H2S is a hazardous gas often present in the vapor space of tanks, trucks, rail cars, and barge compartments containing liquid asphalt. H2S is a flammable, colorless gas with a characteristic odor of rotten eggs. People can smell it at low levels.

Exposure to low concentrations of H2S may cause irritation to the eyes, nose, or throat. It may also cause difficulty in breathing for some asthmatics. Brief exposures to high concentrations of H2S (greater than 500 ppm) can cause a loss of consciousness and possibly death.