When the COVID-19 pandemic emerged in early 2020, ultraviolet radiation became one of the go-to methods for preventing the spread of the SARS-CoV-2 virus, along with facemasks, hand sanitizer and social distancing.
The problem: There was little research showing what UV dosage kills the virus. What wavelength? How long? And could UV systems be installed in public places such as airports, bus stations and stores without causing long-term damage to people?
In a newly published study, researchers from Binghamton University’s Thomas J. Watson College of Engineering and Applied Science answer many of those questions and lay the foundation for health standards about what offers true disinfection.
The paper, titled “Systematic evaluating and modeling of SARS-CoV-2 UVC disinfection” and published in Scientific Reports, is written by Distinguished Professor Kaiming Ye, chair of the Department of Biomedical Engineering; BME Associate Professor Guy German and BME Professor Sha Jin, along with Ph.D. student Sebastian Freeman; Zachary Lipsky, Ph.D. ’21; and Karen Kibler from the Biodesign Institute at Arizona State University.
A number of disinfection technologies have been explored to eliminate SARS-CoV-2 from contaminated surfaces or in the air. SARS-CoV-2 is an enveloped virus. It is generally recognized as being highly susceptible to most cleaning agents2 and alcohol-based hand sanitizing solutions 3.
This approach provides effective disinfection but must be applied thoroughly over all surfaces, and generally by humans. Automated systems such as spray curtains can improve the efficiency 4,5,6 but are not practical for most disinfection applications in the spaces other than in laboratories or healthcare settings.
There is increased interest in developing antimicrobial surface coatings for rapid and sustained disinfection 7,8,9,10. For instance, quaternary ammonium, a well-tested antimicrobial agent, when combined with organosilanes, can achieve a microbial reduction of > 99.9999% even after 24 h after its application 9.
The major disadvantage of these antimicrobial coating approaches is that their effectiveness can be affected by many unpredicted factors. For instance, the quaternary ammonium will lose its effectiveness when mixed with organic matter such as the presence of soil, blood, etc. Also, these agents are lung irritants and can contribute to asthma and other breathing problems.
Lower wavelength between 200 and 290 nm ultraviolet light (UVC) causes DNA and RNA damage, primarily through the mechanism of thymine and pyrimidine dimers, which disrupt nucleic acid replications and therefore inactivate various pathogens, including viruses such as SARS-CoV-2 11,12,13,14,15,16.
Consequentially, UVC-based disinfection systems have become increasingly visible in the public sphere as a reliable method of disinfection, and UVC-based disinfection is a promising tool to reduce SARS-CoV-2 transmission due to its low cost and manageable risk 17.
Measures that reduce transmission rates in public settings are critical to reopening the economy and bringing everyday life back to normal, particularly for in-door activities. An effective UVC disinfection system can eradicate both airborne viruses and contaminated surfaces, helping to prevent human–human transmission. This will enable the restoration of many in-door activities and allow people to return to work before a vaccine is available.
There are three UVC range subsets that have been extensively studied as optimal for pathogen disinfection18,19: (1) 207–222 nm 13,20,21,22, (2) 254 nm 23,24,25,26, and (3) 260–280 nm 24,27,28,29,30. 254 nm wavelength is the most commonly used germicidal wavelength.
However, the 254 nm wavelength can damage skin and eyes 31,32,33.
As such, disinfection methods utilizing 254 nm wavelength UV light must be employed only in unoccupied spaces, or must be designed so as to not cause direct exposure. Wavelengths in the 260–280 nm range have been purported to be more efficient than 254 nm, as these wavelengths are closer to the maximum absorption of the RNA or DNA 34,35.
Furthermore, wavelengths of 207–222 nm have gained an increasing spotlight as a novel disinfection wavelength. Such wavelengths minimally reach the earth’s surface due to attenuation by atmospheric ozone. Recent reports have shown a high germicidal effectiveness of 222 nm wavelength light 13,15,21,36.
Studies also suggested that 222 nm UVC light may be safely used in public settings, as it cannot penetrate tissue as deeply as a higher wavelength UVC can 21,37.
Further complicating the evaluation of the different UVC wavelengths is the widely varying inactivation doses reported to achieve a certain log-reduction for various pathogens. A broad review of coronaviruses and their inactivation doses revealed a wide range of recommended doses from 0.6 to 11,754 mJ/cm2 34.
Such inconsistencies might be caused by varied experimental conditions and setups employed as a potential confounding factor in the determination of the necessary dose. It highlights a substantial need to standardize a testing platform in order to evaluate germicidal inactivation doses while at the same time to understand how environmental factor influence the inactivation doses for a given pathogen.
This is critical not only to the determination of UVC inactivation doses but also to the regulation of UVC products on the market. Un- or ill-regulated UVC products will jeopardize our ability to mitigate the COVID pandemic and also expose the public to risks due to their false safety assurance.
Extended UV exposure is associated with increased skin aging/degradation 40,41, sunburn 42, and an increased propensity to develop skin melanoma 43.
These UV-induced effects can be caused directly by the formation of cyclobutene pyrimidine dimers or photoproducts 42,44,45,46,47, or indirectly through the production of reactive oxygen species, which damage DNA, proteins, and lipids through oxidative stress 38,42,48. The wavelength and exposure dosage of UV light can also influence the method and level of skin damage 45,49.
The pandemic put a great strain on the supply and availability of various personal protective equipment (PPE) such as N95 respirators. Although it is designed for single use and disposable, the desperate situation made it relevant to evaluate the potential reuse after proper disinfection.
Although 254 nm has been evaluated and compared to other methods 6,16,25,48,49, other wavelengths such as LED UVC with relatively high wavelengths that can be used safely were less studied.
Herein, we developed a UVC disinfection dose determination model and used it to determine how required doses changed in different experimental conditions and demonstrate how the effective dose required to completely eradicate SARS-CoV-2 through UVC irradiation changes depending on experimental conditions.
The discovery of the media effect on viral UVC disinfection highlights the need for laboratories to use relevant media when performing UVC disinfection dose determination that will influence public health recommendations. Considerations such as these should be used by industries when designing their UVC products and by agencies when regulating the UVC products designed for disinfecting SARS-CoV-2.
Using this model, we shed some light on the disparate disinfection doses recommended in the literature for SARS-CoV-2. Our study supports the idea that the effectiveness of UVC disinfection is dependent upon wavelength, energy, distance, exposure time, and the media type and volume where viruses are suspended or enclosed.
In addition, we report on important safety considerations when implementing UVC light for disinfection, particular with respect to UVC skin safety, as well as its potential effectiveness for the disinfection of N95 respirators.