What is the required concentration of hydrogen sulfide gas before it affects the sense of smell?

The required concentration of hydrogen sulfide gas before it affects the sense of smell is often considered to be low, actually. Hydrogen sulfide is known for its foul odor, which is commonly compared to that of rotten eggs. Symptoms from exposure to the gas, including the ability to detect its odor, typically occur at concentrations as low as 0.0005 to 0.1 parts per million (ppm). At this level, many individuals can easily smell the gas.

In this context, a very high concentration would not be necessary to perceive its odor. In fact, exposure to particularly high concentrations can rapidly lead to olfactory fatigue, resulting in a loss of the ability to smell the gas even while high concentrations are present, which can create a significant health risk.