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ANSWERS: 3
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Natural Gas is the number one that comes to mind, that's why it has to be scented with that awful odor because it's originally odorless and deadly.
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In a practical sense, any gas that isn't at least 20% oxygen is "poisonous" to humans, because it won't support human life. That includes methane, natural gas, carbon dioxide, nitrous oxides (though I don't know if those are found in mines commonly) and carbon monoxide (which could be present if there is any oxidation going on of, for example, a coal seam in a mine, or a poorly vented internal combustion engine).
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methane.
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