This question may be a bit broad, but I'm basically wondering what minimal things humans need to have in an atmosphere in order to survive. Let's assume we have Earthlike gravity, and are only dealing with perhaps the first 10 or so elements. Life does not have to have developed there. I'm especially interested in what kinds of atmospheric conditions one could expect to find occurring naturally on a solid planet, and what elements/compounds need to be there or not be there for humans to survive, at least for up to an hour.
Essentially, you need the O2 and the CO2 from the air.
- The O2 is vital for basic functional purposes, such as ATP manufacture.
- Trace amounts (parts per million) of CO2 regulate blood pH, and perform a few other vital roles. See the link for a detailed answer.
- Some inert filler to make up the bulk of the gas mix.
In terms of atmospheric pressure, think of the equivalent of being on a tall mountain. So you can probably live around 80kPa, but actually doing any work would be hard, and going much lower you get into the deathzone. Alternatively, you can increase the O2 concentration to increase the partial pressure of O2, but that gives you only a little bit of wiggle room since once you increase it past 30%, humans (especially hair and fat) become about as combustible as a paraffin torch. Not the best environment to be doing welding, say. Not to mention the free radical and vasoconstriction issues that become pervasive once O2 climbs above a certain threshold.
Presumably, you might want to add a no-poison-gas condition, but that can be alleviated by wearing a gas-mask and a basic body-suit.
I should probably add that without a massive oxygen manufacturing process, biological or synthetic, the oxygen would react with (oxidize) exposed iron and other elements and reduce from the atmosphere, so an oxygen atmosphere (beyond trace amounts - ppm) is unlikely to exist on a dead unterraformed world.
What humans need to breathe in order to survive
For the first part of your question, the section of the NASA Manned Systems Integration Standards on atmosphere has comprehensive information on how one would go about designing an atmosphere. It is intended for use in spacecraft but it can also apply to planets, though on a planet you would have to account for gravity as the gasses will separate by density (like oil and water).
The document covers:
- Earth's atmosphere composition
- Human performance limits, which are summarized in this chart:
The way to read this chart is to find the pressure of your atmosphere on the left axis. Then find what percent of the atmosphere (by volume) is oxygen on the bottom axis. If the intersection of those values is in the "Unimpaired performance zone" then your people will breathe normally. Above that, the oxygen is actually toxic and may cause lung damage and eventually death if the concentration is high enough. Below that there is not enough oxygen; their judgement will be impaired and they may suffocate. Note that the acceptable percentage of oxygen changes with total atmospheric pressure.
A list of additional conditions that must be met for an atmosphere to be survivable. Some of these are covered by the previous chart, others include:
- Pressure high enough to prevent the vaporization of body fluids
- Some inert gas to prevent atelectasis, a type of lung collapse
- Low flame and explosive hazard. Hydrogen reacts violently with oxygen, so it should not be in high concentration. NASA considers oxygen concentration over 30% to be a fire hazard, but this limit is arbitrary. Different materials will start reacting with oxygen at different concentrations and may spontaneously combust under the right conditions.
Gas compositions. The Earth's atmosphere has many different gasses, and most of them do not affect us. Like with oxygen, however, they may become toxic at certain pressures.
Human response to diluent gasses. I find this kind of fun, especially if you're inventing an alien atmosphere. You could use nitrogen like Earth, or you could use helium, neon, argon, krypton, xenon, or hydrogen (though hydrogen might explode). The thought of a helium-oxygen atmosphere amuses me, since it would cause everyone breathing it to speak in that hilarious squeaky voice. It might be a problem on a planet though, since gravity would cause the much lighter helium to float to the top.
CO2 toxicity. The short answer is, 1-1.5% is normal range. At 3% people will experience unpleasant side effects for about 3 days but will acclimate. Concentrations 10% or more can kill. Interestingly, returning to a lower CO2 environment can cause withdrawal symptoms.
There is much, much more to this document, but that hits the most important parts. I suggest that anyone interested in realistic science fiction involving human spaceflight browse through it.
What atmospheres can naturally occur on planets
As for the second part of your question, I suggest you look at the atmospheres of other solid bodies in our solar system and exoplanets which we have estimated the atmospheres of. It appears that the most abundant gasses are inert, which makes sense since over millions of years the volatile gasses will react and form more stable gasses. Thus, carbon dioxide, nitrogen, and noble gasses are common, but oxygen is not; Earth has plentiful oxygen because plants produce it. Hydrogen is common because it is the most plentiful element in the universe.
The normal atmosphere is made up of about 20% oxygen, 78.1% Nitrogen, and 1% argon. Humans would flourish at a level of oxygen at or above 21 %. If the Oxygen level is below 17% you start to feel signs and symptoms of hypoxia. At 17% your night vision begins to deteriorate. 16, 15, 14% basically cause you to start seeing things, hearing things, and other physiological effects. At 6-10% you are incapacitated with sicknesses, including vomiting and nausea. If you go any lower than 6% you will be dead in minutes.
This is a link to the CDC on Carbon Dioxide. You might find it interesting, although it is too high level for me to understand easily.
This link has an interesting discussion on the topic of Carbon Dioxide levels, although it is not the most trustworthy site out there. I will add more information if I find any. Source(s):
I'm pretty sure that the only two compounds that humans rely on are oxygen and carbon dioxide.
Oxygen is what the cells of the body use in chemical reactions that produce energy. The partial pressure of oxygen can vary, but the higher the pressure, the greater the chance of oxygen toxicity. Systems for supporting human breathing in extreme environments (SCUBA, space suits) mostly use nitrogen or helium as "filler" gasses depending on the pressure. (SCUBA gas mixtures)
Carbon dioxide is part of the process in the respiratory system for autoregulation of breathing - too little carbon dioxide and a human will not breathe often enough, while too much can cause hyperventilation, and eventually other problems.