Critical System
The Air Constraint
You notice when food is low. You can read water gauges. Air doesn't give you that warning. It's there, then it isn't.
The Baseline Numbers
O2 Consumption
1.83 lb
Per person, per day. That's NASA's standard for a moderately active adult just breathing normally.
CO2 Output
2.20 lb
Per person, per day. CO2 has to be scrubbed continuously. Your thinking gets worse around 1,000 ppm (0.1%). At 2% (20,000 ppm), you get severe symptoms and major cognitive problems. Above 4% is immediately dangerous to life. Sustained exposure at 8-10%+ kills fast.
Lethal Threshold
< 16% O2
Below 16% oxygen concentration, impairment begins. Below 10%, unconsciousness. Below 6%, death within minutes.
Two Things Have to Happen at Once
Problem 1: Oxygen Supply
Oxygen has to be continuously supplied, either from stored tanks or by splitting water molecules using electricity. Both options require mass, power, and backup systems. An interruption of just hours can be fatal.
Problem 2: CO2 Removal
Even with plenty of oxygen on hand, rising CO2 will still kill people. Scrubbers have to run constantly. Lithium hydroxide canisters get used up. Regenerative systems need power and maintenance. And neither type of failure announces itself.
At Scale
4 crew / 21 days
154 lb
Stored tanks. Feasible.
9 crew / 180 days
2,963 lb
Regeneration required.
100 crew / 180 days
16.5 tons
Industrial life support.
150 crew / 365 days
~50 tons
No existing system.
Why CO2 Kills Quietly
Food can be rationed. Water can be stretched. Breathing isn't negotiable. CO2 buildup doesn't feel like anything at first. Then headaches. Then confusion. Then bad decisions. By the time someone notices, the people who'd have to fix it are the ones already impaired.
On the ISS, the CO2 scrubbing system (CDRA) has required frequent maintenance and several partial failures. The ISS gets resupplied constantly and Earth is minutes away on the radio. A Mars-bound crew has neither.
Air is invisible and it can't be rationed. Once an air system fails, the crew has minutes, not hours. That makes it one of the fastest cascade triggers in any habitat.