I’ve done a lot of work near/for/at chemical companies. They all have the regular slow-woop siren telling you to leave, preferably into the wind. And that’s not a fun thing to hear, because they really don’t like to push that button for any old thing.
But there’s also the far worse “shelter in place” siren. When you hear it, it means shit is so fucked you can’t even run away. And most places aren’t exactly suited for riding out a chemical disaster.