"I Can't Lie To You About Your Chances..." - Air, Gravity, Hard Vacuum & Radiation.

Inseminoid (1981) - I've read the novelisation.
It's someone dying in a space suit - for illustrative purposes.

Outer space is one of the ideal survival horror settings, but it's not just about aliens, robots or remote corporate decisions.

This post has sci fi in mind, though I've used fantasy as the base - AD&D Spelljammer and Dungeoneer's Survival Guide were source material. Nothing is meant to accurately reflect IRL conditions, but to approximate genre expectations - your needs may differ.

Intended as system agnostic, it's all clearly D&D adjacent - I'm sure 'save' and 'STR' (etc) map easily enough.

Air.

Fresh Air.

Fine to breathe, even if it is a bit stale and smelly - there is no mechanical effect.

Thin Air.

Still good to breathe, but you need to rest twice as often or twice as long. 

Using BECMI dungeon exploration as a guide, so this is 1 turn of rest after 2.5 (or round in either direction) turns of activity, or 2 turns of rest after 5. 

BECMI has combat take up 1 turn, even when it only lasts a few rounds, so maybe the remainder is a rest/recovery period - total of 2 turns in a Thin Air environment or take fatigue penalties.

Strenuous activity can be treated in a similar way to combat.

Foul Air.

Not fine. Ability Checks and attack rolls are -2 (or disadvantage) until you get some Fresh Air (immediate recovery) or Thin Air (need to rest 1 turn to get the benefit).

As an option, treat Foul Air and Thin Air the same mechanically.

Depleted Air.

Big problem that could kill you.

Same properties as Foul and Thin Air, plus you need to make a save each turn (twice or at disadvantage if you're not taking proper rest) or pass out.

Then, if you don't receive breathable air, save each turn or die. 

Noxious Air.

Polluted with gas, particulate or smoke.

Save each time unit (the smaller, the more noxious) or cumulative -3 Ability Scores and attack rolls.

If any Ability Score hits 0, you collapse helpless. If it's smoke making the air noxious, you start taking lethal damage (or treat as Depleted Air).

You can use Noxious Air for tear gas. 

Recovery rate is 3 points per turn with Fresh Air, half that rate if it's Foul or Thin.

Stench of Death/Rotting Flesh.

Can be treated as any of the other air-types for effect, but won't kill you under normal circumstances. 

As an option, can have the same effect as being Sick (as from Radiation, see below) or any other handy sickness/nausea mechanic you have knocking around.

A sudden rush of air can have a short effect (as little as 1 round) and then dissipate - for when you open damaged hibernation chambers and failed-to-escape pods, for instance.

Gravity.

In High, Low and Zero Gravity you:

• Need to rest twice as long or twice as often.
• Roll for attacks, movement and stability at disadvantage.

High Gravity halves your movement and you cannot jump; fall damage is doubled; you can only act every other round; your encumbrance allowance is halved; STR check to get back on your feet or perform a fancy manoeuvre.

Low Gravity halves fall damage and doubles jump distance; you need a movement/stability check to move at more than half rate; you act last in a round; DEX check to get back on your feet or perform a fancy manoeuvre. Natural healing rate halved. 

Zero Gravity means you cannot fall but you cannot move without something to hold on to; you can only act every other round; you are Sick (as from Radiation, see below); WIS check to right yourself or perform a fancy manoeuvre. Natural healing rate only 10%.

Spending extended periods (more than a week) in Low and Zero Gravity will Weaken you (as from Radiation, see below) and requires x2 time spent in Low Gravity (x4 in Zero Gravity) for recovery in Normal Gravity. A suitable rehabilitation regime can reduce this.

Proper training, special equipment and experience can reduce/eliminate penalties.

Hard Vacuum.

Realistically, you die - no save. But genre and drama have different requirements.

No air, no flame, no sound, no winged flight.

You can use suffocation/drowning rules, or Depleted Air (save per round) if you want airlessness to be the main hazard of Hard Vacuum.

If you want people to boil away into space: fixed or rolled damage per round, but it's un-soakable, permanent hp loss. Saves for half-damage optional. 

Explosive Decompression is save or die, followed by suffocating or boiling away.

Hard Vacuum can also expose you to temperature extremes and Radiation.

Radiation.

The intensity of the Radiation is measured by the length of time it takes to affect you - the shorter, the more intense.

You suffer a unit of harm if you fail a save per time period, or you are exposed to Radiation for time periods equal to your CON (or a fraction of your CON).

1. Poisoned: save & attack at disadvantage
2. Weakened: half move, half damage, STR & CON at disadvantage
3. Sick: disadvantage all Ability Scores, cannot eat/drink without making a save
4. No natural healing
5. No magical healing*
6. Fail all fatigue, disease and poison saves

Units of harm are cumulative but do not stack. You can get them in any order.

For each unit of harm you take, you must also save or your condition is terminal in d6+1 weeks: -1 level/HD and -2 all Ability Scores per week you survive. 

Taking all 6 units of harm is always fatal. In a setting without magical healing, treat the * result as being terminal.

As an option, depending on genre considerations, a crit save (fail or success) gives you a mutation or you can check for psionics/wild talents.

Radiation damage direct to hit points cannot be healed naturally, and magical healing only counts as temporary hit points with regard to these wounds.

Non-sci fi terms for highly radioactive matter include warpstone, cursed metal, soul crystal, negative/positive material, and necrotic/radiant elements.