Small itchy bumps that appear a day or two after a soak in the hot tub, densest where your swimwear was pressed against the skin — that is the classic picture of the condition known as hot tub folliculitis, or hot tub rash. According to the US Centers for Disease Control and Prevention (CDC), it is caused by the bacterium Pseudomonas aeruginosa, which multiplies in warm water when the disinfectant level is inadequate. The good news for hot tub owners: this is a problem of how the tub is run, not a property of hot tubs as such — and how it is run is entirely in your hands.
Important note up front: this article describes what is known about how the problem arises and how to prevent it through water care. It does not make a diagnosis and does not advise on treatment. Any rash that spreads, persists, produces pus, is painful or comes with a fever should be shown to a doctor — in children, pregnant women and people with a weakened immune system, without delay.
What a hot tub rash typically looks like and when it appears
The CDC describes it as an itchy, red, bumpy rash that may develop pus-filled blisters around the hair follicles. It tends to be more pronounced where swimwear held water against the skin for longer — typically on the torso, the buttocks and under the arms. Skin that the water merely washed over and that dried quickly is usually less affected.
The delay is characteristic of this subject. The CDC states that the rash usually appears several days after bathing in a poorly maintained hot tub; clinical sources (Cleveland Clinic) give a timescale of days and note that mild cases tend to clear up on their own within roughly one to two weeks. It is precisely that delay that means people often fail to make the connection with the hot tub at all and look for the cause in a new shower gel or washing powder.
Let us be clear about what does not follow from this: nothing can be diagnosed from a description. Plenty of completely different skin problems look similar. If a rash is bothering you, worrying you or not getting better, the assessment belongs to a doctor, not to the internet.
Why does it happen at all? Warm water is an ideal environment for the bacterium
Pseudomonas aeruginosa is a common environmental bacterium — the CDC lists it among the micro-organisms we routinely encounter in water and in soil. In itself it is nothing exotic. The problem arises when it is given the conditions to multiply and, at the same time, enough time to act on the skin.
Those conditions arise easily in a hot tub:
- Heat. Water at around 37 °C is comfortable for many bacteria and at the same time consumes disinfectant quickly. A hot tub is not a small swimming pool — chemically it behaves completely differently.
- A small volume for a lot of people. Three people in 1,200 litres represent an incomparably greater load than three people in a pool holding 30,000 litres.
- Contact time. The longer you sit in it, the longer the water acts. Wet swimwear then holds water against the skin long after you get out.
- The state of the skin. In warm water the skin softens and the pores open, so the barrier works less well than it does when dry. Freshly shaved or grazed skin is logically more vulnerable — which is why the Cleveland Clinic recommends not shaving immediately before bathing.
The key point is that disinfection can override this equation. The CDC states that P. aeruginosa is reliably inactivated by common chlorine- and bromine-based disinfectants. The rash is therefore not linked to hot tubs in general, but to hot tubs with inadequate or fluctuating disinfection.
Where things go wrong in running a hot tub
When you run into the problem at home, the trail almost always leads to one of the five things below. It is worth going through them honestly — usually several small lapses come together at once.
1. A fluctuating disinfectant level
The most common cause. Warm water consumes chlorine or bromine far faster than a cool pool, so a reading taken on Sunday tells you nothing about Wednesday. If you dose the disinfectant “once a week, by feel”, the tub regularly sinks into periods when it is practically unprotected. You will find a systematic approach in the article on hot tub chemistry step by step, and a comparison of disinfection regimes in the pieces on bromine in a hot tub and active oxygen.
2. Biofilm in the pipework as a reservoir
A hot tub has metres of narrow pipework leading to the jets, where warm water sits still once the pump stops. Over time a biofilm forms on the walls — a slimy layer in which micro-organisms are better protected from disinfectant than they are in free water. It looks as though the water is fine, but every time the jets start up, part of the contents of the pipework is released into it. That is why maintenance includes a regular pipe flush with a biofilm product, typically before every water change.
3. A neglected filter
A clogged cartridge restricts the flow, reduces the efficiency of filtration and itself becomes a place where organic debris is retained. Yet the filter is the cheapest part of the whole system — and the one most often overlooked. The procedure and the intervals are covered in hot tub filter cleaning.
4. A water change that keeps being postponed
The longer the water stays in the tub, the more dissolved substances accumulate in it that bind the disinfectant and reduce its effect. When it takes noticeably more product to reach the same reading than it did at the start, that is a signal, not an invitation to dose more heavily. The article on how often to change hot tub water gives you a sense of the intervals.
5. Testing “by eye”
Clear water with no smell guarantees nothing — Pseudomonas does not make the water cloudy or discolour it. The only information you can rely on is a measured value. How to take the measurement properly and what to avoid with test strips is described in testing pool and hot tub water. If you want the broader picture on hot tub hygiene, you will find it in the article on hot tub hygiene and legionella.
What I do → why it raises the risk → what to do instead
| Common habit | Why it raises the risk | What to do instead |
|---|---|---|
| I dose the disinfectant once a week, “so it lasts” | In warm water it is used up within days; between doses the water is unprotected | Measure before every soak and dose according to the reading |
| I check the water by how it looks and smells | Pseudomonas does not make the water cloudy — clear water is no proof | Test strips or a photometer, with the reading noted in a log |
| After a soak I just towel off and take my swimwear off later | Wet fabric holds water against the skin for hours after bathing | Take swimwear off straight away, shower with soap, wash the swimwear |
| I sit in the tub for an hour or more | Longer contact and softening of the skin weaken its natural barrier | Shorter soaks with breaks; watch how your skin reacts |
| I shave right before a soak | Freshly irritated skin and open hair follicles are more vulnerable | Plan shaving well in advance, not on the day of a soak |
| I only deal with pH when “something is wrong” | Outside the recommended range the efficacy of chlorine disinfection drops | Measure pH together with the disinfectant and keep it in the recommended range |
| I do not flush the pipework, “it looks clean” | The biofilm is inside the pipes; you cannot see it in the water | Flush with a biofilm product before every water change |
| I use the tub even with a graze or when I have a cold | Broken skin and a weakened body carry more risk | Wait until the wound has healed and you are well again |
A preventive routine that works
- Measure the disinfectant before every soak, not once a week. It takes half a minute and it is the one step that nothing else can replace.
- Keep to the recommended range. For hot tubs the CDC recommends free chlorine of at least 3 mg/l, bromine of 4–8 mg/l and pH of 7.0–7.8. The specific target values, however, are always governed by the instructions for the product used, the type of disinfection and the recommendations of the hot tub manufacturer — different figures apply to chlorine-free systems or combined regimes. The instructions take precedence.
- Watch the pH. Chlorine disinfection loses efficacy at a high pH, so “I have plenty of chlorine” does not necessarily mean “I have plenty of disinfection”.
- Shower both before and after a soak. Beforehand you reduce the load on the tub (cosmetics, sweat, creams); afterwards you rinse the water off your skin. The CDC recommends a shower with soap as soon as you get out.
- Take swimwear off straight away and wash it. Drying it on the railing and wearing it a second time because “it is only water” is exactly what needlessly prolongs the contact.
- Do not get into the tub when you are ill or have an open wound. That applies both for your own sake and for everyone else’s.
- Shock-dose and flush regularly. Shock disinfection after a heavier load and a pipe flush before a water change remove what routine dosing cannot handle.
- Keep an eye on the number of bathers and the length of the soak. After several people have visited, the readings should be checked that same evening, not three days later.
- Keep a simple log. Date, chlorine or bromine reading, pH, dose. The trend is more valuable than a single figure — it shows you when the water is nearing the end of its life.
If you have more sensitive skin and chlorine does not agree with you, you will find an overview of the alternatives in the article on sensitive skin and alternatives to chlorine. Beware of one misunderstanding, though: the aim is not to leave out the disinfection, but to choose one you will reliably keep up.
Extra caution with rentals and shared hot tubs
With your own hot tub you know when you last took a measurement. With the tub at a rented cabin, in a hotel or at a wellness centre you do not. That is why the Cleveland Clinic recommends asking the staff how often the water is checked — a daily check is a good sign.
The risky case is mainly the combined scenario: the tub runs intermittently, it is filled and heated in a hurry between guests, the pipework has not been flushed for years and nobody has clearly assigned responsibility for taking measurements. Shared hot tubs also tend to be used more intensively, so the disinfectant disappears faster.
Practical measures for a guest: look to see whether an operating log or a test kit is visible by the tub, opt for a shorter soak, shower with soap after getting out and do not leave your swimwear on. And if the water is cloudy, foaming or you notice a strong musty smell, it is entirely legitimate to give the soak a miss. If you run a hot tub for guests at a holiday property yourself, you will find an operating framework in the article on a hot tub for a holiday rental.
When to see a doctor
Here there are no “it depends” answers. Seek medical help if any of the following occurs:
- the rash is spreading or getting worse over the course of a few days;
- the lesions are producing pus, are painful, warm to the touch, or swelling appears;
- fever, chills, nausea or general weakness develops;
- the rash persists for more than roughly two weeks or keeps coming back;
- the person affected is a child, a pregnant woman, an older person or someone with a weakened immune system, with diabetes or with a skin condition — in these cases do not wait, seek advice straight away;
- you are not sure what you are dealing with. Uncertainty is in itself reason enough.
A doctor will assess what the condition actually is and decide what should happen next. At home, deal with the water, not the diagnosis.
Frequently asked questions
Does clear water mean the hot tub is fine?
No. The appearance of the water says nothing about the disinfectant level or the state of the pipework. Water can be perfectly clear and still have zero free chlorine. The only guide is a measured value.
Can chlorine on its own be behind the rash?
Skin irritation after a soak can have several causes — from a reaction to the chemicals through a high pH to dry skin. They cannot be told apart from a description, and that is not a job for the hot tub owner but for a doctor. From the operating point of view it makes sense to check both the disinfectant readings and the pH at the same time, and not to combine products outside their instructions.
Does it help if I drain the tub after every soak?
It is neither necessary nor practical. What matters more is stable disinfection, a clean filter, a regular pipe flush and a water change at a sensible interval. Draining after every soak addresses the consequence, not the cause.
Do I need to do things differently in the tub with children?
With children a more cautious approach generally applies — a lower water temperature, a shorter stay and a thorough shower afterwards. If a child has any kind of skin reaction, consult a paediatrician; do not wait to see whether it clears up on its own.
How do I know it is time for a water change?
The typical signal is a rising consumption of chemicals to reach the same readings, recurring foaming, or a pH that is difficult to stabilise. The interval according to volume and load is covered in a separate article on changing the water.
Summary
Hot tub folliculitis is no reason to avoid hot tubs — it is feedback on how the tub is run. Stable disinfection measured before every soak, pH within the recommended range, a clean filter, flushed pipework and a shower with the swimwear taken off as soon as you get out cover the overwhelming majority of the risk. And if a rash does appear anyway and it spreads, persists, produces pus or comes with a fever, it belongs with a doctor, not on a discussion forum.
Test strips, disinfection products, pH adjusters and pipe-flush products can be found in the water treatment category. Further practical guides to hot tub care are published on an ongoing basis in the Guide.
