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UV-C Explained: How Short-Wave Ultraviolet Inactivates Microorganisms

UV-C has been used for disinfection for more than a century. It adds no chemicals and leaves no residue, but it only works where the light actually reaches.

What UV-C Is

Ultraviolet light is divided by wavelength into UV-A, UV-B and UV-C. UV-C is the shortest of the three, from about 200 to 280 nanometres. Sunlight contains UV-C, but the atmosphere absorbs it before it reaches the ground, so microorganisms have little natural defence against it.

Germicidal lamps produce UV-C artificially. The most effective wavelengths lie around 260 nanometres, close to the 254 nanometre output of the widely used low-pressure mercury lamp.

How It Inactivates Microorganisms

UV-C is absorbed by DNA and RNA. The energy causes neighbouring building blocks in the genetic strand to bond to each other, which distorts it. A bacterium, virus or fungal cell with enough of this damage can no longer copy itself, and a microorganism that cannot reproduce cannot cause an infection or sustain a colony.

What Decides The Result

The effect depends on the dose, which is the intensity of the light multiplied by the exposure time. Lamp output, distance from the lamp and the type of microorganism all matter.

UV-C travels in straight lines and does not pass through solid material. Shadowed areas and surfaces covered by dirt receive less light. For this reason UV-C works best on a surface that has been cleaned first, and is often combined with a second method, such as ozone, that reaches where light cannot.

Why It Is Used Inside A Closed Chamber

The same property that makes UV-C effective makes it harmful to skin and eyes. Equipment that uses it keeps the lamps inside an enclosure so that people are not exposed while they run.