Is a rotten egg smell in my tap water harmful?

The Minnesota Department of Health lists four sources for hydrogen sulfide in water. It can occur naturally from decay and reactions with soil and rock. It can be made by sulfur bacteria in groundwater, a well or the plumbing. It can be made by sulfur bacteria or chemical reactions inside water heaters. And, rarely, it can come from pollution.

On city water, the two places to check first are inside your own house: the water heater and the drains.

### The water heater

This is the classic case, and it only affects hot water. A. O. Smith's manual says the most common complaint tied to the anode rod is a rotten egg smell in the hot water, and that the odor comes from hydrogen sulfide gas dissolved in the water. It says four factors must all be present for the smell to develop:

1. 1Sulfate in the supply water.
2. 2Little or no dissolved oxygen in the water.
3. 3Sulfate reducing bacteria that have built up in the water heater. A. O. Smith calls this bacteria harmless and nontoxic to humans.
4. 4Extra active hydrogen in the tank, caused by the anode rod doing its job of protecting the tank.

The anode is a metal rod that corrodes on purpose so the steel tank doesn't. Think of it as a bodyguard that takes the hits. The catch is that, as it works, it helps feed the reaction that makes the smell. Rheem says the odor may mean bacterial growth inside the tank between the water and the magnesium anode rod.

### A drain, not the water

The University of Georgia Extension says a rotten egg or sewage-like odor in household water usually comes from bacterial activity, and that the most common cause is organic matter such as hair, soap and food waste building up on the walls of a drain. It gives a simple test. Fill a glass at the sink that smells, step away from the sink and swirl the water. If the water in the glass doesn't smell, the problem is in the drain, and the answer is to disinfect and flush the drain, not treat the water.

Here's the core of the safety answer. The risk is mostly about breathing the gas, not drinking the water.

The Minnesota Department of Health puts it this way: "While sulfur bacteria are not harmful, hydrogen sulfide gas in the air can be harmful at high levels." It says the gas should be removed from the water or vented outside so it can't collect in low spaces such as well pits and basements or in enclosed spaces such as well houses. It also says that in most cases the smell does not relate to the sanitary quality of the water.

The UGA Extension says the bacteria that cause the smell in a water heater are generally not a health threat, though the taste and odor can be very unpleasant. And the EPA's secondary drinking water standards list a limit for odor and name a rotten-egg, musty or chemical smell as the effect people notice above it. EPA says those secondary limits are guidelines for taste, color and odor and are not considered a health risk at those levels.

### What the air levels mean

OSHA's hydrogen sulfide page shows how wide the range is. Its table puts the odor threshold, where some people first notice the rotten egg smell, at 0.01 to 1.5 parts per million in air. It says prolonged exposure at 2 to 5 ppm may cause nausea, tearing of the eyes, headaches or loss of sleep. At 100 to 150 ppm, OSHA lists loss of smell, called olfactory fatigue or paralysis.

That last point is the one to remember. At high levels, your nose stops warning you. A faint smell at the bathroom sink is a long way from those numbers. A strong smell in a small, closed space, like a well pit, a crawl space or a closet with a water heater in it, is the case to take seriously, and a smell that seems to fade there doesn't mean it's gone. Don't go in, and don't stay to look for the source. Leave the space. If it holds a gas water heater, furnace or other gas appliance, treat it as a gas leak and follow the steps above. A well pit is a job for a well contractor. The Minnesota Department of Health says: "Only well professionals using proper safety precautions should consider entering a well pit or other enclosed space where hydrogen sulfide gas may be present."

### When to stop using the water

The UGA Extension draws a clear line. If the drain and the water heater are ruled out, the odor may be coming from the water source, and it says not to use the water because it may contain harmful bacteria. On a public water supply, it says to contact your water provider or county health department right away. On a private well, the Minnesota Department of Health says that in rare cases the gas may come from sewage or other pollution, and it suggests testing for coliform bacteria and nitrate to be safe.

The Minnesota Department of Health gives a simple test. After you've been away from home for a few hours, smell the water at the hot and cold faucets and note which ones have the rotten egg odor. Put the results together like this:

• Hot water only, at every faucet. The water heater. Our page on smelly hot water covers the treatment options.
• Hot and cold at one sink, but the glass test is clean. The drain.
• Hot and cold at every faucet. The supply. Call your water provider, or on a well, test the water.

MDH also lists a few signs that back this up. Black stains on silverware and plumbing fixtures and corrosion on pipes are signs of hydrogen sulfide gas. White, grey, black or reddish brown slime is a sign of sulfur bacteria.

Often it helps, and it's one of the standard fixes. It is not a guarantee, and it needs a steady power supply.

Rheem says the bacterial smell tied to a magnesium anode can be solved by switching to an aluminum anode or a powered anode. A. O. Smith is more careful. It says a special anode rod may be available if water odor or discoloration occurs, but that this rod may reduce the problem without eliminating it. It adds that the water supply may need special filtration equipment to fully remove all odor.

### How a powered anode works

A powered anode is an electrically driven rod that protects the tank with a small current instead of by wearing itself away. In the installation manual for one of its commercial gas water heaters, Bradford White describes its powered anode system this way: it supplies a low voltage current to titanium anode rods and checks that current against the tank wall to make corrections. It says the system is designed to extend tank life without anode replacement.

Look back at A. O. Smith's list of four ingredients. The anode's protective action is one of them, which is why changing the anode is the first lever most manufacturers point to. Swapping it doesn't touch the sulfate in the water or bacteria already in the tank, though.

### The catches

• It needs power all the time. That same commercial manual says the powered anode must have electrical power connected at all times to protect the tank, and that if power to the heater will be off for an extended time, the heater must be drained. It's a commercial unit, so check the instructions for the rod you'd actually install at home.
• The bacteria may already be there. A. O. Smith pairs the anode swap with chlorinating the tank and all hot water lines. The Minnesota Department of Health says that if chlorination doesn't kill all the bacteria, the problem may return within a few weeks.
• Never just pull the anode. A. O. Smith says not to remove the anode and leave the tank unprotected, because doing so voids the tank warranty. MDH says removal may significantly shorten the heater's life.
• Smell in cold water too? Then no anode will fix it, because the tank isn't the only source.

MDH recommends that unless you know water heater work well, you have a plumber or water system professional handle the anode and the tank treatment. MDH notes the job means turning off the water and releasing pressure from the heater before the plug comes out.

We test hot against cold, check the anode, and tell you whether a new rod, a tank treatment or a filter on the supply makes sense. If the heater is old, we'll tell you when replacing it is the smarter move, and our page on whether to repair or replace a water heater lays out that call. HQ Plumbing and Air handles water heater repair and replacement and water filtration, is licensed and insured, and gives free estimates with an upfront price.