Legionella is more often associated with cooling towers and hot water systems, but air handling units can present a risk too — and because that risk is largely invisible, it’s easy to overlook until an inspection or, worse, an incident forces the issue.
The bacteria thrive in stagnant water at the right temperature, and an AHU has more than one place that fits the bill.
Where the risk sits in an AHU:
- Humidifiers — particularly spray or evaporative types, where water is held and aerosolised into the airstream.
- Cooling coil condensate trays — standing water in a poorly draining or dirty tray is an ideal breeding ground.
- Dirty coils and biofilm — microbial build-up on coils and inside the unit supports bacterial growth and gets carried into the air.
Because an AHU’s whole purpose is to move air into occupied spaces, any aerosol generated inside it has a direct route to the people in the building. That’s what makes hygiene a genuine duty, not a nice-to-have.
What’s expected of you
Under health and safety law, whoever is responsible for the building — the “duty holder” — has a legal obligation to assess and manage the risk of Legionella. The HSE’s Approved Code of Practice (L8) and its accompanying technical guidance (HSG274) set out what that means in practice: a risk assessment, a written scheme to control the risk, competent people to carry it out, and proper records.
For an AHU specifically, that translates into regular inspection and cleaning of humidifiers and condensate trays, keeping coils clean, ensuring trays drain properly, and documenting all of it.
Staying on top of it
The practical answer is a planned hygiene and maintenance regime rather than reactive fixes — scheduled inspection and cleaning, with everything recorded so you can demonstrate compliance if you’re ever asked.
It’s the kind of regime we build into our maintenance contracts as standard. On large, compliance-critical sites like our air handling work at Heathrow, hygiene inspection and record-keeping aren’t an add-on — they’re simply part of the schedule, because on a site like that there’s no room for them not to be. The same discipline applies whether it’s an airport or a single office unit.
If you’re not confident your AHUs are being managed for Legionella risk, get in touch and we’ll review where you stand.
