My first thought was actually mixed the wrong cleaning chemicals.

Twice in the last few years I've had to stop someone from mixing bleach into windex. Their plan in both cases was to use it to clean a glass stovetop.

Authorities say its spread by contact, not airborne. Weird.

Dipshits trying to manufacture fentanyl.

This sounds more like an institution, not a residence. My first thought was carbon monoxide, but they said it's not airborne.

N/t

A substance spread by contact to 18 responders? I can't imagine what it could be. Perhaps something that broke down too quickly to be identified?

Praying! (not really religious though)......................

from cadavers and poisoned patients, causing illness among healthcare workers.
It's a component in pesticides. Perhaps the initial patients were farmworkers?