It's not just the fiendish monsters and dastardly traps in dungeons (and ruins) that are out to get curious adventurers and treasure seekers sometimes the environment of the place itself can be hazardous.
Chance of Hazard per Level/Quarter 2 in 6
Chance of Hazard per Suite/Neighborhood 1- additional hazard,2- replacement hazard, 3-5 no change, 6 clear
Chance of Hazard per Room 1- additional hazard,2- replacement hazard, 3-4 no change, 5-6 clear
Environmental Hazards of Dungeons/Ruins (roll 3d8)
3 Chronosis- time displacement disorder, unnatural accelerated aging or rapid youth
4 Arcane Flux- magic acts oddly possibly spell effects halved ,doubled, or reversed
5 Radiation- exposure to radiation, more severe in a zone or Suite , even more severe in a room.
6 Ectoplasm- charges undead, weakness the living, causes psychic disruptions
7 Phantasms- visions and minor haunts confuse and frighten
8 Gremlins/Peskies- annoying semi-sentient mites harass and prank tresspassers
9 Rot- leather and cloth mildew before sloughing off, food molders, wine sours
10 Chiggers- little insects get into your clothing, hair, and even under your skin, difficult to rest and concentrate
11 Echoes- sounds echoes maddeningly, causes confusion and makes communication difficult
12 Dust- stirring the dust up is hazardous causing sneezing and weeping along with coughing and perhaps choking
13 Damp- difficult to rest and amplifies the effects of chill, rot, and rust
14 Chill - induces fatigue and eventually lethal hypothermia
15 Heat- indices fatigue and eventually causes heat stroke
16 Miasma- causes weakness, disease, and even poisons those exposed.
17 Drafts- seems harmless enough but breezes carry scents and extinguish candle, gusts extinguish lamps and torches, gales prohibit missile fire and extinguish lanterns
18 Mist- vision is obscured and some mists have more dangerous chemical properties
19 Rust- ferrous metals begin to stain and rust away
20 Gloom- an overwhelming sense of dread and depression sets in, vitality ebbs and the will to carry on is lost
21 Aural Glow- strange dancing lights confuse and cling to trespassers making it hard for them to surprise others
22 Sticky Darkness- light sources fade, even infra vision weakens, pools of darkness seem to follow
23 Creeping Crust- crusty growth or amber like chrysalis slowly accumulates on all surfaces, doors become hard to open and sleepers can be trapped
24 Electro-Static Charge- minor crackling up to lighting bolt discharges are possible.
Effects are left generic to be adapted to better fit an adventure or campaign. I'd recommend applying environmental effects on a an overloaded encounter die pretty much as described at dungeonofsigns and necropraxis.