Hydrogen Sulfide
The rotten-egg smell. Detectable by nose far below any level that matters to health.
- Measured in
- mg/L
No range is shown because none is published for the compound. EPA sets a secondary standard on ODOUR rather than on hydrogen sulfide itself, and the smell is noticeable far below any health-relevant level.
What it is
Hydrogen sulfide is a dissolved gas produced by sulfate-reducing bacteria, which live in oxygen-poor water and convert sulfate into sulfide. It occurs in wells and, very commonly, inside water heaters, where the magnesium anode rod and warm standing water make ideal conditions for those bacteria.
Why it matters
The nose is a better detector than any instrument here: people notice hydrogen sulfide at concentrations thousands of times below anything with a health consequence, which is why there is no health-based limit and why EPA regulates odour rather than the compound. What it does do is corrode metal — plumbing, heaters, and the silver in cutlery — and it makes water genuinely unpleasant to drink and shower in.
How to measure it
Your nose, first and seriously. The cold-versus-hot comparison costs nothing and answers the question that decides what you do next.
What you can see, taste and feel
- How trustworthy
- Dependable
- Cost
- Free
- Effort
- None
Not a measurement, and it belongs here anyway: it is free, it is continuous, and it is what tells most people to test at all. Treat it as the prompt for a number, never as the number.
Run cold, smell it; run hot, smell it. Hot only means the water heater. Both means the water supply. Cold only is unusual and worth a second check.
Where to find it: A clean, clear glass and a sheet of white paper.
Test strips
- How trustworthy
- Ballpark
- Cost
- Low cost
- Effort
- Under a minute, wherever you drew the sample
A plastic strip with reagent pads on it. Dip it, hold it level, and match each pad against the chart on the bottle at the time the label tells you to. Most strips read several parameters at once.
Sulfide strips exist and are approximate. Useful mostly for confirming that treatment worked, since the before-and-after is easier to judge than an absolute level.
Where to find it: Pool shops, hardware stores, supermarkets and online, sold both per parameter and as multi-packs. Buy strips made for your kind of water: pool strips and drinking water strips read very different scales for the same chemistry.
Certified laboratory, mail-in kit
- How trustworthy
- Reference grade
- Cost
- Most expensive
- Effort
- Careful sampling, then days or weeks of waiting
You are posted bottles and instructions, you fill them exactly as told, and you post them back. Weeks later a report arrives with figures nobody has to interpret by eye. This is what a result you intend to act on should be built from.
Order it with sulfate and iron bacteria if you want the whole picture on a well. The gas escapes readily, so the laboratory's own preserved bottle is not optional here.
Where to find it: State-certified drinking water laboratories; your state's certification officer keeps the list, and many public health departments will point you at one.
Taking the sample
For several of these the sample is where the error is, and no instrument recovers from a bad one.
- • Smell the sample at the tap, not after carrying it indoors. Hydrogen sulfide is a dissolved gas and it leaves the water quickly once it is disturbed and exposed to air.
- • For a laboratory test, fill the preserved bottle to the very top with no headspace and no bubbling. A sample splashed into an ordinary jar reads low no matter how bad the water smells.
Further reading: US EPA: Secondary Drinking Water Standards, the non-enforceable guidance on taste, odour and staining
If it is too low
No odour. Nothing to act on.
- • No action needed.
If it is too high
A rotten-egg or sulfurous smell, tarnished silverware and accelerated corrosion of plumbing. Unpleasant rather than dangerous.
- • Find out whether it is the heater first — run the cold tap and the hot tap separately and smell both. Smell on hot only means the water heater, which is by far the most common case and the easiest fix.
- • For a heater, replacing the magnesium anode rod with an aluminium-zinc one usually ends it.
- • For a well, shock chlorination deals with the bacteria; aeration, oxidising filtration or activated carbon deal with the gas.
- • Test sulfate at the same time — it is what the bacteria are feeding on.
Common questions
Why does only my hot water smell of rotten eggs?
The water heater. Sulfate-reducing bacteria thrive in warm, still water, and the magnesium sacrificial anode gives them what they need to produce sulfide. Replacing that rod with an aluminium-zinc one is the standard fix and usually ends it.
Is the rotten-egg smell dangerous to drink?
At the levels found in drinking water, no — you can smell it at concentrations far below anything with a health consequence, which is why there is no health-based limit. It is unpleasant, it corrodes plumbing, and it is worth fixing for those reasons.
WiseWaterGauge helps you record and understand your own readings. It is not a substitute for a certified laboratory test, and nothing here is medical or regulatory advice. If a health parameter is out of range, confirm it with an accredited lab and speak to your water provider.