Could humans breathe if the average methane content of the atmosphere increased to 1%?
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)