By Devorah Saffern
Ultraviolet (UV) light kills cells by causing thymine bases in the cell’s DNA to interact and form dimers, which are then removed by the DNA’s own correction mechanisms. Increased exposure to UV light increases the chances of these mechanisms incorrectly replacing the dimer or not replacing it at all, which changes the way the entire DNA sequence is read by its polymerase. This impairs the DNA and therefore the cellular functions, which can result in cell death or cause the cell to become carcinogenic (develop into a cancerous cell). Increased exposure to UV light, therefore, can cause cancer, most commonly skin cancers due to direct exposure from UV rays in the sun.
Ultraviolet germicidal irradiation (UVGI) utilizes these destructive properties of UV light to target pathogens. It uses short range UVC light via air or water to kill bacteria and viruses through the DNA interfering cell death mechanism. UVGI is frequently used for air and food purification, and has been used for water treatment since 1955. In 1903 Niels Finsen won the Nobel Prize for Medicine for using UVGI to target tuberculosis of the skin called lupus vulgaris. Since then, researchers have explored the use of UV light in treating disease.
In a study published in June 2016 by Buonanno, M., et al., researchers performed the first in vivo test on mammalian skin using UV light to kill virus and bacterial cells at surgical infection sites, which was not harmful to regular skin cells. The researchers tested UVC/UVGI of 207-nm wavelength on hairless mice skin emitted by an excimer laser, a type of UV laser commonly used for eye laser surgery and in microelectronic devices. 254-nm UVG was used as a control, as well as a negative control of “sham” UV radiation. The 254-nm radiation had pre-mutagenic DNA lesions of the skin, skin inflammation, and epidermal hyperplasia, which is an increase in organic tissue due to cell proliferation, while the 207-nm and sham UV radiated cells showed no significant changes.
Fig 4. from (Buonanno et al., 2016)
Cross-sectional images of the cells showed the presence of two types of dimer DNA mutations found only in the 254-nm UV samples. Pre-mutagenic skin lesions of CPD (cyclobutane pyrimidine dimers, which is typical DNA damage due to UV light) demonstrated that the 254-nm longer wavelength UV exposed was cytotoxic while 207-nm was not. In addition, other images showed keratinocyte cells (skin cells producing keratin, found in the most outer layer of the skin) containing another type of dimer called 6-4PP dimers, found in significant amounts only in the 254-nm UV exposed cells. The results essentially showed that excimer-emitted far-UVC could potentially be used as an antimicrobial agent without posing a threat to the skin’s healthy cells. Future research could perhaps include testing the effects of the radiation beyond the surface of the skin to ensure there are no unknown other potential hazards.
Another Buonanno, M., et al. study, that was published in February 2017, extended their previous experiment by testing 222-nm light in addition to the 207-nm in killing methicillin-resistant Staphylococcus aureus (MRSA) in vitro. The same CPD and 6-4PP dimers were found as before but this time the experiment was done on a 3D human skin tissue model instead of mice skin, a more insightful test. The 222-nm UV light effectively destroys the MRSA and as opposed to the 254-nm UV light, produces hardly any pre-mutagenic DNA lesions, due to its lower wavelength. This inspired more recent work in the area.
A research article by Welch, et al., that was preprinted last December, involved a team at Columbia University Medical Center who looked into the potentials of UV light in preventing microbial diseases. The study used far-UVC light of 222-nm to effectively kill an airborne H1N1 influenza virus in an aerosol UV irradiation chamber, which emitted droplets in the air similar to those produced by humans from coughing and breathing. The study cited the June 2016 article by Buonanno, M., et al. to prove that the light is non-cytotoxic, as well as the Buonanno, M., et al. February 2017 study. The article, which is pending review, concluded that using small quantities of far-UVC light in indoor public spaces would be a safe and economical tool to target airborne microbial viruses.
Infectious diseases can have harmful effects on large portions of the population and some forms, including lower respiratory infections, are even a leading cause of death. Viral and bacterial infections are often contracted from the air – they travel from an ill person through a cough or a sneeze to a healthy individual and enter the person’s mouth or nose. Public spaces such as transit systems and stores are common places where disease can easily spread to many people. Prevention of the spread of viral and bacterial cells in public spaces could therefore significantly reduce the number of illnesses due to infectious diseases among the population. UV lights that target these pathogens are economical, easily implementable, and could be highly impactful in reducing the spread of disease.
Image found on (https://farm5.static.flickr.com/4534/38328874916_d98ee5a388_b.jpg)