Ultraviolet sterilization shines blue UVC light on objects to kill bacteria, viruses, yeast, fungi and their spores. It works by penetrating germs and destroying their DNA which stops them from functioning or reproducing.
This is a quick and reliable method of sterilization but it’s only effective when the object you want to disinfect has direct line-of-sight with the UV source. Forced air systems by design can impede this and prevent effective disinfection.
Viruses
UV light inactivates the genetic material of microorganisms by causing photochemical reactions in their DNA. Specifically, the light changes the bonding of adjacent thymine and cytosine bases within the DNA’s double helix. This renders the cell unable to replicate, thereby preventing the spread of disease. Viruses, bacteria, yeast and fungi are all sterilized in this way.
The first scientific study of the germicidal effects of ultraviolet light was conducted in 1877 by Downes and Blunt. However, it would take many years for scientists to fully understand the powerful disinfectant potential of this technology.
When used properly, UV-C can kill viruses and other microorganisms while posing no threat to humans or pets. Because of the potential for unwanted effects such as sunburns and itchy eyes (photokeratitis), all users of UV-C radiation should follow safety guidelines and use proper personal protective equipment to prevent exposure.
Some pathogens, such as Giardia and Cryptosporidia, form outer coatings or go into cyst states to shield their genetic material from UV light. They are still capable of being inactivated by low doses of UV-C, however. Far-UVC lamps are particularly effective at inactivating these organisms, as shown in a recent study published in Scientific Reports. This makes far-UVC the preferred wavelength for air sterilization applications.
Bacteria
Bacteria are among the most common microorganisms to be sterilized using ultraviolet light. Like viruses, bacteria are tiny organisms that use genetic material to control their growth and function. When exposed to UV light, germicidal rays cause photo-chemical reactions in microorganisms’ nucleic acids (DNA & RNA). This damage prevents them from reproducing properly and eventually kills or disables them.
Unlike eukaryotes, prokaryotes have no cell wall, so they are more vulnerable to UV damage. This makes it easier to destroy them with UV radiation. Typically, bacteria are killed by exposing them to a high dose of UV radiation for an extended period of time. However, a lower dose of UV can also kill them.
The most germicidal UV wavelengths are those in the 100-280 nm range. This is called UVC and it is the kind used in ultraviolet sterilization equipment. UVC is capable of killing all kinds of bacteria and viruses, including some drug-resistant strains.
As with other types of sterilization, UV effectiveness depends on the quality and type of equipment used and a variety of environmental factors. For example, water turbidity and color as well as dust particles on the bulb can reduce or cancel out the sterilizing effect of UV. Also, the rays of UV can be blocked by objects that are placed between the source and the object being treated.
Fungi
Fungi are eukaryotic organisms that include yeasts, molds and fleshy fungi such as mushrooms. Like bacteria, fungi are ecologically important as decomposers and parasites of plants and animals. In addition, fungi produce secondary products that act as antimicrobial agents and are used for alcoholic beverages, medicines, natural fertilizers and cosmetics. Fungi, like bacteria, can also cause diseases in humans and other organisms.
Unlike viruses and bacteria, fungi do not reproduce in the cytoplasm but rather by releasing spores from special sac-like cells called sporangia. Spores may be either asexual (conidia) or sexual in origin, and they are a key identifying feature of many fungi. Asexual structures are referred to as anamorphs and sexual ones are known as teleomorphs.
As a sterilizing agent, UV light can be effective against fungi. In general, it sterilizes the microorganism by breaking down certain chemical bonds and scrambling the structure of DNA, RNA and proteins. In this way, the cell is unable to replicate and the organism dies.
The effectiveness of UV light as a sterilization agent depends on its ability to reach the microorganism and on the microorganism’s ability to withstand the effects of the radiation. As a result, it is important that the UV source be placed so that it can sterilize objects and areas directly in its path of action. Forced air systems by their nature impede line-of-sight and can limit the effectiveness of UVGI.
Other Microorganisms
As the shortest wavelength of the UV light spectrum, UVC is capable of killing bacteria and viruses (also called pathogens). This powerful sterilization process destroys the molecular bonds within a microorganism’s DNA or RNA, thus making it unable to reproduce. UV light also kills “superbugs” that have developed a resistance to antibiotics.
In water treatment, UV can be used to inactivate microorganisms that may cause disease outbreaks in ponds and aquariums. It can also be used to disinfect aquaria and swimming pools, and treat wastewater before it is returned to the water supply or reused in irrigation systems and other municipal applications.
The germicidal effectiveness of UV depends on the degree of UV exposure, which is known as the UV dose. A higher dose is achieved by increasing the intensity of the UV source or the duration of exposure. Within real-world systems, the actual UV dose experienced by a microorganism is difficult to estimate. This is due to uneven distribution of the UV irradiance throughout the treatment volume and differences in local fluid flow paths.
Despite these limitations, it is possible to achieve a high level of germicidal effectiveness in some situations by combining the UV treatment with other disinfection methods. For example, a study done in 2017 found that 222 nm narrow-spectrum UVC light was just as effective at killing methicillin-resistant Staphylococcus aureus (MRSA) bacteria as the more intense 254 nm broad-spectrum UVC.