Natural gas is odorless. Utilities add mercaptan (the rotten-egg smell) so leaks become detectable. If you smell rotten eggs indoors, the protocol is simple: leave first, call from outside. Do not switch lights, do not use electrical devices, do not light matches. Every US gas utility runs a 24-hour emergency line that dispatches a technician for free. The 7-step protocol is the same everywhere and works whether the source is a major leak or a tiny pilot-light extinguishment.
What mercaptan is and why it is added
Mercaptan is a sulfur compound (methyl mercaptan or ethyl mercaptan) added to natural gas at parts-per-billion concentration. The natural gas itself is odorless; mercaptan provides the rotten-egg smell that humans detect at very low concentrations.
Mercaptan is added at the wholesale pipeline level and is identical across every US natural gas utility. The smell is intentional and detectable at concentrations far below the lower explosive limit of natural gas (5 percent gas-in-air).
The 7-step emergency protocol
Step 1: leave the home immediately. Take family members and pets. Step 2: do not switch lights or any electrical devices on or off — the spark could ignite gas. Step 3: do not light matches, cigarettes, or anything else with an open flame.
Step 4: do not use your cell phone while still inside. Step 5: call your gas utility emergency line from outside. Step 6: wait for the utility technician outside. Step 7: do not return inside until the technician confirms safety.
Is it sewer gas or natural gas?
Sewer gas can smell similar to mercaptan-odorized natural gas. The difference: sewer gas is intermittent (usually after running water in a fixture); natural gas is continuous if there is a leak.
If unsure, evacuate first and call the gas utility. The technician can test the indoor air with a combustible-gas indicator and rule out natural gas in 5 to 10 minutes. The call is free regardless of who supplies your gas.
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