How does UVC light kill bacteria and viruses?

The effect of UVC light has been known for over 100 years and was already described in Denmark in 1903, as an option against tuberculosis (Lupis vulgaris) by the Danish Nobel Prize winner Niels Ryberg Finsen. UVC is today used for disinfection in a wide range of industries. UVC will i.a. used in waterworks in Europe for disinfection of drinking water and by medicine manufacturers to ensure that medicine is not contaminated during production.

But what is UVC light and how does it kill bacteria and viruses? That question we are trying to answer here.

What is UV-light?

All radioactivity is a form of energy that is most often invisible to the human eye. UV radiation is just one form of radioactivity in the electromagnetic spectrum. Other types of radioactivity can be, for example, radio waves that send a signal to the mobile phone or microwaves that are used in microwave ovens.

UV is shortwave light and is defined as the light between 100 and 400 nm. Nm is the unit you describe the different wavelengths of light in and refer to the wavelength in nanometers.
The most common form of UV radiation is sunlight, which produces three main types of UV rays:

  • UVA – light from 320-400 nm 
  • UVB – light from 280-320 nm 
  • UVC – light from 200-280 nm 

The shorter the wavelength, the faster the light is stopped by the molecules in the air, such as oxygen, nitrogen, etc. When the sun emits these different wavelengths, it therefore means that only UVA and UVB reach through the ozone layer and the atmosphere down to the earth’s surface. 90% of UVA reaches the surface, while only 5% UVB and no UVC reach the earth’s surface.

What is UVC-light?

UVC light has the highest energy in the UV spectrum. From about 300 nm, UV light is harmful to cells. This is why you can get eye damage and sunburn from UVB light.

However, it is only when we get down to UVC below 280 nm that we have the powerful effect on the various pathogens (bacteria, viruses, yeasts and molds). Therefore, UVC light from 220 nm to 280 nm is referred to as “Germical light”.

The sun’s UVC light does not reach the earth’s surface as it is blocked by the ozone layer. The only way humans can be exposed to UVC radiation is from an artificial source such as a light bulb or laser. Today, a distinction is made between two main types of UVC:

  1. The traditional UVC technology that works with light around 260 nm
  2. Far-UVC at 222 nm which is a relatively new technology and is being investigated as an alternative / supplementary option for the use of UVC

How does UVC light kill bacteria and viruses?

UVC kills microorganisms by penetrating the cells and destroying the hydrogen bonds in DNA and RNA, respectively. The microorganism is then inactivated.

It is proven that this happens most optimally at a wavelength around 260 nm. However, this may vary depending on the different pathogens, so it may be advantageous to have a natural light scattering between 240 and 280 nm.

This means that depending on the structure of RNA and DNA by the individual pathogen, it will require different amounts of energy to destroy these bonds. It is therefore important to know exactly the dose needed to destroy the pathogen you want to inactivate. This can vary greatly.

There are bacteria that can be easily inactivated with a low dose of eg 20 J / m2, but there are also bacteria that require more than 500J / m2. The same goes for viruses, where the differences can be very large. If we look at yeast and mold, a dose of up to 10,000 J / m2 may be needed.

Deactivation rate

It is thus the UVC dose that a UV product releases that is decisive for how effective the disinfection is. When working with inactivation of bacteria and viruses, you always work with how large a part of a given colony you can inactivate.

When looking at standard figures for the use of UVC, these are typically calculated at a power of 90%. This means that the stated dose is necessary to achieve a 90% reduction in a population.

For comparison, here we have data from the International Ultraviolet Association on some common viruses and bacteria:

Name

Hepatitis A

Murine norovirus

Polio (type 1)

Salmonella (typhimuriumspp.)

Ebola (Reston, Sudan, Zaire)

Influenza A

Rhinovirus (B, C)

Required UV dose for 90% reduction (J / m2) 

60 

76 

70 

40 

54 

23 

125