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MOXIE splits two molecules of martian carbon dioxide into two molecules of carbon monoxide and one molecule of oxygen. Breathing elevated levels of CO2 ranges from unpleasant (to say the least and actual footage believe it or not with ASPCA approval) to fatal and of CO ranges from unhealthy to fatal but without warning. This well-sourced answer to How did MOXIE “know” that it produced 5.4 grams of oxygen? Did it measure pressure rise in a container or just vent it? Did it use an oxygen sensor? contains the following image. It shows two gas outputs for MOXIE, a "good" one with an O2 and CO2 sensors, and a "bad" one with CO2 and CO sensors.

Question: Assuming I am thinking clearly and try to use the gas from the good one to build up something breathable in a pinch, would I have to scrub the CO2 or traces of CO from it with an additional system?

MOXIE diagram

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Answer: Yes, scrubbing would be a very wise precaution since MOXIE has no output CO monitor. MOXIE is not designed to deliberately output CO, but CO is produced in high concentration in the process. The internal structure is a stack of plates with 20 seals, so there is ample opportunity for a failed seal to contaminate output O2 with CO.

CO is a cytotoxin with very similar toxicity to CN (cyanide). It is an odorless gas which slowly makes you stupid (neurotoxicity) until your heart stops (cardiotoxicity). Even concentrations as low as 0.02% are fatal. Published standards for breathable compressed air limit CO to 0.001%

NASA says MOXIE outputs “98% pure O2”. Presumably the remaining 2% is N2 and Ar but even a trace of CO would make scrubbing a wise precaution, especially in the absence of a CO monitor.

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    $\begingroup$ Good point. But no one knows. The schematic shows no CO sensor in the output line. Google can’t find NASA data. NASA is being less than honest: “MOXIE makes oxygen like a tree does. It inhales carbon dioxide and exhales oxygen.” Pure marketing B.S. The process bears no resemblance to a tree. We need more data and less marketing. MOXIE is making twice as much CO as O2 but has no CO monitor. Do I see a red flag? To answer the OP question, no, MOXIE’s oxygen is not ready to breath. No regulatory agency would license MOXIE, in its current incarnation, for human use. As rocket propellant, yes $\endgroup$
    – Woody
    Commented Oct 29, 2022 at 0:45
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    $\begingroup$ Air with 800 ppm CO2 is good quality, 1000 ppm medium quality, 1400 ppm low quality. Maximum workplace concentration for 8 hours is 5000 ppm. Fresh outdoor air is about 400 ppm (mean value). 2 % is 20,000 ppm. $\endgroup$
    – Uwe
    Commented Oct 29, 2022 at 0:48
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    $\begingroup$ You must not have looked too hard and google is not the end all of searching. Your rant about NASA dishonesty and "marketing BS" may be based on a misapprehension. There are several MOXIE papers on ntrs. This one dataverse.jpl.nasa.gov/file.xhtml?fileId=61140&version=1.0 claims that the 98% purity was a requirement and that "All recent stacks exceed 99.9%" $\endgroup$
    – Organic Marble
    Commented Oct 29, 2022 at 2:00
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    $\begingroup$ @OrganicMarble ... Thanks for the link… but it doesn’t have data on CO concentration in MOXIE O2. Neither schematic shows a CO monitor on the output. “99.9% O2” allows for up to 0.1% CO which is 5 times the lethal concentration. I wouldn’t breathe that O2 unless it had been scrubbed or analyzed. My answer is still "Yes, scrub before inhaling." $\endgroup$
    – Woody
    Commented Oct 29, 2022 at 3:23
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    $\begingroup$ The point is, you don't know the composition of the product gas so you cannot answer this question. $\endgroup$
    – Organic Marble
    Commented Oct 29, 2022 at 3:44

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