Legionella in tap water: the risks and what actually helps

Reading time: ~11 minutes

What exactly is legionella?

Two things make legionella different from the substances that most tap water questions are about. The first: it is a living organism, not a chemical contaminant. Lead, nitrate and PFAS are in the water and stay there in the same quantity; a bacterium can multiply, so a small, harmless number can grow into a problem under the right conditions. The second: the route into your body does not run through your gullet but through your lungs.

The RIVM records 400 to 680 reports of legionellosis in the Netherlands every year, plus 100 to 150 patients who picked up the infection abroad. The incubation period is 2 to 10 days. Of the people who develop pneumonia from legionella, 2% to 10% die, mainly older people and people in fragile health. Those are serious figures. Set them next to the number of people in the Netherlands who shower every day, and the chance of it happening to you stays small.

Important to hold on to: those reports by no means all come from homes. A large share can be traced back to hot tubs, wet cooling towers, pressure washers, garden centres and travel. Your own shower is a possible source, not a likely one.

How does legionella get into tap water?

Not through your water company. The Dutch mains network works without chlorine and is therefore monitored intensively: in 2022 the water companies carried out 559,933 measurements, 99.9% of which met the standards, reports the industry association Vewin on the basis of the inspectorate’s findings. The water delivered at your front door is cool, fresh and moving, and those are exactly the three conditions in which legionella does not feel at home.

Evides, one of the Dutch water companies, sums it up neatly in a conversation about tap water behind the front door: the water company delivers up to the door, but what happens beyond it is up to you. In your own indoor plumbing all three conditions change at once.

1. The temperature enters the growth zone

According to the RIVM, the legionella bacterium thrives best in standing water between 20 and 50 degrees, writes Brabant Water (a regional Dutch water company). That is exactly the range your house operates in: a cold water pipe running alongside a hot water pipe, a pipe in a warm meter cupboard, a boiler set to 45 degrees, a hose lying in the sun. How tap water warms up indoors and what that means, you can read in more detail in our piece on the temperature of tap water.

2. The water stands still

A pipe to a guest shower, a second toilet, an outside tap or a holiday home can stay unused for weeks. Standing water is the most important risk factor, and it is also the reason every official piece of advice starts with flushing. How quickly the quality of standing water deteriorates, we describe separately in how long you can keep tap water.

3. Biofilm forms

On the inside of pipes, in shower heads and in hoses a thin layer of micro-organisms forms. That biofilm is the hiding place in which legionella can settle and multiply. A shower head that has not been cleaned for months is therefore a more relevant point of attention than the water in the street.

4. The water is turned into mist

No mist, no infection. Showers, hot tubs, pressure washers, garden sprinklers and humidifiers turn water into millions of tiny droplets that can reach deep into the lungs. The Rijksoverheid (the Dutch central government) names hot tubs explicitly as a risk because the water there often sits between 25 and 45 degrees, right in the growth zone.

How big is the risk in your own home?

For an ordinary home that is used daily: small. The legislation says the same, and that is telling. According to the Informatiepunt Leefomgeving (the Dutch information point on the living environment), the legionella prevention rules from the Drinking Water Decree and the Drinking Water Act apply to priority locations: hospitals, nursing homes, saunas, hotels, campsites, larger bed and breakfasts and truck stops. There many people live side by side, pipes stand still for long stretches and visitors are more often vulnerable. Ordinary homes fall outside that obligation, and the Inspectie Leefomgeving en Transport (the Dutch human environment and transport inspectorate) therefore supervises those institutions, not your bathroom.

That does not mean you have nothing to do as a resident. Brabant Water puts it sharply: in managing the risks around legionella, the responsibility lies with the owner of the indoor plumbing. That is you, or your landlord. In a rented home that is a useful conversation to have, certainly if there are pipes that rarely run. We wrote earlier about what you may and may not change yourself as a tenant in a water filter in a rented home.

Run through these five questions. The more often you answer yes, the more it pays to spend half an hour on flushing.

• Standing water: is there a tap, shower or outside tap you have not used for more than a week?
• Temperature: is your boiler or combi boiler set lower than 60 degrees?
• Mist: do you use a hot tub, pressure washer, garden sprinkler or humidifier?
• Age: are the shower head and the hose more than a year old and never descaled?
• Vulnerability: does anyone live in the house who is older or has reduced resistance?

Five times no and a shower used daily? Then this is mainly useful background knowledge. Two or more times yes, and certainly on the last question, and it is worth actually carrying out the measures below.

Can you get ill from drinking water with legionella?

No. This is the misunderstanding that comes back most often, and the answer is unusually clear for a health subject. The RIVM writes that infection happens by breathing in water droplets or soil particles, and that people do not get ill from drinking contaminated water. The only exception the institute names is choking in cases of severe swallowing problems, where water ends up in the lungs instead of in the stomach.

Brabant Water says it just as directly: you do not get infected by drinking water, but by breathing in water droplets when water evaporates or is turned into mist. The bacterium does not survive the stomach; the lungs are the only target that counts.

That is different, by the way, for bacteria that do come in through the gastrointestinal tract, such as E. coli and enterococci. Those sometimes turn up in a boil water advisory from your water company. How that works and what to do then, we explain in enterococci and E. coli in tap water and in what to do when there is a boil water advisory.

What does help against legionella at home

The measures that work are free, boring and effective. They all turn on the same three knobs: take the standing water away, take the temperature out of the growth zone, and prevent yourself from breathing in mist from a tap that has not been opened for a long time.

Flushing after a week or longer away

Waternet (the Amsterdam water company) gives the most concrete step-by-step plan for anyone who has been away from home for more than a week. Start at the tap closest to the water meter and work your way through the house. Let every cold and hot tap run for at least a minute, put mixer taps in the middle position and flush the toilets once. Brabant Water keeps to two minutes per pipe; feel free to take the longer of the two, water is not the expensive part here.

The shower head in a bucket

This is the step that is skipped most often and that actually makes the most difference. A shower is by definition a mist maker, so the first seconds after a long standstill are the moment at which you could breathe something in. Waternet and Evides therefore both advise the same thing: put the shower head in a bucket of water, with a cloth or a floor cloth over it, and only then turn on the tap. The water flushes through, the mist stays under the cloth. More about what plays a role in shower water is in our article on a water filter for the shower.

Boiler at a minimum of 60 degrees

The RIVM advises setting the boiler or combi boiler to at least 60 degrees. Above 50 degrees growth stops, at 60 degrees and higher the bacterium dies off. If you set the appliance lower to save energy, you create exactly the lukewarm conditions in which legionella does well. Do not confuse this with drinking water: hot water from the tap stays unsuitable for drinking for other reasons, as we explain in drinking hot tap water.

Disconnect the garden hose and outside tap

A garden hose lying full of water in the sun is the ideal breeding ground: standing, lukewarm and with a spray head that blows everything into the air as mist. Disconnect the hose after use so the water runs out, and flush it through without the spray head before you use it again.

Let everything run once a week

The RIVM recommends flushing rarely used water pipes thoroughly at least once a week. For a newly built home the advice is stricter: flush every morning before use for the first three months. For new taps alone, ten seconds is enough, for three months. Put it in your diary next to the moment when you walk through the house anyway; after a few weeks it is a habit.

What a water filter does and does not do here

We have to be clear here, even though we sell water filters. A water filter is not an answer to legionella. The PureFilter Mineral+ is made for Dutch tap water and targets chemical contaminants; the filter makes your water better and better-tasting, but it does not disinfect. On top of that, a filter on your cold water line changes nothing about the two places where the legionella risk actually sits: your hot water system and your shower.

There are indeed filters that stop legionella. Those are sterile membrane filters mounted directly on the tap point in hospitals and nursing homes. They work, but they are made for a different situation than a family with a kitchen tap: they have a short, prescribed service life, are thrown away afterwards, and they belong in a management plan with periodic sampling. That is healthcare engineering, not a household solution, and we do not supply them.

What each approach does, in a row.

That last row deserves attention. A filter jug on the worktop stands at room temperature for hours, and that is exactly the combination of standing water and lukewarmness you want to get away from. That is no reason to panic, but it is the opposite of what a legionella leaflet advises. A fixed filter in the line does not have that drawback, because the water only flows through the filter at the moment you turn on the tap. How that works technically, we explain in how a water filter works, and which filter types there are and what they do and do not do, we set out in full on our page about water filters.

When a fixed filter is worth it

So do not choose a water filter against legionella. Choose one if you want better drinking water for other reasons, because there the gain is measurable. At PureFilter we see that customers almost never come to us with a worry about bacteria, but with a taste problem, a PFAS question or simply the wish to stop hauling bottles.

The PureFilter Mineral+ works with five filter layers: a pre-filter, two carbon fibre layers, Swedish Mineral Rock and a post-filter. The filter removes all 40 chemicals tested, from 75% to 99.97%, tested by Equinox Labs, an independent laboratory (2026). Specifically: 83% to 85% less PFAS, including PFOA and PFOS, 99.97% less chlorine, 99% less nitrate, and lead, mercury, arsenic and cadmium reduced to below the detection limit. In the last step Swedish Mineral Rock adds calcium, magnesium and potassium, which makes the water fresh-tasting.

In practice: the filter costs 149.00 euros including the first filter cartridge and you fit it yourself on the cold water line in 5 to 10 minutes, with a pair of water pump pliers. A replacement filter costs from 65.00 euros and lasts up to 12 months for 1 to 3 people, or up to 6 months for 4 or more. If you would rather not think about it, the cartridge arrives automatically and on time for 48.00 euros per year with the Filter Plan for 1 to 3 people. A step-by-step explanation of the installation is in our guide to installing a water filter, and how often you need to change the cartridge you can read in how often you replace a water filter.

Do you have a boiling water tap? It combines well with a filter on the cold water supply, but it does not filter anything itself. What a Quooker does and does not do, we have worked out on the page about water filters and the Quooker. If you are still hesitating between systems, our overview of the best water filters will take you further, and in the drinking water filters from our webshop you can see the full range.

Frequently asked questions

Conclusion

Legionella in tap water is a real but manageable subject, and it is often misunderstood. You do not get ill from drinking it, and the problem does not arise at your water company but in your own pipes, as soon as water becomes lukewarm and stands still. That makes the solution simple too: flush, set the boiler to at least 60 degrees, keep the shower head clean, and empty the garden hose.

So do not let yourself be talked into a water filter against legionella, not by anyone, and not by us either. A filter on your cold water line does nothing about it, and we would rather say that out loud than have you find out later for yourself. What such a filter does do is rid your drinking water of PFAS, chlorine, nitrate and heavy metals, and refine the taste. That is a good reason to get one. Legionella is not.

Want to know more about what is in your tap water and what a filter changes about it? On purefilter.nl you will find the whole range and all the answers in one place.