The global crisis that arose from the COVID-19 pandemic highlights the scientific and engineering potentials of applying ultraviolet (UV) disinfection technologies for bio contaminated air and surfaces as the major media for disease transmission. Nowadays, various environmental public settings worldwide, from hospitals and health care facilities to shopping malls and airports, are considering the implementation of UV disinfection devices for disinfection of frequently touched surfaces and circulating air streams. Moreover, the general public utilizes UV sterilization devices for various surfaces, from doorknobs and keypads to personal protective equipment, or air purification devices with an integrated UV disinfection technology. However, a limited understanding of critical UV disinfection aspects can lead to improper use of this promising technology. 

Coronaviruses are a large family of viruses that can cause illness, ranging from the common cold to more severe diseases like the Middle East Respiratory Syndrome (MERS-CoV) or the Severe Acute Respiratory Syndrome (SARS-CoV). First reported in Asia in February 2003, SARS spread over the next few months to more than two dozen countries on various continents, including North America, South America, Europe, and Asia, before the SARS global outbreak of 2003 was contained. The novel coronavirus (SARS-CoV-2) is a new strain from the CoV family that has not been previously identified in humans. The emergence of COVID-19, the disease caused by SARS-CoV-2 in late 2019 in Wuhan, China, created a pandemic that resulted in large-scale scientific, economic, and public efforts to contain viral transmission. Thus far, the novel CoV is being transmitted directly from person to person, among other routes. The best way of dealing with the pandemic is first simply to reduce the risk of being infected by the virus by blocking the transmission routes. Pathogens, including viruses, are able to spread and be transmitted by environmental routes, including air and inert surfaces or indirectly through touching a contaminated surface. In this regard, avoiding close contact with anyone showing COVID-19 symptoms, such as coughing, sneezing, fever, and difficulty breathing, is strongly recommended by various infection control agencies. WHO urges everyone, particularly those in high-risk areas, to prevent infection spread through regular hand washing and wearing facial masks or any other physical transmission barriers. However, the efficacy of these preventative actions is limited, particularly in indoor environments where bio contaminated circulating air or frequently touched surfaces can mediate transmission.

Coughing by a COVID-19 infected individual can produce about 3000 droplets in a wide size range (10–1 to 102 μm). Droplets larger than 100 μm deposit rapidly on surfaces. According to a study published in the New England Journal of Medicine, SARS-CoV-2 can live on surfaces for several hours to days, depending on the surface material, similar to durations previously reported for SARS-CoV-1. Tiny droplets (0.1–5 μm) are capable of dissolving with the aerosol, remaining airborne, and travelling hundreds of meters. Intermediate-size range (5–100 μm) droplets also shrink to tiny sizes due to evaporation, and the peak concentration of droplets in bioaerosols are in two diameter ranges: 0.25–1.0 μm and 2.5–10 μm. Factors such as air current, temperature, and humidity can also affect bioaerosol transmission rates. The risk of viral infection could be reduced through many control techniques, including heat sterilization, chemical disinfectants, filtration, and ultraviolet (UV) irradiation. The possible material damage caused by heat sterilization, in addition to the shortages of consumer chemical disinfectants and filters on the market, poses a critical challenge throughout pandemics leading to demand more sustainable disinfection systems. Disinfection using UV radiation has been a fast-growing chemical-free technology over the past decades. UV radiation is highly efficient at controlling microbial growth in any medium, such as water and air, as well as on any type of surface.

During the pandemic, UV air and surface disinfection have attracted tremendous attention, and many products became available on the market. Various public places with different levels of contaminated air and surface probabilities, from hospitals and health care facilities to restaurants and cafeterias, started using UV surface disinfection systems. Widespread use of UVC disinfectors is also advised to limit the spread of the virus after reopening public places. However, limited understanding of the critical aspects of UV disinfection, not only among the majority of general public but also with some of the UV surface disinfection manufacturers, has led to inappropriate use of this promising technology. Dubious and nonscientific performance claims by some of the UV system designers and manufacturers are unfortunately widespread. 

Fundamentals of UV Disinfection

UV disinfection has been a validated technology for the disinfection of pathogens on surfaces, as well as in air and water, for several decades. A particular spectrum of UV radiation between 200 and 280 nm, the so-called UVC spectrum, has been employed extensively as the germicidal range of UV radiation. Over the UVC range, a more detrimental effect on microbial cells occurs because the intercellular components of microbes (e.g., RNA, DNA, and proteins) can sensitively absorb UVC photons. Absorbed UVC photons cause critical damage to the genomic system of microorganisms (nucleic acid and micro organismal proteins), preventing them from replicating and surviving, where the adenine–thymine bond is collapsed and a covalent linkage, pyrimidine dimer, is generated between two adenines leading to inability of the cell to replicate. Therefore, the effect of UV irradiation on microorganisms is called “inactivation” and not “killing”. Although the effectiveness of UV irradiation on the infectivity of viruses and the nucleic acid of the virion is well documented, an increased environmental UV dose is likely to lead to an increased rate of viral mutation. the virus can replicate even in the presence of induced mutations, but the effect on the viral genes could be different. The lethal effect of the UV-induced nucleic acid (DNA or RNA) damage depends on the location of changes within the viral genome. In addition, many mutations will not have any discernible effect on the virus, as they are repaired by the host nucleic acid repair mechanism. The majority of the mutations diminish the infectivity of the viruses since most viral genes have a specific role to perform. However, some mutations may lead to the evolution of more pathogenic viruses. For instance, a novel receptor-binding protein can be synthesized within the virus structure that enables the virus to infect a different cell type in host. It is also likely that some UV-resistant strains of viruses will emerge, perhaps as a result of evolving a thicker capsid structure to protect the nucleic acid from UVC damage.

The UV disinfection mechanism is absorption-based ruled by the susceptibility of microbe genetic material to UV wavelength. This susceptibility varies widely among species of microbes. Viruses, as an example, are composed of a nucleic acid identified as either double-stranded DNA (dsDNA), single-stranded DNA (ssDNA), double-stranded RNA (dsRNA), or single-stranded RNA (ssRNA). In general, single-stranded viruses are more sensitive to UV irradiation because of lacking the redundancy of genetic information in a second strand that allows double-stranded viruses to repair the damage.  The viral nucleic acid, independent of its type, is encased in a protein capsid, and in some viruses, such as the influenza virus and SARS-CoV-2, the protein capsid is itself encased in a lipid envelope. Non- enveloped viruses are typically more UV resistant than enveloped viruses, since proteins and lipids of the envelope may be disrupted more easily than other viral parts.  

Safety considerations during UV disinfection

UV has many effects on skin physiology, with some consequences occurring acutely and others in a delayed manner. One of the most apparent acute effects of UV on the skin is the induction of a cascade of mediators in the skin that together causes “sunburn”. UV radiation is also classified as a “complete carcinogen” because it is both a mutagen and a nonspecific damaging agent and has properties of both a tumor initiator and a tumor promoter. The risk of skin cancer is heavily influenced by UV exposure and skin pigmentation. Moreover, if the eye is exposed to excessive UV radiation, several severe consequences are likely to take place, including photo keratitis, erythema of the eyelid, cataracts, solar retinopathy, and retinal damage. Dangerous UV exposure to human skin or the eye includes direct irradiation, in addition to secondary exposure due to the UV reflection from surfaces. The secondary exposure from materials with high reflectance in the UV region must be a crucial consideration in designing UV surface disinfection devices. For instance, PTFE, aluminium, and stainless-steel surfaces can reflect up to 95%, 90%, and 50% of UVC radiation, respectively. If the dose of UV exceeds, severe sunburn-like reactions could be initiated, leading to “sunburn cells” on the skin. In addition to the use of appropriate PPE, such as UV protective goggles and gloves, providing some safety features such as a child lock and motion sensors, as well as designing a shield for the UV exposure area could significantly diminish the chance of human exposure.

About the author

Mr. Jibi T is a specialist & senior preventionist, infection control at King Saud Medical City, MOH, Riyadh, KSA.