Instead of methane, why not methanol?
Methanol has a high toxicity in humans. If as little as 10 mL of pure methanol is ingested, for example, it can break down into
formic acid, which can cause permanent blindness by destruction of the
optic nerve, and 30 mL is potentially fatal,
[9] although the median lethal dose is typically 100 mL (3.4 fl oz) (i.e. 1–2 mL/kg body weight of pure methanol
[10]).
Reference dose for methanol is 0.5 mg/kg/day.
[11] Toxic effects take hours to start, and effective antidotes can often prevent permanent damage.
[9] Because of its similarities in both appearance and odor to
ethanol (the alcohol in beverages), it is difficult to differentiate between the two (such is also the case with
denatured alcohol). However, there are cases of methanol resistance, such as that of
Mike Malloy, who was the victim of a failed murder attempt by methanol in the early 1930s.
[12]
Inhalation risk is mitigated by a characteristic pungent odor. At concentrations greater than 2,000 ppm (0.2%) it is generally quite noticeable, however lower concentrations may remain undetected while still being potentially toxic over longer exposures, and may still present a fire/explosion hazard.
Methanol fuel - Wikipedia, the free encyclopedia