Inert gas asphyxiation may be effectively painless, and certainly more humane than death by extreme temperature; see http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1789805/ for detailled information.
ʷᶦᵏᶦᵖᵉᵈᶦᵃ ˢᵒᶜᵏᵖᵘᵖᵖᵉᵗ
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