Could an atmosphere of 50% O₂ 25% Neon 23% Nitrogen and 3% trace gasses be survivable for a human being at the similar or slightly less atmospheric pressures as Earth?
Some of the information in this thread is questionable, so I'm adding my 2 cents (as a qualified rebreather diver):
- rebreathers use soda lime absorbent material to remove the CO2 in their breathing loop. The CO2 passing through the scrubber absorbent is removed when it reacts with the absorbent in the canister; this chemical reaction is exothermic, which results in heat and moisture.
- we target a PPO2 of 1.2 for most dives. Going above 1.6 is where toxicity starts to become an issue. That said, many divers report blurry vision after long (4+ hours) dives at 1.3.
This environment is almost certainly fatal, due to Oxygen Toxicity:
When the exposure to oxygen above 0.5 bar (50 kPa) is intermittent, it permits the lungs to recover and delays the onset of toxicity.
0.5 bar would be your 50% O2 level at atmospheric pressures, so current medical knowledge says exposure to this environment must be intermittent.
Oxygen toxicity occurs at points above 0.3 bar (30% at 1atm). It's more trouble as one progresses to higher levels, but that shows that such high oxygen levels are going to cause problems.
Even if the toxicity didn't do them in, this atmosphere would turn a small firecracker into a grenade. Higher concentrations of oxygen make things burn and/or explode more intensely.
Don't believe just because I'm saying this. Watch this video (you may jump to 0:50).
Humans are dumb, and explosions are a recurring theme in the Darwin Awards. Any large group of people wouldn't survive in that atmosphere even if they were breathing an Earth-like mix of gases from a scuba gear.
If 3% traces are not a problem. Then the only thing that rises concern is an oxygen levels. Oxygen toxicity, as stated, will be part of your problems, as there are more, like production of oxygen species, sight defects and blindness in infants and more.
- Oxygen at 50% is around levels that have no prominent toxicity for humans. So any healthy adult human will be able to live in such an atmosphere.
- Such levels will be a problem for the weaker ones: children, elderly, sick. Your population will age faster and live less, your child mortality and birth defects most likely will push them to extinction.
Good options are:
- We live high. With altitude there is less air, so less oxygen partial pressure. Living 4000 m - will deal with most of problems, around 6000 meters will be Earth like. Do not forget your pressure cooker.
- Balance atmosphere pressure and composition. You would like your oxygen partial pressure to be less than 30 kPa.
Of note such an atmosphere is a fire hazard and promotes corrosion.
It would be survivable for moderate durations, but probably not for many days at a time.
A partial pressure of oxygen (PPO2) of 0.5 (50% of O2 multiplied by 1atm of pressure) is considered the cusp of noticeable effects of oxygen toxicity and the equations used in diving for computing the allowable time limit works from that, so a PPO2 of 0.5 wouldn't return a valid result. The closest data point is the first entry in diving tables at a PPO2 of 0.6, which recommends a time limit of 720 minutes.
If you wanted to increase the atmospheric pressure to 2atm, the new PPO2 would be 1, as 0.5*2. Using that you should be able to find the allowable time for any combination of oxygen percentages and atmospheric pressures.