Treat it as a natural gas leak first. Both natural gas and sewer gas smell like rotten eggs, and a gas leak can explode. If the smell is strong or near gas equipment, evacuate, then call 911 and Southwest Gas at 877-860-6020. If it is near drains, it is likely sewer gas.
First, rule out a natural gas leak
Your nose cannot tell mercaptan from sewer gas, so safety comes before diagnosis. Southwest Gas, the gas utility across the Phoenix metro, adds mercaptan precisely so people notice a leak. The company's guidance is clear about what to do when you smell it: leave the building right away, and do nothing that could create a spark.
That means you should not:
- Flip light switches, thermostats, or appliance controls on or off.
- Use a phone, lighter, or match inside the building.
- Start an engine or use an automated garage or dock door.
Once everyone is outside and upwind, call 911 and Southwest Gas at 877-860-6020. That line is staffed 24 hours a day, and Southwest Gas will respond whether or not you are a customer. Other signs that point to gas rather than sewer gas include a hissing or roaring sound near a line or appliance, dirt blowing from the ground outside, or dead vegetation near the gas meter. In a kitchen, a pilot light that keeps going out is another warning.
Do not try to find the leak yourself, and do not assume it is "just the drains" because the kitchen sometimes smells. A wrong guess here is the one mistake in this whole topic that can hurt someone.
Restaurants carry more risk than a home for a reason worth keeping in mind. A commercial kitchen often runs many gas appliances at once: ranges, ovens, fryers, broilers, griddles, and a gas water heater, each with its own connection and shutoff. More connections mean more places a fitting can loosen or a flex line can fail. The same kitchen also moves a lot of air through hoods and makeup-air fans, which can spread a smell or thin it out, making the source hard to pin down by nose alone. That is one more reason to let the utility find and verify a leak instead of guessing in a room full of ignition sources.
How natural gas and sewer gas smells differ
Once the building is safe and the gas utility has cleared a leak, location and timing usually tell you which gas you smelled. The two have different homes in the building.
| Clue | Points to natural gas | Points to sewer gas |
|---|---|---|
| Where it is strongest | Near the range, oven, water heater, or gas meter | Near floor drains, the mop sink, or the three-compartment sink |
| Other signs | Hissing sound, pilot lights out, dead plants outside | Gurgling drains, slow drains, drain flies |
| When it appears | Steady, or when a gas appliance runs | After drains sit unused, or worse on hot dry days |
| What fixes it briefly | Nothing you should do yourself | Pouring water down a drain |
Sewer gas tends to come and go with how the drains are used. Natural gas tends to stay put near the equipment it leaks from. If pouring a bucket of water into a floor drain makes the smell fade within a day, you were almost certainly dealing with sewer gas, not a leak.
If it is sewer gas, here is the usual kitchen cause
A confirmed sewer smell in a restaurant nearly always comes from a dried-out P-trap. Every drain has a U-shaped trap that holds a few inches of water, and that water blocks sewer gas from rising into the room. The International Plumbing Code, Section 1002.4, requires a trap seal of "not less than 2 inches and not more than 4 inches" of water. Floor drains in a kitchen often sit unused for days, so in Phoenix heat the seal evaporates and gas pushes through.
The fast test is to pour about a quart of water into every floor drain, the mop sink, and any fixture that rarely runs. If the smell clears, a dry trap was the cause, and a trap seal primer can keep low-use drains filled automatically. If the smell stays, the next suspects are a grease trap that needs pumping or a blocked vent that is siphoning the traps dry. A constant, building-wide sewer smell can also mean a cracked sewer line under the slab, which a plumber confirms with a camera. For the full walk-through of these causes, see our page on why a restaurant smells like sewer.
Why you cannot trust your nose for long
Smell is a poor safety tool, and both gases prove it in different ways. With sewer gas, the danger chemical is hydrogen sulfide. According to the ATSDR, people can smell it at concentrations as low as 0.0005 to 0.3 parts per million, far below harmful levels, which is good. The problem is the opposite at high levels: the same agency reports that people lose the ability to smell hydrogen sulfide after 2 to 15 minutes of exposure around 100 ppm. So a smell that seems to "go away" is not always proof the air is clear.
Natural gas has its own catch. The U.S. pipeline safety agency, PHMSA, warns about odor fade, where the added scent weakens as gas passes through soil or rust, so a leak can be present with little smell. The lesson for both gases is the same. Use your nose to notice a problem, then rely on action and equipment, not your sense of smell, to decide whether the air is safe.
What to do right now
Run this order every time, and you protect your people first and your building second.
- 1If the smell is strong, sudden, or near gas equipment, evacuate, avoid anything that sparks, and call 911 and Southwest Gas at 877-860-6020 from outside.
- 2Let the gas utility confirm the air is safe before anyone goes back in. This step is not optional in a building full of staff and guests.
- 3If a leak is ruled out and the smell sits near drains, pour water into every floor drain and unused fixture. A fading smell means a dry trap.
- 4If the smell survives full traps, schedule a plumber to check the grease trap, vents, and main line.
It also helps to train your staff on this order before they ever need it. A line cook who smells sulfur at 7 p.m. should know to pull people out and call, not to keep working and hope it passes. Post the Southwest Gas emergency number where the team can see it, and know where your main gas shutoff is, even though the utility will handle the valve once they arrive.
The bottom line: a rotten-egg smell is the one odor in a restaurant you never wait on. Assume gas, get everyone out, and confirm before you go back. Once a leak is off the table, the smell is almost always a simple dry trap you can refill in seconds and prevent for good with a primer.