Gas smell or sewer smell? If in doubt, treat it as gas
Updated October 30, 2026 · 3-minute read · Old House Sewer Lines
When a smell could be natural gas or sewer gas, treat it as natural gas and get everyone out now. Touch no switches, phones or appliances on the way, and call 911 or Con Edison only once you are outside and away from the house.
The two can be hard to separate by nose. On its own, natural gas is odorless. Con Edison adds a chemical called mercaptan so leaks can be detected, and lists an odor "similar to rotten eggs" as the first sign of a leak (Con Edison, Gas Safety). Hydrogen sulfide, the gas most associated with sewers, also smells like rotten eggs, according to ATSDR (ATSDR).
The order of steps
Con Edison's guidance is a sequence: notice the signs, leave immediately and take others with you, then call 911 or Con Edison. Its gas-safety page says not to light a match, smoke, flip a switch, ring a doorbell, or touch appliances or electronics, "including your phone." Its FAQ adds that the ban covers flashlights and ovens too, and that you should not try to find the leak yourself (Con Edison FAQ).
Two more points from the same pages:
- Faint still counts. A weak odor is still grounds to get out, per Con Edison, and nobody should assume a neighbor already reported it.
- No smell does not mean no leak. Soil around an underground leak can strip the odorant, Con Edison says, so if a leak is suspected without a smell, call from a phone away from the area.
NYSEG, which serves parts of New York outside Con Edison's area, gives the same order: get out and away, then call from a safe location or a neighbor's phone, and do not operate switches or appliances that could spark (NYSEG, archived). The CDC says the same: leave and contact emergency authorities, and do nothing that could cause a spark.
Signs that point to gas
Con Edison lists sight and sound as well as smell: a white cloud, bubbles in water, blowing dust or dying plants, and a roar, hiss or whistle. Any of those, with or without an odor, is a reason to leave and call. Con Edison's gas detector instructions add that no one should go back in until the authorities say so.
If the problem turns out to be in the owner's own gas piping, Con Edison says it will turn off the gas until a plumber or technician makes repairs. Indoor gas piping from where it enters the building to the meter is the property owner's to maintain.
After the all-clear
Once the gas utility or the fire department has checked and found no gas leak, the sewer causes are worth looking at, starting with traps that have lost their water, caps that are not tight and vents that are plugged. Those are covered in sewer gas smell in the house. A smell only from the hot tap points to the water heater; see hot-water odor versus sewer odor. Where a sewer clog is being cleared outside the house, read cross bores first.
Con Edison also notes that a carbon monoxide detector is not a gas leak detector. For homes with gas appliances, it describes detectors listed to UL 1484, mounted within 12 inches of the ceiling in the room with the gas pipe or appliance. More hazards are on the safety topic page.
Video from ConEdisonNY, an outside channel: A Con Edison gas worker explains the rotten-egg odorant and what to do if gas is smelled at home. Whatever the strength of the smell, leave the building first, then call from outside. Nothing loads from YouTube until you press play.
Key facts
- Con Edison's order: notice, leave immediately with others, then call 911 or Con Edison (Con Edison).
- Mercaptan is added to natural gas so it can be smelled (Con Edison).
- Hydrogen sulfide, often called sewer gas, also smells like rotten eggs (ATSDR).
- Soil can strip the odorant from an underground leak (Con Edison).
- A carbon monoxide detector is not a gas leak detector (Con Edison).