The UV region of electromagnetic radiation extends from 200-400 nm and is divided into UVA. (320-400 nm), UVB (290-320 nm) and UVC (200-290 nm). Below 200 nm, the radiation is absorbed by air molecules; hence, it is called vacuum ultraviolet, as one needs to pull a vacuum to observe it. The ozone layer absorbs the UVC region of the Earth entirely and partially absorbs UVA and UVB. About 80% of UVA and 20% of UVB make it to the Earth's surface. Bacteria and viruses have not been exposed to natural UVC radiation; therefore, UVC can act as a germicidal radiation that disrupts their DNA and prevents them from multiplying.
It is well known that viruses are obligate parasites that attach to host cells and cannot multiply without the support of host cells (Reference 1). In any environment, different surfaces can become contaminated when body fluids infected with the virus come into contact with them, or when aerosol particles emitted through sneezing and coughing settle on them. The virus survives on the surface for a few hours until it finds a suitable host in the form of another person touching that surface, followed by touching the mouth, nose, or eyes. Recently, there have been a number of outbreaks due to viruses such as Hepatitis Virus, West Nile Virus, Enterovirus (Related to the intestines), Severe Acute Respiratory Syndrome (SARS), and, most recently, COVID-19, which has caused a worldwide pandemic. The most threatening viral emergence is enteric viruses and SARS, which have posed significant public health threats. The ability of these viruses to spread through close interpersonal contact and also through droplets when a person touches a contaminated surface with infected droplets and then touches his/her face makes them very dangerous.
Many measures can be taken to control the spread of viral diseases, such as SARS-CoV, which occurred in 2003, and SARS-CoV-2 (COVID-19), which is happening now, by disinfecting surfaces where these viruses reside. These measures could include heating the surface to sterilize it, using chemical disinfectants such as Lysol, and Ultraviolet Germicidal Irradiation (UVGI). However, the first two methods could potentially damage surfaces, as some surfaces cannot be sterilized by heat, and chemical solutions may also damage them. Unlike the first two methods, UVGI has proven to be an effective disinfection method that does not damage surfaces. The absorption peak of a DNA molecule is around 264 nm, and using a mercury lamp that emits at 253.7 nm generates dimers that can interfere with DNA replication and also destroy nucleic acids, effectively destroying viruses.
Viruses can be divided into four groups: single-stranded RNA (ssRNA), single-stranded DNA (ssDNA), double-stranded RNA (dsRNA), and double-stranded DNA (dsDNA). In one study, it was found that the double-strand versions of RNA and DNA are less sensitive to UV decontamination than the single-strand versions and require two to three times the UV irradiation to be deactivated. The radiation dose and humidity levels also played a part. To calculate the radiation dose, the irradiance at a given distance is multiplied by the exposure time in seconds, as shown below.
UV dose (J/cm 2)=Irradiance (W/cm 2 )*Time (s)
It turns out that a dose of 2-5 mJ/cm 2 will reduce the virus population by 90% (Reference 1, Reference 4). For example, for a lamp with 200 mW/cm 2 irradiance at 10 ft, it will take 10 to 25 seconds to disinfect a surface 10 ft away. Increasing humidity will also reduce the effectiveness of UV disinfection, as the study found that at 85% humidity, it was harder to disinfect the surface than at 55%. This could be due to the absorption of UV radiation by the water covering the surface, reducing the dose that reaches the virus and deactivating it.
Aside from disinfecting surfaces, UV germicidal lamps have also been used to clean N95 masks and hospital equipment. Due to the current pandemic, there is a shortage of N95 masks, and one strategy is to reuse them by disinfecting them with UVGI.
In this project, two robust UVGI systems, each having eight 34 Watt mercury lamps emitting at 254 nm, were placed at the opposite vertices of a rhombus, 8 feet apart. Strings were drawn across the longer diagonal of the rhombus, which was 13 feet long, and N95 filters were hung across it. The units would provide 200 mW/cm 2 at a distance of 10 ft. A UV detector shown at the right of the figure measured 400 mW/cm 2 from the two units, and a dose of 800-1200 mJ/cm 2, which took 15 to 20 minutes of irradiation, was sufficient to disinfect the masks for reuse.
Aside from discharge lamps such as a Mercury lamp (254 nm) and a Xenon pulsed lamp (200-320 nm, UVC-UVB) with millisecond pulses, UV LED panels are also used for disinfection applications to kill bacteria and viruses. UV LEDS at 266, 270, 275 and 279 nm have been fabricated and used for killing bacteria in food. There are a few advantages that UV LEDs have over discharge lamps, as follows:
- The wavelength is not fixed (254 nm for mercury lamp) and can be tailored-made. Some wavelengths, such as 260 nm, are more effective in killing bacteria and viruses.
- Discharge lamps have low activity in refrigerated environments
- Risk of mercury exposure
However, discharge lamps are generally more powerful and preferred for decontaminating a spacious hospital room.
UVC radiation is hazardous to the skin and eyes, and the rooms must be fully evacuated during disinfection operations. If the UV lamps are being operated by staff, they need to wear proper Personal Protective Equipment (PPE) to protect their skin and eyes. It might be easier to use robotics to move the UVC lamp across the room to disinfect surfaces and avoid hazards to human operators.
Allied Scientific Pro has introduced a line of UVC germicidal lamps mounted on very useful robots.
References:
1 - Inactivation of viruses on surfaces by Ultraviolet Germicidal Irradiation, Chun-Chieh Tseng et.al, Journal of Occupational and Environmental Hygiene, 4, 2007.
2 - https://memoori.com/can-uv-light-kill-coronavirus-in-our-contaminated-buildings/
3 - Ultraviolet germicidal irradiation of influenza-contaminated N95 filtering face-piece respirators,
Devin Mills et.al, American Journal of Infection Control, 46 (2018)
4 - N95 filtering face-piece respirator UVGI process for decontamination and reuse, Nebraska Medicine, 2020.
5 - Using UVC Light Emitting Diodes at wavelengths of 266 to 279 nm to inactivate foodborne pathogens and pasteurize sliced cheese, Soo-Ji Kim et.al, Applied and Environmental Microbiology, volume 82, number 1, 2016.