Yellowstone Super-Volcano, USA, Hydrogen Sulfide 21.5 ppm, Canyon, Madison River  Hydrogen sulfide (H2S) is very toxic in high concentrations. Hydrogen sulfide is a colorless, flammable gas with a strong, offensive odor. It is sometimes referred to as sewer gas. Interestingly, the human nose is more sensitive to H2S than any gas monitoring instrument we have today: air mixtures with as little as 0.000001% H2S are associated with a rotten egg smell. Unfortunately, however, our sense of smell is not a reliable alarm - at mixing ratios above about 0.01%, H2S becomes odorless and very toxic, causing irritation of the upper respiratory tract and, during long exposure, pulmonary edema. Exposure to 500 ppm can cause a human to fall unconscious in 5 minutes and die in an hour or less.