10
DOSAGE DETERMINES EFFICACY
The amount of UV-C energy required to inactivate a given micro-organism is measured on the basis of the dose,
which is determined by a combination of radiation energy and exposure time. A key difference between surface
inactivation and airflow inactivation of micro-organisms is exposure time. The residence time in the UV field for
micro-organisms in the airflow is in the order of seconds and would require a much higher UV-C dose compared
to surface application. Scientists have determined the rates at which various microbial populations decrease due
to exposure to biocidal factors such as UV-C irradiation. Organisms differ in their susceptibility to UV-C inac-
tivation; in general, viruses are the most sensitive to UV-C rays followed by bacteria with fungal moulds and
spores that are the least sensitive.
UV-C irradiation also obeys the inverse-square law of light, where the intensity at a given point is inversely
proportional to the square of its distance from the light source.
To date, the dose in Joules has not yet been officially established to inactivate COVID-19 because to obtain the
exact data means waiting for the technical laboratory times, which should be made official by the end of October.
However, all bacteria and viruses tested to date (many hundreds over the years, including coronaviruses) are
eliminated by UV disinfection. UVC rays (or germicidal range), inactive (i.e., “kill”, “eliminate”) at least two
other coronaviruses that are the closest relatives of the COVID-19 virus: SARS-CoV and MERS-CoV (“Large-
scale preparation of UV-inactivated SARS coronavirus virions for vaccine antigen”, Tsunetsugu-Yokota Y et al.
Mol Biol Methods. 2008; 454:119-26. doi: 10.1007/978-1-59745-181-9_11.; “Efficacy of an Automated Mul-
tiple 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 (https://www.infectioncontroltoday.com/covid-19/focus-surface-disin-
fection-when-fighting-covid-19). However, the efficacy condition of UVC light remains related to THE DOSE,
i.e. the energy with which the light reaches the target to inactivate it. UVC systems must therefore be designed
to ensure the DOSE required to inactivate the viruses for the environments and applications for which they were
designed.
For example, an excerpt from the Virus table ( at this link:
https://www.iuvanews.com/stories/pdf/archives/180301_UVSensitivityReview_full.pdf
the full table).
