Do shower filters work? What the evidence really says

Do shower filters work? Yes, for one job: shower filters are designed to reduce free chlorine, the disinfectant that water companies keep in tap water. The chemistry behind the usual filter media is documented. Shower filters do not soften water or remove limescale, and a lab result is not a promise about your own shower.

Below: what the chemistry clearly shows, how a claim is tested, what the evidence doesn’t tell you, and a two-minute way to check any claim.

Most answers to this question, this one included, come from companies that sell shower filters. We sell a filter shower head, so every figure here links to its source.

Do shower filters work? Yes, for one job

Shower filters are designed for one job, reducing free chlorine in the water that reaches you, and the chemistry behind that job is documented. None of the evidence covers softening water or removing limescale.

Two questions hide inside this one:

• Does the chemistry work? For the media used in shower filters, the sources below say the reactions with free chlorine are real.
• Does a given filter work in your shower? Only evidence for that model, under stated conditions, can show it, and the answer changes as its cartridge is used up.

A third question, whether one is worth it for you, depends on your shower and the running cost rather than on the chemistry. Our guide to filter shower heads in the UK covers fit, fitting and running costs.

What is a shower filter actually for?

A shower filter is for the chlorine that water companies deliberately keep in tap water to protect it on its way through the pipes. The Drinking Water Inspectorate (DWI) says companies in England and Wales typically keep that residual at 0.5 mg/l or less.

Disinfection is a regulatory requirement, the DWI notes, and at these very low levels chlorine is perfectly safe to drink. Bristol Water, for example, says the chlorine reaching homes in its area is typically 0.1 to 0.8 mg/l. A typical swimming pool, it adds, has 1.5 to 5.0 mg/l.

Bristol Water also explains why chlorine can be more noticeable in hot water. Hot water holds less chlorine, so the smell can be stronger when water is heated directly by a combi boiler or an electric shower. None of these sources measures what a filter does to that smell, and we make no claim about it.

Levels also shift with distance from the treatment works, the season and the time of day, Bristol Water adds. For how much chlorine reaches UK taps, and how chloramine differs, see our guide to chlorine in UK tap water.

So a shower filter answers a preference, not a safety problem. Tap water in England and Wales is regularly tested and safe to drink, the DWI says in its advice on domestic water filters and softeners.

What the chemistry clearly shows

Shower filter chemistry is the strongest part of the evidence: vitamin C, KDF, calcium sulfite and activated carbon each react with free chlorine or hold on to it. The sources vary in weight, from a US government note to a maker’s own page and a patent.

Vitamin C neutralises chlorine, and fast

In an April 2005 technical note, the USDA Forest Service reported that two forms of vitamin C neutralise chlorine: ascorbic acid and sodium ascorbate. The reaction, it said, is very fast. The note was written for operators dechlorinating water in water systems, not for showers, but it describes the reaction that vitamin C filter media are designed to use.

KDF converts chlorine by redox

KDF is a granular mix of copper and zinc used in filter media. Kymera International, which makes it, says KDF works by redox, a transfer of electrons, and can convert chlorine into chloride. The maker adds that KDF tolerates high temperatures and is used in shower head filters. That is a maker describing its own product, not independent proof.

Activated carbon holds chlorine, but less well in hot water

Activated carbon is porous carbon that holds substances on its surface, a process called adsorption. The DWI suggests a simple jug filter with an activated carbon cartridge if you dislike the taste of chlorine in drinking water.

The shower is harder on carbon. A 2000 US patent for shower filter media says carbon is quite effective at room temperature (25 °C) and low flow. At the higher temperatures and flows of a shower, it says, carbon is less effective.

Calcium sulfite: chosen for hot, fast water

Calcium sulfite, also spelt sulphite, reacts with chlorine, and the same patent sets out the reaction. Its inventor paired ceramic-type calcium sulfite beads with copper-zinc media for hot water at high flow rates.

A patent sets out an inventor’s reasoning. It is not independent evidence that any product works, and we treat it that way.

Taken together, these sources show the reactions are real. They do not show how much free chlorine a given cartridge reduces in your shower, or for how long. Our plain-English guide to each filter medium goes into more detail.

How is a shower filter claim tested?

A shower filter claim can be tested by running water with a known chlorine level through the filter under fixed conditions and measuring what comes out. NSF/ANSI 177, a US standard for shower filters, is one published method, described by Plumbing & Mechanical in 2005.

According to that description, the free chlorine test uses public-supply water with 2.0 mg/l of free chlorine at 40 °C. Water runs at the maker’s flow rate, which must be at least 1 US gallon (about 3.8 litres) a minute at an initial 80 psi (about 5.5 bar). The test cycles 15 minutes on and 15 minutes off, and it stops if the flow falls below that minimum.

A filter passes if it cuts the challenge by 50%. Capacity is set by the last of at least five consecutive passing samples. The article notes it can come out higher or lower than the maker’s estimate.

The standard’s full title is Shower Filtration Systems – Aesthetic Effects. Makers who claim to meet it must use a set statement, which says the system has not been evaluated for free chlorine reduction where chloramine is present. Later editions of the standard may differ from this 2005 description.

That is the level of detail to look for whenever a seller quotes a result: the method, the challenge, the temperature, the flow and the capacity.

What the evidence doesn’t tell you

The evidence on shower filters doesn’t tell you how a given filter performs in your own shower, over the whole life of its cartridge. Lab methods fix the conditions; your bathroom doesn’t.

Lab conditions are not your shower

The 2.0 mg/l challenge in NSF/ANSI 177 is at least four times the 0.5 mg/l or less that the DWI says is typical in England and Wales. Your temperature and flow differ from the lab’s too, and Bristol Water notes that chlorine levels shift with the season and the time of day. A lab figure is a reference point, not a forecast.

Performance changes as the cartridge is used

Filter media are used up as they react. The USDA note says about 2.5 parts of ascorbic acid are needed to neutralise 1 part of chlorine, so the vitamin C is consumed as it works. That is why test standards measure capacity and makers set replacement schedules.

The DWI’s general advice for home filters is to change cartridges as the manufacturer says, or they stop working. A schedule offered as a suggestion is not the same as a tested capacity, so check which one you are given.

A percentage without its conditions says little

A reduction figure only means something next to its starting level, temperature, flow, volume of water and sampling point. Without those, two percentages from two sellers can’t be compared.

Reviews are experiences, not evidence

Reviews and forum posts tell you how people feel about a product, which can be useful, but they are not proof of a claim. Advice from the ASA and CAP on substantiation says testimonials alone are unlikely to be enough to support objective claims.

What shower filters don’t do

Shower filters don’t soften water or remove limescale. Hardness is dissolved calcium and magnesium, the DWI’s guide to hard water explains, which is a different problem from chlorine and needs a different fix.

The DWI’s bands run from soft, up to 100 mg/l, to very hard, over 300 mg/l, measured as calcium carbonate. Citing the World Health Organization, the DWI adds that water of 200 mg/l or more will produce scale. A domestic water softener tackles hardness by ion exchange, replacing calcium and magnesium with sodium.

In a hard water area, clean any shower head, filtered or not, as its leaflet says. If limescale is your main concern, our comparison of shower filters and water softeners shows which one your home needs.

Waterhush makes one claim only: it reduces free chlorine.

How to check any shower filter claim in two minutes

Checking a shower filter claim takes about two minutes if you ask seven questions in order. A clear claim names one substance, one test method and the conditions it was measured under.

1. What exactly is claimed? Note the verb, such as ‘reduces’ or ‘removes’, and any number.
2. Which substance? Free chlorine, chloramine and hardness are three different things.
3. Which method, and which lab? A named standard, such as NSF/ANSI 177, tells you how the test was run.
4. Under what conditions? Look for the starting chlorine level, the water temperature and the flow rate.
5. When, and on which model? Check the test date and that it covers the product you are buying.
6. For how long? Look for a tested capacity or a replacement schedule, and whether that schedule is only a suggestion.
7. Can the seller show the evidence? Under CAP Code rule 3.7, marketers must hold documentary evidence for objective claims before publishing them.

The DWI’s advice for anyone buying water treatment products is short: be wary of claims made when buying online.

Where Waterhush stands

The Waterhush Shower Kit makes one performance claim: it reduces free chlorine. We don’t quote a percentage or a certification for it, and we claim nothing beyond free chlorine.

Its cartridge, fitted in the handle, includes four filter media: calcium sulfite, vitamin C, KDF (copper-zinc granules) and carbon. We suggest a new cartridge every 3 months, or every 2 months for households of two or more; that is a suggestion, not a promise of how long a cartridge lasts.

Each Waterhush refill cartridge is the same cartridge, in its own airtight sachet. How the head, cartridge and handle work together is shown on a single page.

The bottom line

Shower filters are designed for one job, reducing free chlorine, and the chemistry behind it is documented. No source can tell you how a given filter performs in your shower, at your temperature and flow, as its cartridge is used up. Shower filters don’t soften water. Judge any product by its claim, its test conditions and its replacement schedule, not its adjectives, using the seven questions above.