Hydrogen sulfide

At a glance: A rotten egg smell in water is hydrogen sulfide, a gas usually produced by sulfur-reducing bacteria in low-oxygen groundwater, wells or water heaters. It has no federal limit because the smell makes water unpleasant long before it becomes harmful, but it corrodes plumbing and blackens silver. Find the source first: if only the hot water smells, the water heater is the likely cause, and an anode rod swap is far cheaper than a filter.

If both hot and cold taps smell, the gas is in the well water, and treatment depends on how much there is.

What it is and where it comes from

Hydrogen sulfide is a dissolved gas. The Minnesota Department of Health lists its sources as natural decay, sulfur bacteria living in groundwater, wells and plumbing, reactions inside water heaters, and rarely pollution. Penn State adds that it is common where sulfur-reducing bacteria thrive in low-oxygen water, such as wells drilled into shale and sandstone.

The water heater is a frequent culprit. MDH explains that magnesium anode rods, which protect the tank from corrosion, can supply electrons that help convert sulfate in the water into hydrogen sulfide. That is why some homes only notice the smell from the hot tap.

MDH suggests a simple check: after you have been away for a few hours, smell the hot and cold water separately. Our sulfur smell guide walks through what each result means.

Health and household effects

At the levels found in well water, hydrogen sulfide is mainly a nuisance. Penn State says most people can smell it at levels well below 0.5 mg/L, and that it has no drinking water standard because it renders water “aesthetically undrinkable long before it reaches harmful concentrations.” MDH notes the gas can be harmful at high levels, so it should be vented rather than allowed to collect in enclosed spaces such as well pits.

The household effects are the real cost:

• Corrosion. Penn State says it corrodes metal plumbing, and MDH lists pipe corrosion as a sign.
• Stains. Penn State describes yellow or black greasy stains on fixtures and inside pipes. MDH notes black stains on silverware and plumbing fixtures.
• Taste. Coffee, tea and cooking water pick up the flavor.

The sulfur bacteria that produce it are not harmful themselves, according to MDH.

Limits and what they mean

Standard	Level	What it means
EPA	No limit for hydrogen sulfide	No federal health or secondary standard
EPA secondary standard for odor	3 threshold odor number	Guideline for any objectionable smell
EPA secondary standard for sulfate	250 mg/L	Salty taste; a different compound
WHO	No health-based value	Taste and odor noticeable at 0.05 to 0.1 mg/L

Sulfate and hydrogen sulfide are often confused because both get called “sulfur.” Sulfate has no smell. The two are linked in one place, the water heater, where sulfate can become hydrogen sulfide.

How to test

Hydrogen sulfide escapes from an open bottle, so a normal sample can read low. Penn State says it has to be measured on site or collected with chemical preservation for the lab, and not every state-accredited lab offers the test. Ask your lab how to sample before you collect anything.

Test iron, manganese and pH in the same round. Oxidizing filters are rated for all four, and iron often comes with sulfur in the same well. MDH also recommends testing wells for coliform bacteria every year regardless of smell. Our well testing guide covers labs and costs.

Treatment options

Treatment is chosen by level. The ranges below come from Penn State and MDH.

Method	How well it works	Certification to look for	Cost range
Activated carbon	Moderate; below about 1 mg/L	None common	$500 to $3,000 whole house
Aeration	Moderate; below about 2 mg/L (Penn State)	None common	$800 to $4,000
Oxidizing media such as greensand (specialty media)	High; about 2 to 6 mg/L depending on source	None common	$1,500 to $3,000
Air injection oxidation with filtration	High within the maker’s rating	None common	$2,231 for the system we track
Chemical injection plus filter, often catalytic carbon	High, including above 6 mg/L	None common	$500 to $2,500; $2,255 to $2,983 for peroxide systems we track

Cost ranges are Minnesota Department of Health estimates based on 2017 and 2018 quotes, low end self-installed and high end professionally installed. Figures for systems we track are prices checked September 2026.

Shock chlorination of the well can kill the sulfur bacteria and is a sensible first step, but Penn State cautions that the relief is often temporary without continuous treatment. If the smell returns, treat by level. Our well water filter picks list each system’s hydrogen sulfide limit, and the filter finder sorts by problem.

When to call a professional

If only the hot water smells, call a plumber about the water heater anode. MDH recommends having the anode replaced professionally, and Penn State warns that removing it without a replacement voids the manufacturer’s warranty.

Call a water treatment professional if hot and cold both smell and your level is above about 1 mg/L, or if you also have iron or low pH, because the equipment must be matched to all of them. Call a well contractor if the smell appeared suddenly after well work, or comes with slime and other signs of bacteria.