Damage from hazards

If a hazard (improvised or prepared) deals damage, ask yourself what it could likely do to a normal person.

worst outcome	die
Bruises & scrapes, pain, light burns	d4
Nasty flesh wounds, bruises, burns	d6
Broken bones, bad burns, debilitating pain	d8
Death or dismemberment	d10

Then, assign effects to the hazard according to its capabilities (choose all that apply):

if...	effect
... armor can't protect against it	ignores armor
... it slices through leather/hide	1 piercing, messy
... it can tear metal apart	3 piercing, messy
... it knocks them down or around	forceful
... it's big/vicious/scary	+2 damage
... the PCs have taken precautions	disadvantage
... it catches them off-guard	advantage

Apply disadvantage to the damage roll if the PCs knew (or suspected) about the hazard and proactively did something to protect themselves. For example, if the hazard is a poisonous gas and they cover their faces and mouths with damp cloth before entering it, impose disadvantage on the damage roll. By contrast, if the hazard catches them flat-footed, hurting them even before they realize it's there, then apply advantage to the damage roll.