Hydrogen sulfide (also known as H2S, sewer gas, swamp gas, stink damp, and sour damp)

It is extremely flammable and highly toxic.
Hydrogen sulfide is used or produced in a number of industries, such as
Oil and gas refining
Mining
Tanning
Pulp and paper processing
Rayon manufacturing
Hydrogen sulfide also occurs naturally in sewers, manure pits, well water, oil and gas wells, and volcanoes. Because it is heavier than air, hydrogen sulfide can collect in low-lying and enclosed spaces, such as manholes, sewers, and underground telephone vaults. Its presence makes work in confined spaces potentially very dangerous.
The health effects of hydrogen sulfide depend on how much H2S a worker breathes and for how long. However, many effects are seen even at low concentrations. Effects range from mild, headaches or eye irritation, to very serious, unconsciousness and death.