Humans breath Air which is a mixture of about 78% Nitrogen, 21% Oxygen and trace other gases including 0.03% Carbon Dioxide. Much higher levels of Oxygen would make it easy for things to catch fire. More than 2.0% or 3.0% Carbon Dioxide causes headaches and higher levels can cause death. Plants can tolerate a higher level because they use the Carbon Dioxide. and water to make sugars and cells.

So would we bring tanks of some other gas to mix with the O2? Or is there a plan to generate Nitrogen (or some other gas) on the surface of Mars?

Our body’s CO2 detection system would go nuts: https://www.healthline.com/health/hypercapnia#copdand-hypercapnia

“American Mercury, Gemini and Apollo spacecraft contained 100% oxygen atmospheres, suitable for short duration missions, to minimize weight and complexity.”

“The Space Shuttle was the first American spacecraft to have an Earth-like atmospheric mixture, comprising 22% oxygen and 78% nitrogen.”

“The life-support system on the Soyuz spacecraft is called the Kompleks Sredstv Obespecheniya Zhiznideyatelnosti (KSOZh).[citation needed] Vostok, Voshkod and Soyuz contained air-like mixtures at approx 101kPa (14.7 psi).” (I assume air-like means roughly same composition as air at sea level.)

“Skylab used 72% oxygen and 28% nitrogen at a total pressure of 5 psi.”

“The Salyut and Mir space stations contained an air-like Oxygen and Nitrogen mixture at approximately sea-level pressures of 93.1 kPa (13.5psi) to 129 kPa (18.8 psi) with an Oxygen content of 21% to 40%.”

From https://en.wikipedia.org/wiki/Life-support_system

No, it’s lethal. That’s why Apollo 13 was in so much danger. They weren’t in danger so much of the oxygen running out (despite the oxygen tank that blew up), but the CO2 levels building up.

And to be clear, the reason why it’s lethal is that you will develop a severe respiratory acidosis. That’s what’s lethal.

@Caravanfan Even beyond the explosion risk, isn’t pure O2 unhealthy for extended periods of time? Is nitrogen the best inert gas for diluting the O2 levels on Mars or are there other options that may be more practical?

No. I worked on a portable rebreather for a customer. The CO2 removal was the most challenging part.
Assuming you have adequate oxygen, here some rough numbers for you. they vary with duration and other factors but this will give you an idea. 1% CO2 you feel it. 3% you are confused and unable to work controls reliably. 5% dizzy and might lose consciousness. 10% is lethal.

Here is an experiment for you. Humans exhale ~4% CO2 when they breathe.
Right now, hold your breathe for a bit. Wait a few seconds. Do you notice the panicky feeling? That is your body’s “internal CO2 sensor” alerting you to do something about it.
OK you can breathe now.

Here are some options used in medicine and deep sea diving.

Argox – Argon and oxygen gas mixture occasionally used by scuba divers for dry-suit inflation
Nitrox – Breathing gas, mixture of nitrogen and oxygen
Heliox – Breathing gas mixture of helium (He) and oxygen (O2).
Hydreliox – breathing gas mixture of helium, oxygen and hydrogen
Hydrox – Breathing gas mixture experimentally used for very deep diving
Trimix – Breathing gas consisting of oxygen, helium and nitrogen

Probably.

@gorillapaws Pure oxygen is okay at low PSA, 3–5 psi. That’s what Apollo used. So outposts will use that. Bigger cities will likely need imported nitrogen.

GN2 would be relatively cheap and easy to take. We installed three big 2700 psi tanks of it into the Psyche spacecraft, which was just delivered to JPL.

So it sounds like nitrogen is the best choice, and that there is no practical means of producing it on Mars? This is acceptable though because, unlike the O2, that gets converted to CO2 and will require a MOXIE to replenish, the nitrogen level will remain constant. Do I have that right?

Yes, nitrogen would be stable, assuming it’s not vented. But it would need to be imported from Earth as there are no nitrogen stores on Mars.

That’s what they do on ISS – bring GN2 and add to the atmosphere as required. It is fewer kg per day activity than replenishing the O2 and removing the CO2, though.

@RocketGuy What is the G part of the GN2?

Gaseous. Liquid nitrogen (LN2) is harder to store long term.

@RocketGuy ” That’s what they do on ISS – bring GN2 and add to the atmosphere as required.”

Assuming they’re not venting GN2 into space, where is the GN2 going? Are wthe astronauts metabolizing small quantities of it somehow?

There are small leaks here and there, plus they lose a bit when they use the various airlocks. I forget whether they pump the air back into the station or just vent it out, but even pumping does not get all the air back in.

@RocketGuy Makes perfect sense. Thanks so much for all of your helpful answers.

One more thing, if plants are to be grown on Mars, they will require significant quantities of nitrogen as part of the nitrogen cycle, right? That would mean GN2 will always be a significant limiting factor for Martian colonists?

@gorillapaws – I am not up on my agricultural biology, but I am guessing colonists will need to bring some GN2 but also a lot of nitrogen-containing fertilizer to work the nitrogen cycle: https://en.wikipedia.org/wiki/Nitrogen_cycle#:~:text=The%20nitrogen%20cycle%20is%20the,both%20biological%20and%20physical%20processes.
Also, they might need the GN2 just to get up to 14.7 psi to support various plant growth processes such as transpiration: https://en.wikipedia.org/wiki/Transpiration
Water evaporates quickly at 5 psi.