the science behind the uvc effect on bacteria and viruses
UV light has been known for over 100 years as being really effective in destroying bacteria and inactivating Virus. Most are familiar with UV-A and UV-B light, which is naturally present on earth from the sunlight and are known to cause sunburn.
The energy of UVC light disrupts the DNA and RNA of bacteria and viruses exposed to it, disabling their vital functions such as reproducibility and protein production. Unable to replicate their DNA and RNA, bacteria and viruses can no longer reproduce nor produce proteins required for them to infect a host cell. This is when they are considered neutralized.
UVC light has been used extensively for more than 40 years for disinfecting drinking and wastewater, air, pharmaceutical products, and surfaces against a whole suite of human pathogens.
UV light, specifically between 200-280nm (UVC or the germicidal range), inactivates (aka, ‘kills’) at least two other coronaviruses that are near-relatives of the COVID-19 virus: 1) SARSCoV-1 and 2) MERS-CoV 
COVID-19 infections can be caused by contact with contaminated surfaces followed by facial areas touching. This case is less common than person-to-person transmission, but still an issue . Minimizing such risk is key, since the COVID-19 virus can live on plastic and steel surfaces for up to 3 days . Normal cleaning and disinfection practices may leave behind some residual contamination, which UVC can treat suggesting that a multiple disinfectant approach is prudent. UVC has also been shown to achieve a high level of inactivation of a near-relative of COVID-19’s virus (i.e., SARS-CoV-1, tested with adequate dose of UV while suspended in liquid).
how is germicidal effectiveness determined?
Power intensity, wavelength and exposure duration are the three parameters used to determine germicidal effectiveness. For instance, in water there may be multiple germs with different optimal absorption wavelengths. For any given wavelength of germicidal UV light, the power and duration of exposure would need to be calculated in order to achieve the desired level of disinfection.
Six levels of disinfection can be identified, based on the reduction factor. In the hypothesis of an initial population of one million bacteria, the disinfection effectiveness can be measured as follows:
- Reduce the colony to 100,000 bacteria – 90% reduction
- Reduce the colony to 10,000 bacteria – 99% reduction
- Reduce the colony to 1,000 bacteria – 99.9% reduction
- Reduce the colony to 100 bacteria – 99.99% reduction
- Reduce the colony to 10 bacteria – 99.999% reduction
Reduce the colony to one bacterium – 99.9999%reduction
Partly text from IUVA (ultraviolet association, Advancing the sciences, engineering & applications of ultraviolet technologies to enhance the quality of human life & to protect the environment.), Covid 19
 “Miscellaneous Inactivating Agents – Guideline for Disinfection and Sterilization in Healthcare Facilities (2008);” Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), Division of Healthcare Quality Promotion (DHQP) (Link)
 “Large-scale preparation of UV-inactivated SARS coronavirus virions for vaccine antigen,” Tsunetsugu-Yokota Y et al. Methods Mol Biol. 2008;454:119-26. doi: 10.1007/978-1-59745-181-9_11.
 “Efficacy of an Automated Multiple Emitter Whole-Room Ultraviolet-C Disinfection System Against Coronaviruses MHV and MERS-CoV,” Bedell K et al. ICHE 2016 May;37(5):598-9. doi:10.1017/ice.2015.348. Epub 2016 Jan 28.
 “Focus on Surface Disinfection When Fighting COVID-19”; William A. Rutala, PhD, MPH, CIC, David J. Weber, MD, MPH; Infection Control Today, March 20, 2020 (Link)
 “Preventing the Spread of Coronavirus Disease 2019 in Homes and Residential Communities”; National Center for Immunization and Respiratory Diseases (NCIRD), Div. of Viral Diseases (Link)
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 “Inactivation of SARS coronavirus by means of povidone-iodine, physical conditions and chemical reagents;” Kariwa H et al. Dermatology 2006;212 (Suppl 1): 119 (Link)
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