Summary
Ultraviolet germicidal irradiation (UVGI) is a method of decontamination that utilizes short-wavelength UV light known as UVC. Among UVA and UVB light, UVC is the only one with germicidal properties because of its ability to deactivate the infectious and reproductive abilities of cells. The UVC wavelength measures between 200-280nm which means it is absorbed by the ozone layer, thus UVC is produced exclusively by artificial sources on Earth. The most common source of UVC is a low-pressure mercury lamp that has a main emission of 254nm.
Due to its carcinogenic properties, UVC is generally used within closed containers where humans don't experience direct exposure. Alternatively, a specific band known as far-UVC (202-222nm) may be safe for human cells while still retaining its germicidal properties, although research for this theory is still underway.
Affected Microbes
Viruses
Bacteria
Fungi
Parasites
Bacterial Spores
Mold
Protozoa
Implementation
Air Decontamination
The germicidal properties of UVC require direct and prolonged exposure, therefore it cannot properly clean moving air like standard HVAC filters. Alternatively, upper-room UV devices can be used to clean slower, horizontally-flowing air near the ceiling while maintaining safe levels of UVGI in the lower portion of the room.
Water Purification
UVC light is used on both large and small scale water cleaning. Several thousand water treatment plants world-wide expose contaminated water to UVC light to deactivate hazardous protozoa, most commonly, Cryptosporidium and Giardia. Smaller, but quite similar methods are used to treat local water within residential or commercial buildings. Smaller UV devices can be used on personal water supplies which are particularly useful for cleaning fresh water directly sourced from nature.
Surface Disinfection
Frequently touched items can be placed within closed containers fitted with UVC lights to remove microbes residing on the surface. On a larger scale, entire rooms or bathrooms can be fitted with strategically placed UVC light systems to cleanse the air and exposed surfaces between human occupancy.
How does UVGI work?
UVA, UVB, and UVC light is produced naturally from the sun, however UVC is absorbed by the ozone layer before reaching organic material on the Earth’s surface. Therefore, UVC is produced artificially by a variety of lamps that range from 200-280nm wavelengths, but the majority of UVC light comes from low-pressure mercury that has a main emission of 254nm. Due to its short, high energy wavelength, UVC can cause both short and long-term damage to cells, most notably, human skin and eye cells; cumulative exposure to UV lights is widely considered to be a carcinogen.
When a microbe is exposed to UVC light, electromagnetic radiation is absorbed by the cells’ macromolecules like proteins and nucleic acids. Direct absorption into genetic material will excite DNA or RNA with photons and cause unwanted bond mutations known as pyrimidine dimers. Sufficient dimerization of the double helix deactivates the ability for a cell to reproduce, and thus, losing its infectious properties. Indirect absorption occurs when photons hit other molecules within a cell and form free radicals which go on to negatively interact with DNA and RNA. Research also suggests that UV lights can kill cells by rupturing membranes, although this is a less studied interaction.
Far-UVC works in a similar way, but evidence suggests that it does not cause significant damage to human cells, but still retains its germicidal properties. Wavelengths between 207nm and 222nm are absorbed by proteins in mammalian cells, which are significantly larger than bacteria and viruses. In theory, most of the human body is protected by proteinous, keratin-rich skin cells, so the far-UVC radiation does not have the opportunity to reach our DNA, but is still capable of reaching the genetic material of microbes.
Concerns
Some research shows that microbes like E. Coli are capable of acquiring resistance to germicidal UV, however it tends to be weak. At this time, it is unknown if microbes can develop full resistance to UV lights like bacteria do to antibiotics.
Sustainability
UV lights used regularly should be changed about every 12 months to maintain optimal effectiveness, but otherwise have a 2-3 year lifespan. The cost of UVC systems is varied. Personal germicidal lights or boxes are fairly inexpensive for a business or individual, usually between $20 and $300. Upper-room UVGI systems and air purifying units typically range from about $500 to $1500 and cost far less to maintain annually. Equipping an entire building with UVC lights is a costly and likely unnecessary investment for non-medical spaces upwards of several thousand dollars. Environmental considerations include potentially harmful components like mercury and the energy requirements needed to emit UV light. Generally speaking, UVC lights are not particularly harmful to the environment.