No. Carbon monoxide has no smell at all. A rotten egg smell in the house is usually natural gas, which Southwest Gas scents with mercaptan so you notice a leak, or hydrogen sulfide from a drain or a water heater. Treat a gas smell as a leak, and count on a CO alarm, not your nose, for carbon monoxide.
Why do people confuse carbon monoxide with a rotten egg smell?
People confuse them because both come up around gas appliances, and only one of them has a smell. Natural gas has no odor of its own. Southwest Gas explains that "In its untouched state, natural gas is actually odorless. For safety reasons, Southwest Gas injects an element called mercaptan, which emits an odor similar to sulfur or rotten eggs."
Carbon monoxide is a different gas with a different source. Southwest Gas says it "is produced by the incomplete burning of fuel, including natural gas." In a home, that means a furnace, water heater, range, or other burner that is not burning cleanly or not venting properly. Nobody adds an odorant to it, because it isn't a product anyone delivers. It is a byproduct.
The two can show up in the same house on the same day, which is where the mix-up starts. A gas smell means gas is getting out before it burns. Carbon monoxide means gas is burning badly. One doesn't cause the other, and smelling one tells you nothing about the other.
| Gas | Smell | Where it comes from | What to do |
|---|---|---|---|
| Natural gas | Rotten eggs or sulfur (added mercaptan) | A leak in piping, a connector, or an appliance | Leave, then call 911 and Southwest Gas |
| Carbon monoxide | None | Poor combustion or venting at a fuel-burning appliance | Get to fresh air, call 911 |
| Sewer gas (hydrogen sulfide) | Rotten eggs | Drains, dry traps, sewer lines | Find the dry trap or the leak |
| Hydrogen sulfide in hot water | Rotten eggs, only at hot taps | Bacteria reacting with the water heater's anode rod | Water heater service |
What should you do if you smell rotten eggs near a gas appliance?
Treat it as a natural gas leak and leave. Southwest Gas's rule is short: "If you smell rotten eggs, leave the area, then call us for help, whether you have natural gas at your home or business or not." Its FAQ says to act on the smell "even if it's faint or momentary."
The steps from its safety page:
- 1Exit the building immediately. Tell others to leave, and leave doors open.
- 2Don't create a spark. Don't turn on or off any electric switches, thermostats, or appliance controls, and don't smoke or use matches or lighters.
- 3Don't start or stop an engine or use an automated garage door.
- 4Call from a safe place. Call 911 and Southwest Gas at 877-860-6020, day or night.
Don't try to decide whether the smell is gas or sewer gas while you are standing next to the water heater. Get out first. The utility can check it with a detector, and if it turns out to be a drain, that's a good outcome.
How do you know if carbon monoxide is present?
You know by an alarm or by symptoms, not by smell. That is the whole point of a carbon monoxide alarm: it senses a gas your eyes and nose can't.
The symptoms matter because they are easy to mistake for a cold or the flu. The CPSC lists them as "headache, dizziness, weakness, nausea, vomiting, sleepiness, and confusion." Southwest Gas adds flu-like symptoms, fatigue, shortness of breath, loss of muscular coordination, and loss of consciousness. None of those comes with a smell to warn you first.
If you suspect carbon monoxide, the CPSC's instruction is: "get outside to fresh air immediately, and then call 911." Southwest Gas adds: "Don't re-enter the building until an emergency official has determined the building is safe."
Our page on where to put carbon monoxide alarms covers placement, testing, and replacement. For the appliance side, see whether a water heater can cause carbon monoxide.
What if the rotten egg smell isn't a gas leak?
Then it is usually hydrogen sulfide, from a drain or from the water heater. OSHA describes hydrogen sulfide, also called sewer gas, as "a colorless gas known for its pungent 'rotten egg' odor at low concentrations," and says it "occurs naturally in sewers, manure pits, well water, oil and gas wells, and volcanoes." Two plumbing sources account for most household cases.
### Sewer gas from a drain
A drain trap holds a plug of water that blocks sewer gas. The plumbing code Phoenix uses requires every fixture to be "separately trapped by a liquid-seal trap," with a seal of at least 2 inches and no more than 4 inches of water, and it allows trap primers and barrier-type seal devices to keep rarely used floor drains from drying out. When a trap dries out, in a guest bath, a floor drain, or a sink that rarely runs, sewer gas comes up through it. The smell is strongest near that drain. Running water into the drain refills the trap, and if the smell comes back fast, a cracked pipe, loose toilet seal, or vent problem is more likely.
OSHA adds a warning worth knowing: "After a while at low or more quickly at high concentrations, you can no longer smell it to warn you it's there." A smell that fades while you stand there hasn't gone away. Your nose has stopped reporting it.
### Hydrogen sulfide in the hot water
If the smell shows up only when you run hot water, and the cold water smells fine, the water heater is the source. A.O. Smith's manual says the most common anode rod complaint is "a 'rotten egg smell' in the hot water," and it needs four things at once: sulfate in the supply water, little or no dissolved oxygen, sulfate-reducing bacteria in the tank ("this harmless bacteria is nontoxic to humans"), and excess active hydrogen from the anode.
The fixes from the manufacturers:
- A different anode rod. State's service handbook says the standard anode "can be replaced with a special zinc anode rod which may help reduce or eliminate the odor." It also says an anode must stay in the tank "at all times or the warranty is void."
- A higher tank temperature. Both manufacturers say 140°F can reduce the smell, with a mixing valve to limit the temperature at the taps, since hotter water raises the scald risk.
- Chlorination. State calls chlorinating the water "the only sure treatment."
This smell comes from the water, not the gas line, so it doesn't mean the heater is leaking gas. Our guide to a house that smells like gas or rotten eggs walks through telling the three sources apart.
Can you have a gas leak and carbon monoxide at the same time?
Yes, and that's why neither check replaces the other. A leaking connector behind the range and a furnace with a blocked vent are two separate problems that can exist together. The gas smell warns you about the first. Only a CO alarm warns you about the second.
So keep both lines of defense in place:
- Trust the smell for natural gas. Any rotten egg smell near gas equipment means leave and call.
- Trust the alarm for carbon monoxide. Southwest Gas notes that it "does not inspect for the absence of carbon monoxide detectors," so having working alarms is on you.
- Watch the flame. Southwest Gas says a properly working gas appliance "should have a clear, steady blue flame," and that "Yellow flames may indicate improper combustion and can be a safety concern."
HQ Plumbing & Air repairs gas water heaters, gas lines, and the drain problems behind sewer smells across the Phoenix area, with 24/7 emergency service. See our gas line and water heater services.
Rotten eggs means gas or sewer gas. Carbon monoxide means nothing at all, which is why the alarm has to be there.