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Below is a table of atmospheric composition. Could humans breathe if the average methane content of the atmosphere increased to 1%?

Table 7a-1: Current Average composition of the atmosphere up to an altitude of 25 km.

Gas Name          Chemical Formula    Percent Volume

Nitrogen          N2                  78.08%
Oxygen            O2                  20.95%
*Water            H2O                 0 to 4%
Argon             Ar                  0.93%
*Carbon Dioxide   CO2                 0.0360%
Neon              Ne                  0.0018%
Helium            He                  0.0005%
*Methane          CH4                 0.00017%
Hydrogen          H2                  0.00005%
*Nitrous Oxide    N2O                 0.00003%
*Ozone            O3                  0.000004%

* variable gases
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    $\begingroup$ Could you provide a link for the table? $\endgroup$
    – Michael_A
    Commented Oct 22, 2017 at 21:14
  • $\begingroup$ 1.0 what? Percent, ppm, or? $\endgroup$
    – jamesqf
    Commented Oct 23, 2017 at 18:41
  • $\begingroup$ Not sure what you had in mind, but if the atmosphere was 1% methane there would be a massive contribution to global warming. I don't know if anyone has run a model with that much methane but it could very well be an unsurvivable level for human's time on the planet, regardless of direct biological effects. $\endgroup$
    – Bryan Krause ♦
    Commented Oct 23, 2017 at 21:24
  • $\begingroup$ See sciencedirect.com/science/article/pii/S1871174X16300488 $\endgroup$
    – Bryan Krause ♦
    Commented Oct 23, 2017 at 21:27

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Methane is biologically inert and can only act as an asphyxiant at high concentrations by displacing oxygen. An environmental exposure limit has been set at 5,000 ppm, though 10,000 ppm (1%) had no effect:

It is obvious that an exposure limit that presents an explosion hazard cannot be recommended, even if it is well below a concentration that would produce toxicity; thus, exposure limits should not exceed 5% by volume in air.

Animals exposed to methane at 10,000 ppm showed no toxic [effects]; an uncertainty factor of 2 is suggested to derive an EEL—5,000 ppm. There is no evidence that duration of exposure is important in methane toxicity.

Source: Methane in Emergency and Continuous Exposure Limits for Selected Airborne Contaminants Volume 1 (1984)

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