Hopefully wouldn’t be too hard, but it could be a smoke grenade that emits smoke when thrown. Anybody within the smoke will start coughing (just plays a coughing sound) and their health will drain slowly. If they die from it, then it will say “You’ve died from second hand smoke”, or something along those lines.
If I may add: An overlay of smoke would be cool as well, with the transparency of the overlay depending on how far in/away you are from the center.
Just a thought.