UV-C light genuinely kills bacteria and viruses. It is used in hospitals and water treatment, and it works. But there is one thing about how it works that almost everyone gets wrong, and it completely changes how you should judge anything that shines a UV-C light at a surface.

The mistake is thinking UV-C works instantly, like flicking a switch: point it at something, and whatever is there is dealt with. It does not. UV-C works by building up a dose, and a dose takes time.

Dose is just intensity times time

UV-C damages the genetic material of microbes so they cannot reproduce. But one flash of light does not do it. It takes enough light landing on the same spot to add up to the damage needed. That accumulated amount is the dose, and it comes down to two things multiplied together: how strong the light is, and how long it shines on that exact spot.

Both matter, and the second one is the part people forget. A UV-C lamp can be switched on and glowing brightly, but if it only passes over a spot for a fraction of a second, the dose that spot receives is tiny, nowhere near enough to do the job. The lamp is on. The dose is not there.

The tanning-bed test

Wave your arm under a tanning bed for half a second and you will not get a tan, no matter how powerful the bed is. Your skin has to stay under the light for a sustained stretch to build up the exposure. The lamp being on is not what matters; the time under it is.

A flame works the same way. Hold it near a pot of water for a moment and nothing happens. The heat has to stay in one place long enough to add up. Sweep it past quickly and the water stays cold, even though there was definitely fire.

UV-C is no different. The light has to dwell on a spot, up close and holding still, long enough for the dose to build. Move it past quickly, or hold it too far away, and you simply do not deliver enough, however bright the lamp looks.

Two things quietly destroy the dose

Distance. UV-C gets weaker fast as it moves away from its source. Move the lamp a few centimetres further back and the amount reaching the surface drops sharply. A lamp held well above a surface is already delivering far less than one held right against it.

Movement. If a lamp is swept across a surface, the way you would move a vacuum, any given spot is only under the light for the instant it passes over. That might be a tenth of a second. But to actually sanitise that spot, the light usually needs to sit on it for several seconds. The gap between a flash as it sweeps by and ten seconds held in place is the whole ballgame.

A lamp that is both held at a distance and moving fails on both counts at once. The light is real. The dose is almost nothing.

The numbers, briefly

It is worth seeing how big this gap is, because it is not close.

Killing the common bacteria found on bedding takes a dose of roughly 6 to 14 units (millijoules per square centimetre, the standard measure).

  • A lamp held close and still for ten seconds can deliver around 200 units, comfortably past what is needed, with plenty to spare.
  • A lamp held further away and swept past in a fraction of a second delivers a fraction of a single unit, far less than a hundredth of what is required.

Same light. Wildly different result. The only things that changed were how close it was held and how long it stayed, which is exactly the point.

The question to actually ask

None of this means UV-C does not work. It plainly does, when it is delivered properly. It means the right question to ask about anything using UV-C to sanitise a surface is not does it have a UV-C light? It is does it actually deliver enough dose?

And that comes down to three simple things: is the light held close, is it held still, and does it stay there long enough? A glowing lamp tells you it is switched on. It tells you nothing about whether it is doing the job.

The full science behind the dose figures, with every source, is set out in our technical reference papers.