Short definition
A gas leak is the uncontrolled escape of natural gas or propane from a piping system, fitting, or appliance. The added odorant (mercaptan) is what gives leaking gas a “rotten egg” smell — the gas itself is odorless. Any time you can smell gas in a house, treat it as an evacuation event, not a “let me check that fitting” event.
What it is
Distributed natural gas and propane both have a sulfur-based odorant — typically ethyl mercaptan — added at the utility level so a leak is detectable long before concentrations approach the explosive range. Natural gas becomes flammable at about 5% in air (the lower explosive limit, or LEL); propane at about 2.1%. Mercaptan is detectable to the human nose at roughly 1 part per billion — meaning if you can smell it, you can almost always still leave safely.
That margin is the safety system. The rule is built around it: if you smell gas, leave. Don’t troubleshoot. Don’t switch lights. Don’t use a phone inside the house. Don’t start a vehicle in an attached garage. The cumulative spark count of normal household activity is what turns a small leak into a fire.
Sources of leaks include threaded-fitting failures (old hardening pipe dope, wrong tape), under- or over-tightened flare fittings at appliance valves, earthquake or settlement damage to buried lines, lightning-pinholed CSST, corroded black iron in damp crawlspaces, and disconnected lines that were capped but not sealed and tested.
Why it matters to a homeowner
Accumulated gas in a closed structure can ignite from an electrical arc or pilot light. Documented annually in US fire-investigation reports, gas-leak fires are among the most destructive residential events because the entire structure is the fuel container. The good news: the response is simple, rehearsable, and works.
The other reason this matters: gas-leak repair is unambiguously a pro job. There is no DIY-acceptable scope for repairing a leaking gas line. Soap-water testing of a known fitting after a pro has shut down and re-pressurized is reasonable. Testing or fixing an active leaking system is not.
What to do if you smell gas (the order matters)
- Leave immediately. Get everyone — family, pets — out. No detours.
- Don’t touch light switches, thermostats, garage door openers, or appliances. Don’t use a landline or cell phone inside the house. Don’t start a vehicle in an attached garage.
- From outside, call your gas utility’s emergency line. Then call 911 if the smell is strong or you suspect a fire.
- Stay out until the utility says it’s safe to return. If the utility shuts off the meter, only the utility (or a licensed gas-piping contractor with a permit and pressure test) should re-pressurize.
If you can close the manual quarter-turn shutoff at the meter on your way past, do it. If that adds time, just leave.
When you’ll encounter this term
- Smell of gas anywhere in the house
- Hissing sound near an appliance valve, meter, or fitting
- Dead vegetation along an outdoor gas-line route — buried-leak indicator with no indoor smell
- After a Cascadia event: assume pipe damage; check the meter shutoff; if you have an earthquake gas shutoff valve, it should have tripped
- After a remodel where a gas-using appliance was removed: confirm the disconnected line was capped, sealed, and leak-tested
Common variants and disambiguation
- “Gas leak” (raw fuel gas) vs. “carbon monoxide leak” (combustion exhaust). Different sensors detect them. CO alarms do not detect natural gas. Combustible-gas alarms do — they’re a separate device.
- “Mercaptan smell after a long absence.” Sometimes mercaptan permeates iron pipe slowly, and a strong rotten-egg whiff after vacation is residual rather than active. Treat it as a leak until verified, but expect it might not be one.
- “Dead vegetation in a yard line.” An underground leak indicator that doesn’t show up as indoor smell. Worth noting if grass dies in a strip following the gas-line route.
Common leak sources
- Threaded-fitting compound failure — old, hardened pipe dope or the wrong tape (white PTFE instead of yellow gas-rated) at a union or appliance shutoff.
- Flare fitting under- or over-tightened at appliance gas valves.
- Earthquake, settlement, or impact damage to buried service lines or in-home runs.
- CSST puncture from a lightning strike on an unbonded system.
- Black iron corrosion at threads in damp crawlspaces.
- Appliance internal failure — gas valve diaphragm, regulator vent line, pilot orifice.
- Disconnected line capped but not sealed after appliance removal.
Washington note
Each WA utility maintains a 24-hour emergency line for gas leaks; the call is free regardless of who pays the bill. Verify the current number against your utility’s website at the moment you need it — these numbers change rarely, but they do change.
- Puget Sound Energy: 1-888-225-5773
- Cascade Natural Gas: 1-888-522-1130
- Avista (Spokane / E. WA): 1-800-227-9187
- NW Natural (SW WA): 1-800-882-3377
For propane, the supplier’s emergency number is on the tank.
After a leak is shut off, any repair to gas piping requires a permit and a pressure test through the AHJ. In Seattle, a Gas Piping Mechanic License is required. Statewide, paid gas-piping work requires a registered contractor under chapter 18.27 RCW. Inside-the-home repair after the utility shuts off the meter typically runs $200–$800; a pressure test for re-pressurization runs $100–$250.
If your home is served by buried iron pipe and was built in the 1950s or earlier, a gas pressure-drop test ($100–$250 add-on at any home inspection) is cheap relative to the cost of discovering a slow leak after closing.
FAQ
What does a gas leak smell like?
Like rotten eggs or sulfur. The odorant added to natural gas and propane (mercaptan) is detectable at very low concentrations — well below the explosive level. If a faint smell lingers near a stove or appliance, treat it as a leak first; check the burner controls and pilot afterward, only with a pro present if the smell persists.
Should I open windows to “air out” a gas leak?
No — leave first. Opening windows on the way out is fine, but don’t stay inside to ventilate. Don’t operate any switch, lighter, or device that could spark. The utility will ventilate the house safely once they’ve shut off the supply.
What if I’m not sure whether it’s gas or something else?
Treat it as gas until proven otherwise. The cost of evacuating and being wrong is a free utility visit. The cost of staying and being right is catastrophic. Mercaptan can sometimes be confused with sulfur water or sewer gas, but the response is the same — leave, then troubleshoot from outside.