Since 2020 I’ve had the same conversation in a dozen office kitchens. A facilities manager points at a glowing tower in the corner, or a lamp bolted above head height, or a wand still in its box, and asks whether the thing actually works. The honest answer is: sometimes, under conditions the person who sold it rarely spelled out. UV-C is real science with a genuine germicidal effect, and it’s also one of the most oversold products to reach the cleaning industry in years. The whole task is telling those two apart.
What is UV-C actually doing when it “disinfects”?
Ultraviolet light comes in bands, and the germicidal one is UV-C – the short, energetic wavelengths between about 200 and 280 nanometres. Shine enough of it on a bacterium or a virus and it wrecks the organism’s genetic material, so it can’t replicate. No replication, no infection. That’s the whole mechanism, and it’s been used to disinfect water and hospital air for the better part of a century. The workhorse lamp emits at 254 nanometres, and it does the job well.
There’s a catch, and it’s a big one. The same wavelengths that shred microbial DNA are hard on human skin and eyes. A 254-nanometre lamp will give you sunburn and a painful dose of what the eye doctors call photokeratitis – arc-eye, essentially – if you sit under it. Conventional UV-C can’t simply be switched on in a room full of people.
Why the light that kills germs also burns you
This single fact shapes every honest use of UV-C. Because 254-nanometre light is hazardous to people, it has to be kept away from them – run inside sealed air-handling ducts, or mounted high on the wall to treat only the upper air while the room’s convection currents carry microbes up into the beam. Or fired around an empty room after everyone’s gone home. Every legitimate deployment is really an exercise in keeping the light and the humans apart. The moment a product promises to break that rule cheaply, your guard should go up.
Where does UV-C genuinely work?
Against all that, there are still places UV-C earns its keep, and I’ll happily say so. Upper-room systems have decades of evidence behind them in tuberculosis control, where high-mounted lamps disinfect the air people are breathing without touching the people themselves. In-duct UV-C, built into the air handling, is a sensible layer in a building already moving a lot of air. And in hospitals, wheeled robots that flood an empty room with 254-nanometre light after the cleaners have finished do measurably cut surface contamination – one study found high-touch surfaces testing positive dropped from around two-thirds to under a fifth once the robot had run.
Read that last part again: after the cleaners have finished. The clearest finding across the whole surface-disinfection literature is that UV-C works best bolted onto a proper manual clean, never in place of one. It’s a second pass, and it can’t be the first.
The winners tend to be unglamorous
None of these winning applications looks like the thing being wheeled across an office floor. They’re ducts and ceiling-mounted fittings run to a schedule – infrastructure, not gadgets, scoped by someone who measured the space. The version sold to a general office – a portable tower you roll out at five o’clock, a handheld wand still in its packaging – is where the science and the marketing part company.
What’s really behind the “99.9%” on the box?
A UV-C dose is irradiance multiplied by time – how bright the light is where it lands, times how long it lands there. Both halves matter, and both fall apart in a real room.
Brightness drops with the square of the distance. Double the gap between lamp and surface and you’ve quartered the dose. Robot trials show it plainly: strong kill rates within a couple of metres of the lamp, then a steep fall-off – one set of measurements went from over ninety per cent effective at under three metres to the low forties at five. The desk in the far corner is not getting what the near one gets.
Then there’s shadowing. UV-C travels in straight lines and barely reflects off ordinary surfaces, so anything the light can’t see directly, it doesn’t treat. The underside of a desk, the far side of a monitor, the keyboard tray, the gap behind the bin – all shadowed, all missed. A tower in the middle of the floor cleans the tops of things and leaves the rest.
And “99.9 per cent”? That’s a three-log reduction. The bar for genuine disinfection in a hospital is usually five-log – a hundred-thousand-fold cut, not a thousand-fold – and it’s measured on clean, exposed test surfaces at a set distance, held for a set time. The number on the box is a best-case laboratory result. Your office is not a laboratory.
Dose is the number that’s missing from the box
I was called to a tenant’s office up at Here East in Stratford, where the facilities manager showed me two UV-C towers bought in a panic in 2021, still boxed in a cupboard. She wanted to know whether to start running them. I asked the only question that matters: what dose, at what distance, for how long? Nobody had ever told her, the brochure didn’t say, and without those numbers the towers were furniture. We worked out that to dose the far side of that floor properly she’d have needed to leave each tower running in each zone for the best part of an hour, shifting it by hand between cycles, every single night. They went back in the cupboard.
Is far-UV-C (222nm) the game-changer the reps claim?
The clever answer to the burns-you problem is a different wavelength. Far-UV-C, at 222 nanometres, comes from krypton-chloride lamps, and its selling point is real: the light is so short it can’t push past the dead outer layer of your skin or the tear film on your eye, yet it still reaches bacteria and viruses, which are far smaller. In 2022 the main industrial-hygiene body in the US, the ACGIH, weighed the evidence and raised the permitted daily eye-exposure limit for 222-nanometre light around sevenfold, from roughly 23 to 160 millijoules per square centimetre. Lab and room studies show it knocking airborne bacteria down by well over ninety per cent at levels people can sit under. On the face of it, this is the one that lets you disinfect a room full of working people. The reps love it, and I understand why.
The brochure stops there. What it leaves out is that a 222-nanometre lamp doesn’t only make germicidal light – it makes ozone. Measurements in working offices have clocked meaningful ozone production, pushing indoor levels up by several to twenty-odd micrograms per cubic metre depending on how many lamps run and how many people are in the room. Ozone irritates the lungs on its own, and it doesn’t stop there: it reacts with the volatile organic compounds already drifting round an office to form ultrafine particles and secondary aerosols – the very things sound indoor-air practice tries to cut down.
There’s a wrinkle here that ought to bother anyone in my trade especially. A lot of those airborne compounds come from cleaning products and air fresheners, and the citrus-scented ones are loaded with limonene, which is exactly what ozone loves to react with. So the gadget sold to purify your air can, in a sealed and busy room, quietly turn your lemon-scented surface spray into fine particulate. The ozone dose is usually small, and in a well-ventilated room it may not matter at all. But “may not matter, if ventilated” is a long way from the promise on the stand at the facilities show.
My own view here is blunt. I won’t recommend a 222-nanometre system for an occupied office I clean until the ozone and air-chemistry questions are properly settled, and any rep who tells you they already are is selling, not informing. The science is real and the promise is genuine. Promise is a long way from proven-in-your-boardroom.
The ozone the sales sheet forgets
Far-UV-C’s two headline properties pull against each other. To kill enough airborne virus you want plenty of light in the room, and the more light, the more ozone and downstream chemistry. Ventilation fixes the ozone – but ventilation on its own also clears the airborne virus, which begs an awkward question about what the lamp is adding on top. The researchers still thrashing this out in the journals haven’t reached a settled answer. A salesperson quoting you one reassuring figure has simply chosen the study that suits.
So should your London office buy any of it?
For the great majority of London offices, the honest answer is no – or at least, not yet, and not the portable version. The things that reliably lower the germ load in an office are dull and already to hand: fresh air moving through the space, and a cleaner who actually lifts the keyboard and wipes under it. Spend the UV budget on better ventilation and a proper cleaning spec before a penny goes on a glowing tower. Run a hospital or a genuinely high-risk, high-occupancy space and the sums change – validated, engineered systems have a real place there. A normal office is not that place.
Four questions that separate the engineers from the opportunists
If someone is selling you UV-C, four questions do the sorting. What dose does your system actually deliver to a surface two or three metres away, and how long does it need to run to get there? Where’s the independent validation – not a lab certificate for a petri dish, but evidence of a five-log reduction in a room like mine? If it’s far-UV-C, what’s the measured ozone output, and are the lamps properly filtered? And what happens to every surface the light can’t see? A good supplier answers all four without flinching. The rest reach for the brochure, and that tells you what you need to know.
