Many a writer on survival will invoke the Rules of Threes and how it relates to survival.
In extreme survival situations you can typically not survive more than:
- 3 seconds without paying attention
- 3 minutes without air
- 3 hours without shelter
- 3 days without water
- 3 weeks without food
- 3 months without hope
Paying attention is obvious, certainly so in worlds where everything seems set out to kill you.
For mutant future the ML need to be diligent with sticking to players described actions or lack of actions in resolving situations where a 3 seconds lapse in attention could mean life or death.
Lack of air is a big problem and technically people can last longer then 3 minutes without breathing but if you find yourself in that situation you are likely to still be in serious lack of air over the next 3 minutes and die.
PCs aren't harmed by lack of air for as many rounds as they have points of CON. After that they are required a CON check each round or they suffer 1d6 of damage. Three consecutive failed checks and they collapse into unconsciousness and will automatically suffer 1d6 damage each round until death.
Lack of shelter is an issue in areas of extreme heat and extreme cold.
PCs can survive in extreme cold or heat for as many 10 minute turns as they have points of CON.
Each 10 minutes afterward requires a CON check or the victim loses 1d6 HP and is incapacitated by 3 consecutive failed checks. Appropriate clothing can extend this duration by tripling the time allowed before a check is required and giving the wearer a 4pt improvement on CON for the means of making this roll. If one becomes wet in extreme cold they are required to make a save after a number of rounds equal to CON.
Trying to get by without water can be a trick indeed. A PC can survive for as many hours as they have points of CON between drinks. Each hour afterward requires a CON check or they suffer 1d6 damage. If three checks are failed in a row the character loses a point of STR, INT and WIL each time a successive roll is failed.
A pint of water will offset an hour of dehydration. A quart will offset 3 hours.
Starving takes a while. A character can survive without eating for as many days as they have points of CON with no ill-effects aside form being unable to heal if they have been without food for 3 or more days. Each day after their CON score the character suffers 1d6 damage if a CON check is failed. If three CON checks are failed in a row the character loses a point of STR, INT and WIL on each successive day.
Once one has eaten 4 meals or more over a period of 2 days they are no longer starving.
Lack of hope is a bit odd to define. Seems to me surviving in the deadly worlds of the Mutant Future would require a certain degree of optimism, even so situations like capture could have an impact. A character can survive unharmed for 3 weeks while captured. Each following week requires a WIL check or a point of WIL is lost. If one passes 3 WIL checks in a row they are no longer at threat for loss of hope. If three checks in a row fail each successive failure causes a 1 point loss of CHA as well.
Keeping track of the above and applying the effects will make the realities of survival brutal and almost certainly gritty.