I wanted to reassure people, I'd say which gas it is. You know, something like: "We're going to simulate a chemical attack on the subway to test if we can deal with it. We'll be using helium as a test gas. If your voice sounds a bit funny, that's why. Don't worry, it's not dangerous." Or something like that.
I note they're not saying which gas it is.
If I were organizing a test like this and I wanted to reassure people, I'd say which gas it is. You know, something like: "We're going to simulate a chemical attack on the subway to test if we can deal with it. We'll be using helium as a test gas. If your voice sounds a bit funny, that's why. Don't worry, it's not dangerous." Or something like that.
Hopefully it's not a hallucinogen. They wouldn't even be able to tell if it was working in a NYC subway.