Pictures published on the internet show UVC light robots in China disinfecting huge spaces with the purpose of preventing coronavirus to be spread among people.

The germicidal properties of UVC light have been used since 1909 for disinfection of drinking water, air and surfaces, primarily for killing pathogenic bacteria. Ultraviolet light also has a very powerful germicidal effect against viruses. The short answer to the question asked above in the article is “yes”, UVC light kills viruses including coronavirus.

Coronavirus has a very high susceptibility against UVC light (wavelengths below 280 nm). The UVC doses necessary to obtain a 90% kill (D90: standard reference number) of coronavirus in both water and air are comparable to and even smaller than the D90 doses needed to kill harmful bacteria such as Pseudomonas a. and Staph. aureus.  D90 doses replicated experimentally and reported in the literature for the same species of bacteria and viruses vary substantially. This variation is caused by many different reasons such as the specific experimental setup (reactor), detector systems, light source optics and the variation of the microbiology procedures used for collecting and handling the UVC treated samples.

D90 doses for viruses and bacteria are tabulated in W. Kowalski’s reference book ‘Ultraviolet Germicidal Irradiation Handbook’. Tabulated D90 doses (in J/m2 for coronavirus are below 10 J/m2 in both air and water. These low doses are comparable to those we should expect for Pseudomonas a. and Staph. aureus in air. In water, the reported range of D90 doses varies substantially for Pseudomonas a. and Staph. aureus (approximately between 10 and 60 J/m2). The reasons for these variations in the reported D90 values are probably caused by the above mentioned possible variations of the experimental parameters.

Rate constants are tabulated and references are made to the original scientific reports. The rate constant ‘k’ can be used to calculate the doses required to obtain an even higher disinfection rate. The expression is:

S = e-kD

S is the survival rate (10% for a D90 dose), k the rate constant and D the applied UVC dose. From Kowalski (page 476) the rate constant for coronavirus in water is 0.3210 m2/J. A survival rate of 10% of the coronaviruses requires then a D90 dose of 7.2 J/m2. The tabulated rate constant for coronavirus in air is 0.3770 m2/J with reference to the work of C.M Walker et al: ‘Effect of ultraviolet germicidal irradiation on viral aerosols’, Environ. Sci. Technology. 2007, 41, 5460-5465. Notice that this rate constant results in a D90 dose of 6 J/m2 (and probably not 3 J/m2 as tabulated, please comment). Walker presents the required UVC D90 dose for approximately 10% survival of coronaviruses in air to be 599 μW×s/cm2, which corresponds to ~ 6 J/m2.

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