>Two of six mice exposed to 70% methane in air died in 18 min, whereas mice exposed to 70% nitrogen in air developed only ataxia. Animals exposed to 50-90% methane in oxygen showed mild depression and a marked decrease in locomotion, but no ataxia. Thus, the toxic effect of methane is much greater than that of nitrogen when available oxygen is low, but methane has little effect when oxygen is readily available.

It seems that the toxicity of methane should be discussed not alone, but rather with respect to the partial pressure of oxygen in the atmosphere in question.

https://www.ncbi.nlm.nih.gov/books/NBK208285/#:~:text=Thus%2....