How Does UV Light Kill Germs?

UV light is mentioned as an effective disinfectant for disinfecting. But have you ever thought, how does it kill germs? Let’s find out.

uv ray


What are Germs?

The germ is a term for various types of microorganisms such as bacteria, fungi, and viruses, especially those that can cause disease.

Where Are Germs?

Bacteria are generally found everywhere. Within the scope of our homes, for example, bacteria are found in sinks, TV remotes, door handles, laptop keyboards, cellphones, and even toothbrushes. In our body, there are also many bacteria such as in the skin and mouth, especially in the digestive tract. From the description above, that means without us knowing, bacteria are literally everywhere.

Although the fungus is not as abundant as bacteria, this type of microorganism is also commonly found in our daily lives, such as trash cans and gutters.

As for viruses, they can only live and multiply in other living things (for example humans, animals and plants). Viruses can survive outside of other living things, but not for a long time.

Are All Microorganisms Dangerous?

Of the types of microorganisms above, not all of them are dangerous and cause disease. From types of bacteria such as E. coli bacteria that help us digest food in our intestines, from types of fungi such as the fungus Saccharomyces cerevisiae which is used in making bread.

UV Light (Ultraviolet)

What Exactly Is UV Light?

Just like Gamma rays, Ultraviolet Light is also a type of electromagnetic radiation. Electromagnetic radiation is an energy-carrying wave, the most common example of electromagnetic radiation is visible light. Please pay attention to the following electromagnetic radiation spectrum.

electromagnetic spectrum
The more upward, the greater the energy level

From this spectrum, we can see that UV (Ultraviolet) rays have wavelengths that are slightly shorter than visible light. This means that UV light has higher energy than visible light.

You can also read more about Gamma Rays at the following source.

Gamma Ray Burst: Natural ‘Sniper’ From Space

What Are the Types of UV Rays?

uv light spectrum
The Ultraviolet rays (A, B, and C) frequency position in the electromagnetic radiation spectrum


UVC has a wavelength of 100 nm to 275 nm, which means it has the highest energy compared to the other two types of UV light. Because the energy level is quite high, UVC has the highest ionizing radiation properties among the three, in other words, it is riskier to break or trigger the formation of chemical bonds of the molecule in our cells. This ionizing property is very dangerous because it can damage DNA which generally causes cancer. This makes UVC harmful to all living things and is often used as a disinfectant for germs.

However, the shorter the wavelength, the ability to penetrate something will decrease. UVC generally can only penetrate to the outer layer of the skin, the layer of dead skin cells. Therefore, shining UVC light (for example with a UVC flashlight) into the wound area is very dangerous. Besides, UVC rays originating from sunlight are all absorbed by the atmosphere.


UVB with wavelengths of 275 nm to 325 nm has higher energy than UVA but is lower than UVC. UVB can penetrate the first layer of our skin (epidermis) and is generally the cause of sunburn and skin cancer. Because it can penetrate something better than UVC, UVB is more at risk of causing skin cancer than UVC. However, most of the UVB rays from the sun are absorbed by the ozone layer, only 5-10% can reach the earth’s surface.


UVA has a wavelength of 325 nm to 375 nm, so it has the lowest energy compared to UVB and UVC. UVA can penetrate the skin to the second layer (dermis) and generally causes premature aging and is one of the causes of certain skin cancers. Only a small portion of UVA from the sun is absorbed by the atmosphere, about 95% of which reaches the earth’s surface.

How Do UV Rays Kill Organisms?

Ultraviolet (UV) rays kill organisms by damaging their DNA or RNA. One of the result is causing the formation of pyrimidine dimers.

PairAdenineThymine (in DNA) / Uracil (in RNA)

Table 1. DNA base purine-pyrimidine pairs

DNA is a string of proteins consisting of 4 types of bases, namely Adenine (A) and Thymine (T) / Uracil (U) which are paired together, and Guanin (G) and Cytosine (C) which are paired together. The two pairs are attached to a hydrogen bond.

pyrimidine dimers formation
Efek UV menyebabkan terbantuknya dimer pirimidin

Pyrimidine dimers are formed when UV photons make two thymes (T) or cytosine (C) bases that are next to each other to react and bind. The formation of this pyrimidine dimer makes that part of DNA cannot be replicated (duplicated) or transcribed which makes the organism’s protein-making system disrupted so that it cannot function normally. Pyrimidine dimers are more easily formed when organisms are exposed to UVC compared to two other types of UV light.

Generally, cells/organisms have a recovery mechanism against this problem. For example, by breaking the bond between the two thymine/cytosine with the photolyase enzyme or by cutting a strand of the damaged DNA section then reconnecting it.

The repair process generally runs effectively but does not rule out the possibility of an error during the repair. The longer an organism is exposed to UV light, the more pyrimidine dimers are formed, and the greater the chance of errors in DNA repair. DNA damage like this can trigger abnormalities ranging from mutations to cell death.


UV light is a type of electromagnetic radiation that has a high enough energy. Therefore, these rays have ionizing properties, which are able to break or trigger the formation of chemical bonds.

From this ability, UV light can cause abnormalities in DNA which can cause mutations to cell death. Although every organism cell has the ability to repair damaged DNA, the DNA repair system is not perfect. Prolonged exposure to UV light can cause an increase in damaged DNA and increase the risk of errors in DNA repair. More errors in DNA repair can cause mutations and generally cell death.

Sources and References:

  1. Apa Itu Kuman? | Perlindungan Kuman – Dettol
  2. Yeast: Saccharomyces, Cryptococcus and Candida – University of Edinburgh
  3. What Is the Difference Between UVA, UVB, UVC and UVV – DoctorUV
  4. What Is Ultraviolet Radiation? – Government of Canada
  5. Effects of UVC on Humans – The Health Physics Society
  6. Pasangan Basa – Wikipedia
  7. Biological Consequences of Ultraviolet Exposure – Kansas State University
  8. Is UVC Safe? – Klaran University

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