A “rotten eggs” smell—often described as sulfur or hydrogen sulfide—coming from your tap water is a common complaint. While the odor itself can be unpleasant, the underlying cause matters: it may be something as simple as a temporary plumbing issue, or it could be related to the water source or treatment process.

Hydrogen sulfide is the most frequent culprit. It can dissolve in water and release a strong odor when water is running, especially when hot water is used. People also report that the smell may come and go, worsen after water has been sitting, or show up more in one faucet than the rest.

Common reasons for sulfur (rotten egg) odor

1) Hot water heater problems
Because hydrogen sulfide can build up in heated water, the smell may be strongest at showers or hot taps. A failing anode rod, corrosion inside the tank, or sediment buildup can contribute. If cold water smells normal but hot water smells bad, this is a prime suspect.

2) Plumbing or household plumbing materials
Older pipes, iron corrosion, or buildup in internal plumbing can release sulfur-like odors. Trapped water in certain sections of your system may smell more than water that has been freshly flushed.

3) Changes in the water supply or treatment
Municipal systems sometimes adjust disinfection or treatment steps, or water sources can vary seasonally. These shifts can temporarily alter how much hydrogen sulfide (or related sulfur compounds) is present in the distribution system—leading to odor complaints across a neighborhood.

4) Well water and natural sulfur
If you use a private well, sulfur odors can be natural. Bacteria, low oxygen conditions, or certain geological formations can increase sulfide in groundwater. In these cases, the problem often persists and may worsen with changes in pumping rates or pressure.

How to identify where the smell is coming from

To narrow down the cause, observe patterns across time and faucets. Start with a quick check: does the odor appear only on hot water, only on cold water, or on both? Does it show up at every faucet or just one? Is it stronger after the water has sat overnight?

• Hot-only smell: focus on the water heater.
• Whole-house cold smell: consider supply or upstream conditions, or corrosion in plumbing.
• Single faucet smell: check aerators, valves, or localized piping.
• After sitting: trapped water in pipes or a storage component may be releasing odor.

What to do next

If the smell is new, sudden, and affects multiple neighbors, contact your local water utility. Ask whether there are recent treatment changes, flushing activities, or reports of odor from the same area. Utility companies can also advise on whether any testing is underway.

If the odor is isolated to your home, consider having a licensed plumber inspect the water heater and visible plumbing for corrosion or sediment. If you use a private well, consult a water testing lab about hydrogen sulfide and related parameters (such as bacteria and other dissolved gases). Treatment options vary widely depending on the cause, so targeted testing is key.

In the meantime, you can reduce exposure to the odor for routine use by letting cold water run briefly if the issue is strongest after it sits. However, avoid relying on “flushing” as a permanent solution if the odor persists—especially if you notice any changes in taste, color, or your water’s overall appearance.

When to take it more seriously

Hydrogen sulfide can be harmful at high concentrations, though typical household tap-water levels are usually not the same as dangerous air concentrations. Still, if the odor is extremely strong, occurs indoors near plumbing vents, or is accompanied by other water quality changes, stop using the affected source for drinking and cooking and contact professionals for assessment.

Odors in water are rarely something to ignore. With a few observations and, when needed, proper testing, you can pinpoint whether the issue is tied to your water heater, your plumbing, or your water supply—and choose the right fix.