Mercaptan is added to natural gas supplies so leaks can be detected by smell.
Natural gas is largely methane, and methane itself is colorless and odorless. Gas utilities therefore add tiny quantities of intensely odorous compounds called mercaptans, also known as thiols. Their strong sulfur-like smell warns people about a possible leak before the gas reaches a dangerous concentration.
The best-known odorant is often tert-butyl mercaptan, although utilities may use blends of sulfur-containing compounds. The odorant is not intended to improve combustion or energy content; it is a safety additive used for detection.
People sometimes call the additive simply “rotten-egg gas,” but that description refers to the smell rather than a single universal chemical. Odor can also fade or become harder to notice in some circumstances, so suspected gas leaks should never be investigated with a flame or electrical switch. Immediate evacuation and contacting emergency services are safer responses.