Why does my house smell like gas or rotten eggs?

When in doubt, treat a rotten-egg smell as a gas leak until you know otherwise. A gas leak can ignite, so the cost of guessing wrong is far higher than the cost of an unnecessary call. Southwest Gas, which serves the Phoenix area, adds mercaptan exactly so you will notice a leak, and the company's safety guidance is direct about what to do.

Get everyone out of the house first. On your way out, do not do anything that could create a spark:

• Do not turn light switches, thermostats, or appliances on or off.
• Do not use a phone, lighter, or match inside.
• Do not start a car in an attached garage or use an automatic garage door.

Once you are outside and away from the house, call 911 and Southwest Gas at 877-860-6020. The line is answered 24 hours a day, for customers and non-customers alike. Other clues that point to gas rather than a drain or water heater include a hissing sound near a gas line or appliance, a pilot light that keeps going out, or dead plants and blowing dirt near the gas meter outside. Do not try to find the leak yourself. Let the utility locate and confirm it.

If the smell shows up only when you run hot water, and never in cold, the source is almost certainly your water heater, not a leak or your drains. This one is harmless but unpleasant, and it has a clear cause.

The Minnesota Department of Health explains the chemistry well. A water heater holds a anode rod, usually magnesium, that protects the tank from rusting. In some water, that rod feeds a reaction with naturally occurring sulfur bacteria, and the warm tank "can supply electrons that aid in the conversion of sulfate to hydrogen sulfide gas." The result is the rotten-egg smell, produced inside the tank and carried out only through the hot side.

The common fixes, from simplest to most involved:

1. 1Flush the tank to clear sediment and bacteria buildup.
2. 2Raise the temperature to about 160 degrees Fahrenheit for several hours to kill the bacteria, then turn it back down. Use care, because water that hot can scald.
3. 3Replace the magnesium anode with an aluminum or zinc-alloy rod, or a powered anode, which protects the tank without feeding the smell.
4. 4Disinfect with a chlorine solution if the odor keeps returning.

If your home is on a private well rather than city water, the smell can also come from the well itself, so test there too if it shows up in cold water as well. Most Phoenix homes are on city water, where this smell almost always points to the water heater rather than the supply, so the tank is the right place to start.

If the smell is strongest near a sink, shower, or floor drain, you are smelling sewer gas, and the cause is usually a dry P-trap. Every drain has a U-shaped bend that holds a few inches of water, and that water seals the pipe so sewer gas cannot rise into the room. The International Plumbing Code, Section 1002.4, sets that seal at "not less than 2 inches and not more than 4 inches" of water. When the water evaporates, the seal is gone and the gas comes through.

This happens most in drains you rarely use: a guest bathroom, a basement floor drain, a laundry sink, or a tub in a spare room. In Phoenix's dry heat, an unused trap can lose its seal in a few weeks. The fix is as easy as the cause: pour a quart or two of water down each unused drain, and add a little water to toilet bowls in rooms you do not use. If the smell clears within a day, a dry trap was the answer.

This is also where the word "methane" comes up. People often call a sewer smell a "methane smell," but methane is actually odorless. Sewer gas is a mix, and the part you smell is the hydrogen sulfide, not the methane. So a "methane smell" in a house is really sewer gas, and the cause and fix are the same as above. If refilling the traps does not help, the smell could come from a dry or broken toilet wax ring, a blocked plumbing vent on the roof, or a cracked sewer line under the house, all of which need a plumber.

Use where and when you smell it to narrow the cause in under a minute.

You can safely handle two of these three yourself. A dry trap just needs water, and a smelly water heater often clears with a flush or an anode swap, though replacing an anode rod is a job many homeowners hand to a plumber. The natural gas case is the one you never handle alone. If the smell is strong, spreading, or near gas equipment, the only correct move is to leave and call.

Here is the danger you should know about sewer gas, so you take a lingering smell seriously. According to the ATSDR, people can smell hydrogen sulfide at very low levels, far below where it harms you, which is why even a faint odor is worth fixing. But at high concentrations the body loses the ability to smell it within 2 to 15 minutes, so a strong smell that seems to fade is not proof the air is safe. The takeaway for any rotten-egg smell: identify the source, do not just cover it. Rule out gas first, check whether it is tied to hot water or to a drain, and you will land on the right fix quickly. When in doubt, or when the smell will not go away after the easy fixes, call a licensed plumber to find the source for certain.