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Imagine that humanity found a rocky planet with earth like gravity, atmospheric pressure, and temperature, orbiting a sun-like star. The only problem is that the planets atmosphere is that it is composed of 20% nitrogen, 60% hydrogen, 14% ozone, 3% argon, and 3% other gasses.

Assuming that the colonists to this planet had sufficient amounts of oxygen with them, could they breath a mixture of 80% the planets natural atmosphere and 20% pure oxygen safely? If not, what would be the effects of this natural-atmosphere+oxygen mix on the human body? Whether or not these gasses are a probable/viable atmosphere are unimportant for the scope of this question.

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    $\begingroup$ Your parameters do not work well together. Habitable zone and Earth like gravity cannot hold hydrogen. For that you need either larger gravity or colder temperatures. $\endgroup$ – L.Dutch Jan 16 at 20:20
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    $\begingroup$ Another problem: ozone is O3. Oxygen and Hydrogen tend to not exist together for very long. Plus what @L.Dutch said. $\endgroup$ – ShadoCat Jan 16 at 20:22
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    $\begingroup$ The problem has very little to do with the effect on the human body, and a great deal to do with the mixture of the planet’s atmosphere and pure oxygen being violently explosive. $\endgroup$ – Mike Scott Jan 16 at 20:25
  • $\begingroup$ All good points. I didn't think of the ozone problem. Thanks. $\endgroup$ – John Doe Jan 16 at 20:29
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The first thing I was taught on my safety training was: never mix hydrogen and oxygen.

For more info read this question

Now while the explosive limits of hydrogen in air range from about 18 -- 60 % the flammable limits are from 4 -- 75 %, in oxygen the limit of flammability goes all the way from 4% to 95% read: for practical purposes, hydrogen in oxygen is always at least a flammable mixture.

Also, hydrogen safety states

  • "Hydrogen-air mixtures can ignite with very low energy input, 1/10 that required igniting a gasoline-air mixture. For reference, an invisible spark or a static spark from a person can cause ignition."
  • "Although the autoignition temperature of hydrogen is higher than those for most hydrocarbons, hydrogen's lower ignition energy makes the ignition of hydrogen–air mixtures more likely. The minimum energy for spark ignition at atmospheric pressure is about 0.02 millijoules."
  • "The flammability limits based on the volume percent of hydrogen in air at 14.7 psia (1 atm, 101 kPa) are 4.0 and 75.0. The flammability limits based on the volume percent of hydrogen in oxygen at 14.7 psia (1 atm, 101 kPa) are 4.0 and 94.0."

Therefore your guys breathing a mixture of oxygen and hydrogen are doing a really unsafe thing. They are at risk of flaming their lungs at every breathe.

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  • $\begingroup$ "never mix hydrogen and oxygen" I'm glad NASA moved beyond the first lesson, then. $\endgroup$ – a CVn Jan 16 at 20:43
  • $\begingroup$ @aCVn, NASA was not intentionally putting humans, hydrogen and oxygen in the same environment. $\endgroup$ – L.Dutch Jan 17 at 4:13
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Assuming that you can actually create that atmosphere, you would have to first filter out the ozone (and wear full body enclosure). Ozone is toxic. It will also tend to bleach things. So, if you go walking outside without full body protection, you will notice that all of your fancy clothes are now very fading to white (though you won't have to worry about your underwear turning grey in normal washes). Though, you will only notice that after you are done dealing with the full body rashes that you now have as the ozone starts burning through your skin. NIH1 and NIH2

Also, any spark that happens in the air that you add O2 to will likely flash all your air. O2 + 2xH2 = 2xH2O.

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