>The gas [natural gas] replaced, "coal gas" or "illuminating gas" was another matter entirely. It was manufactured locally at "gasworks" from coal heated in airtight chambers. The gas produced, a mixture of methane, hydrogen and carbon monoxide, not only burned beautifully, but was perfect for the suicidally-inclined. The active ingredient was, of course, the carbon monoxide.
With blood having more than 200 times the affinity for carbon monoxide than oxygen, it doesn't take much to saturate the blood and starve your brain and nervous system of oxygen.