Yeah they are useless, you need to throw them ahead of time (it's ok, it' gas grenade) but you also need to know where your enemy will end his turn, because they only do something if someone start/finish his turn in it.
I think it would be better to apply the chill effect by just passing through it and the hypothermia effect only for standing in it at the end of your turn.
Then it could be use as a mass slow and "packer" of enemy, working nicely with cooked shot in a chemical focused build.
Also damage is ridiculous. Maybe like the burrower caltrops that deal less damage than regular burrower poison, you could introduce a weaker version of the chill debuff, like 5 movement points instead of 10. That way it would be like a caltrop but with a lingering effect on the enemy and the added possibility of constitution debuff.
