---
title: "Portable and evaporative air coolers: the heatwave Legionella risk that isn't a cooling tower"
source_url: https://legionella.io/articles/evaporative-air-cooler-legionella/
canonical_url: https://legionella.io/articles/evaporative-air-cooler-legionella/
pillar: "Building Types & Use Cases"
summary: "A portable evaporative cooler's tank can grow Legionella when left warm and stagnant. Here's why it isn't a cooling tower, and how to empty and dry it safely."
primary_keyword: "evaporative air cooler Legionella"
date_published: 2026-06-02
date_reviewed: 2026-06-26
author: "Legionella.io editorial team (REMOTE TECH LTD)"
reviewed_against: "HSE L8 and HSG274 guidance"
region: "United Kingdom"
license: "(c) REMOTE TECH LTD. Quote freely with attribution and a link to source_url."
---

# Portable and evaporative air coolers: the heatwave Legionella risk that isn't a cooling tower

Yes — the water tank that makes a portable evaporative cooler work can grow Legionella if it sits warm and stagnant, but it is a small aerosol risk, not the notifiable cooling tower people picture. The fix is unglamorous: empty it, dry it, and don't let lukewarm water stand in it between hot days.

That one-line answer hides a few useful distinctions, because "air cooler" covers very different machines, and the word "evaporative" sets off alarm bells it doesn't always deserve.

## How an evaporative cooler actually cools

A portable evaporative cooler — sold as a swamp cooler, a desert cooler, or just an "air cooler" — is a fan with a water tank. A pump lifts water from the reservoir over a wetted pad or honeycomb filter; the fan pulls warm room air through the damp pad; some of the water evaporates, and evaporation steals heat, so the air leaving the front is cooler and more humid. No refrigerant, no compressor. The cooling effect is the evaporation itself.

That design matters for risk, because most of what leaves the unit is true water vapour, and water vapour cannot carry bacteria. Legionella infects people when they breathe in tiny liquid droplets — an aerosol — small enough to reach the lungs [3]. Pure vapour is not that. So an evaporative cooler is inherently less of an aerosol generator than a misting fan or a humidifier that atomises water on purpose.

"Less" is not "none", though. Cheap units splash, and the pump agitates the reservoir; a wetted pad can shed fine droplets into the air stream; and you create your own little aerosol every time you slosh the tank out or top it up. The hazard lives in the standing water, and the standing water is the part you can fix.

## Why it isn't a cooling tower — and why that still matters

A cooling tower or evaporative condenser is a fixed part of a building's air-conditioning or process plant. It moves a large volume of warmed water through a tower, sheds a visible plume of drift, and is high enough risk that operators must notify the local authority of its existence and manage it under specific guidance [2]. Those are the systems behind the outbreaks people remember. A plug-in cooler on a warehouse floor is not one of them, is not notifiable, and shouldn't be assessed as if it were. For the genuine article, see [Cooling towers and evaporative condensers: high-risk systems](https://legionella.io/articles/cooling-towers-and-evaporative-condensers-high-risk-systems/).

It also helps to separate it from two neighbours people lump it in with. A split-unit air conditioner — the wall-mounted box with no water reservoir — is a different question again, covered in [Does air conditioning cause Legionnaires' disease? Split units vs cooling towers](https://legionella.io/articles/does-air-conditioning-cause-legionnaires-disease-split-units-vs-cooling-towers/). And a misting humidifier deliberately throws breathable droplets, which is why it sits higher up the list in [Misting systems and humidifiers: Legionella in unexpected places](https://legionella.io/articles/misting-systems-and-humidifiers-legionella-in-unexpected-places/). The evaporative cooler sits between them: a real but modest risk, driven almost entirely by what happens to the water in its tank.

What puts any of these on the risk list is the same recipe: water held roughly between 20 and 45°C, something that disperses it as a breathable spray, and the nutrients and time for the bacteria to multiply [1]. A portable cooler has all three ingredients in miniature.

## The bit the box doesn't tell you: the danger is the lukewarm tank it leaves behind

Here is the part that surprises people. The riskiest moment for one of these units is not when it is roaring away in a 32°C heatwave. While it runs hard, the water is moving and turning over fast. The hazard is the half-full reservoir it leaves behind when the weather breaks — sitting at 25 to 28°C room temperature, undisturbed, for the four cooler days until the next hot spell. That is the warm, stagnant, nutrient-rich window the growth range describes, and the "use it on the hot days, leave it the rest" pattern walks straight into it. The same trap shows up across [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/).

Two more things the instructions skip. First, the cooling pad or honeycomb filter is the part that harbours biofilm, and a clogged, slimy cellulose pad is almost impossible to disinfect properly — replacing it is cheap and beats scrubbing a degraded one. Second, an "antibacterial" tank tablet or a capful of disinfectant is not a control on its own; it does nothing for the pad, and it is no substitute for the water simply not being there. Dry surfaces do not grow Legionella. An empty tank is the most reliable disinfectant you own.

## The empty, dry, clean routine that actually controls it

This is the whole of air cooler cleaning, and it is simple:

- **After each use, or when you finish for the day:** tip the reservoir out. Do not leave water standing in it overnight.
- **Whenever it's idle for more than a day or two — including between heatwaves:** empty it fully and leave the tank and the pad to dry out completely before storing or parking it.
- **In continuous summer use:** once a week, drain it, wipe and descale the tank, and rinse the pad per the maker's instructions.
- **Each season:** replace the cooling pad or filter rather than trying to resurrect a clogged one.
- **Filling:** use fresh mains water. Don't top it up from a water butt, a bucket, or a jug that's been standing.
- **Siting:** don't point the outlet straight into someone's face at close range, particularly anyone older or with a weakened immune system, and keep the space ventilated.

If this is a workplace, record it like any other recurring task — even a line in a shared log reading "emptied and dried" shows the unit is managed, not ignored.

## Who should take it more seriously

For a fit adult in a workshop, a well-emptied evaporative cooler is a minor concern. The people who change the calculation are the susceptible: older residents, anyone immunocompromised, and clinical settings. The bacteria behave the same everywhere; it is the lungs at the receiving end that differ [3]. In a care home, a healthcare space, or a room where a vulnerable person sleeps, the honest answer is often to prefer a water-free cooling method, or else to be religious about the empty-and-dry routine and keep the unit well away from the person's breathing space. In a workplace, that judgement belongs in your written risk assessment, because the duty to identify and manage a reasonably foreseeable risk doesn't stop at the things that happen to be notifiable [4].

## A genuine caveat

None of this turns a £60 plug-in cooler into a notifiable plant, and nothing here is legal or medical advice. Where these units belong in a building's Legionella risk assessment is a judgement for a competent person who has seen your site and knows who breathes the air near them; at home it is basic hygiene rather than compliance. Treat the routine above as a sensible default, not a replacement for an assessment that considers how your specific unit is used and by whom.

## FAQ

### Do portable air coolers legally need a Legionella risk assessment?

At home, no — there's no duty, just good hygiene. In a workplace they don't need their own separate assessment, but the duty holder should account for them within the building's existing Legionella risk assessment, especially where vulnerable people are present.

### Is a "swamp cooler" the same risk as a desert cooler or a bladeless air cooler?

If it cools by evaporating water from a tank, the risk profile is the same whatever it's branded — swamp cooler, desert cooler, or a sleek tower unit. A bladeless fan with no water reservoir is a different, lower question entirely.

### Can I just put disinfectant tablets in the reservoir and leave it?

No. Additives don't clean the pad and don't compensate for water sitting warm for days. Emptying and drying between uses is the control; a tablet is, at best, a minor extra.

### Does using ice packs or chilled water make it safe?

Cold water in the tank helps performance and briefly lowers the temperature, but the reservoir warms back to room temperature within hours, so it doesn't remove the need to empty the unit when you're done.

## Do this before the next hot spell

Walk round and look at every cooler in the building today. Tip out any standing water now, note the make and the pad type so you can buy replacements, and tell whoever fills them that the rule is "empty and dry between uses". If any unit sits near a bed, a desk used by an older or immunocompromised person, or anywhere in a care setting, add it to your risk assessment this week rather than waiting for the annual review.

## Sources

[1] HSE, "Systems most likely to create legionella risk". https://www.hse.gov.uk/legionnaires/risk-systems.htm
[2] HSE, "Other duties: RIDDOR and notification of cooling towers or evaporative condensers". https://www.hse.gov.uk/legionnaires/what-you-must-do/duties.htm
[3] CDC, "How Legionella Spreads". https://www.cdc.gov/legionella/causes/index.html
[4] HSE, "Legionnaires' disease - what you must do". https://www.hse.gov.uk/legionnaires/what-you-must-do/index.htm