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Extreme Exposure to Filtered Far-UVC: A Case Study


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Far-UVC devices are being commercially sold as "safe for humans" for the inactivation of SARS-CoV-2, without supporting human safety data. We felt there was a need for rapid proof-of-concept human self-exposure, to inform future controlled research and promote informed discussion. A Fitzpatrick Skin Type II individual exposed their inner forearms to large radiant exposures from a filtered far-UVC source. No visible skin changes were observed at 1,000 mJcm-2 , whereas skin pigmentation that appeared around 2 hours and resolved within 24 hours occurred with an 8,000 mJcm-2 exposure. These results combined with Monte Carlo Radiative Transfer computer modelling suggest that filtering longer ultraviolet wavelengths is critical for the human skin safety of far-UVC devices. 3
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Extreme Exposure to Filtered Far-UVC: A Case Study
Ewan Eadie*1, Isla M. R. Barnard2, Sally H. Ibbotson3, Kenneth Wood2
1. Scottish Photobiology Service, Photobiology Unit, NHS Tayside, Ninewells Hospital and
Medical School, Dundee, DD1 9SY
2. SUPA, School of Physics & Astronomy, University of St Andrews, St Andrews, KY16 9SS
3. Scottish Photobiology Service, Photobiology Unit, University of Dundee, Ninewells
Hospital and Medical School, Dundee, DD1 9SY
*Corresponding author e-mail: (Ewan Eadie)
Far-UVC devices are being commercially sold as “safe for humans” for the inactivation of
SARS-CoV-2, without supporting human safety data. We felt there was a need for rapid proof-
of-concept human self-exposure, to inform future controlled research and promote informed
discussion. A Fitzpatrick Skin Type II individual exposed their inner forearms to large radiant
exposures from a filtered far-UVC source. No visible skin changes were observed at 1,000
mJcm-2, whereas skin pigmentation that appeared around 2 hours and resolved within 24 hours
occurred with an 8,000 mJcm-2 exposure. These results combined with Monte Carlo Radiative
Transfer computer modelling suggest that filtering longer ultraviolet wavelengths is critical for
the human skin safety of far-UVC devices.
The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the virus responsible
for the current global COVID-19 pandemic. Estimates as of the 8th of September 2020 indicate
27.5 million confirmed coronavirus cases and approximately 900,000 deaths globally
( As of May 2020 the pandemic had also
resulted in an estimated 3.8 trillion dollars of global consumption losses and 147 million job
losses (1). As a consequence, it is imperative to employ measures that inactivate or destroy the
virus and limit its transmission.
Ultraviolet-C (UVC) radiation covers the wavelength range of 100 nm to 280 nm and has a
known germicidal effect (2). UVC irradiation is a well-established technology used for the
destruction of bacteria and viruses and employed in a range of industries (36). The
established UVC wavelength routinely used for germicidal tasks is the mercury emission
wavelength of 253.7 nm, which has been shown to inactivate SARS-CoV-2 but also results in
acute adverse reactions in the skin and eyes (7, 8).
Far-UVC is a term, which loosely incorporates wavelengths between 200 nm and 225 nm.
Current far-UVC published research is dominated by Krypton-Chlorine (KrCl) excimer lamps,
which emit predominantly at 222 nm but can include low-power long-wavelength emissions. It
has been demonstrated that far-UVC, emitted by KrCl excimer lamps, inactivates SARS-CoV-
2 on surfaces as well as human coronaviruses alpha HCoV-229E and beta HCoV-OC43 in air
(9, 10). However, it does not induce pre-mutagenic DNA lesions in mouse skin, even when
chronically irradiating mice particularly susceptible to ultraviolet radiation (1113). These
laboratory data are being used commercially to intensively promote and sell far-UVC systems
to the global public. At the beginning of the COVID-19 pandemic, the only published study
investigating a far-UVC system in humans had contradicted the laboratory results, showing
skin damage in the form of erythema and cyclobutane pyrimidine dimer (CPD) formation (14).
The authors of this study hypothesized that it may be longer wavelengths present in the lamp
spectrum that caused the adverse effects, a hypothesis supported by subsequent computer
modeling (15).
Due to the unsupported but widely disseminated commercial claims of far-UVC systems being
“safe for humans” it was felt that there was a need for rapid proof-of-concept testing on human
skin with an appropriately filtered far-UVC device. This proof-of-concept testing could then
inform future detailed and controlled assessment.
In-vivo exposure: A 37-year-old male, Fitzpatrick Skin Type II, performed self-exposure with
a filtered far-UVC source (SafeZoneUVC, Ushio Inc., Tokyo, Japan). On Day 1 the left inner
forearm was exposed for 250 seconds and then assessed visually at hourly intervals from zero to
eight hours and 17 to 24 hours. On Day 2 the right inner forearm was exposed for 33.3 minutes
and again assessed hourly from zero to 10 hours and at 24 hours. Both exposure areas were
covered between time point assessments. The irradiance of the filtered far-UVC source was
determined with a broadband radiometer (International Light IL1400A meter with SEL220 sensor,
QNDS2 filter and quartz diffuser. International Light Technologies. Massachusetts, USA) and the
spectral distribution with a double-grating spectroradiometer (DM150, Bentham Instruments Ltd,
Reading, UK). The broadband radiometer was calibrated against the double-grating
spectroradiometer, which is itself calibrated against both a deuterium and quartz halogen tungsten
lamp with traceability to national standards.
Monte Carlo Radiative Transfer (MCRT) computer modelling: MCRT codes, previously
used to study an unfiltered far-UVC device, were used to investigate depth penetration of light
from a filtered far-UVC source as utilized in the self-exposure (15). Optical properties of the skin
layers and structure of the 5-layer skin model were as previously described (15, 16). Results
between filtered and unfiltered far-UVC sources were compared.
In-vivo exposure
Average irradiance on the skin surface from the filtered far-UVC source was 4.3 mWcm-2 in
the wavelength range 200 nm to 400 nm. The exposures to the left and right inner forearms
were 1,075 mJcm-2 and 8,600 mJcm-2 respectively. The normalized spectral distribution of the
filtered far-UVC source is presented in Figure 1.
Figure 1 Spectral distribution of the filtered far-UVC source used in the self-exposure (orange). For comparison the
unfiltered far-UVC source used in the study by Woods et al. is also plotted (blue).
There were no visible changes to the skin on the left inner forearm, at any point during the
observed period.
The right inner forearm developed pigmentation, which became visible at two hours
post-exposure, peaked around 5 hours post exposure, was almost resolved by 10 hours and was
fully resolved by 24 hours (Figure 2).
No erythema was evident at any time point on either forearm site.
Figure 2 Right inner forearm following exposure to 8,000 mJcm-2 filtered far-UVC a) 2 hours post exposure, first
appearance of pigmentation, b) 5 hours post exposure, peak pigmentation, c) 10 hours post exposure, pigmentation is
MCRT computer modelling
Figure 3 details the fluence rate incident on different layers within the epidermis as defined by
the MCRT computer modeling. There is roughly 100 times less incident on the basal layer
between 240 nm and 320 nm, when comparing the filtered far-UVC to the unfiltered source.
Figure 3 Results from monte carlo radiative transfer (MCRT) modeling of the filtered far-UVC source (orange). These
are compared to the MCRT modeling of Barnard et al. (blue) (15). Unfilt = Unfiltered, Epi = Epidermis.
These self-exposure results have demonstrated that large radiant exposures (“doses”) of 1,000
mJcm-2 of filtered far-UVC can be delivered to pale skin without induction of visible changes.
Based on the results of Buonanno et al., such a dose within an 8 hour limit would allow for an
approximately 99.9% inactivation of airborne human coronavirus alpha HCoV-229E in less
than 1 minute (9). Similarly, SARS-CoV-2 on a surface could undergo a 99.7% inactivation in
less than 1.5 minutes (10). A dose of 1,000 mJcm-2 is much larger than the 23 mJcm-2 limit of
exposure in the International Commission on Non-ionizing Radiation Protection (ICNIRP)
guidelines (17). The ICNIRP limit of exposure represents “conditions under which it is
expected that nearly all individuals may be repeatedly exposed without acute adverse effects
and, based upon best available evidence, without noticeable risk of delayed effects” (17). This
proof-of-concept study in no way replaces these guidelines and associated national legislations
but is a baseline for further explorative, controlled research studies. The ICNIRP limits of
exposure also apply to the eye, which this study has not investigated.
At much larger doses, 8,000 mJcm-2, of filtered far-UVC a reaction in the skin was observed,
with pigmentation first appearing around 2 hours and fading by 10 hours post exposure. This
pattern is similar but not identical to immediate pigment darkening (IPD), which is the photo-
oxidation of existing melanin, routinely seen with exposure to ultraviolet-A (UVA) and UVA1
radiation. However, the small UVA radiant exposure from the filtered far-UVC lamp is much
lower than would normally be required to induce IPD and so this requires further investigation.
In the 2015 study by Woods et al., the Minimal Erythema Dose (MED) from exposure to the
unfiltered far-UVC device was 40 50 mJcm-2, whereas in the current report, no erythema was
observed with the filtered far-UVC device self-exposure of 1,000 mJcm-2 (or 8,000 mJcm-2).
This difference would support the hypothesis from Woods et al., and subsequently reinforced
by Barnard et al., that longer ultraviolet wavelengths were responsible for the skin damage
seen in the 2015 Woods et al. study (14, 15). It is well recognized that it is important, when
assessing the hazard from an ultraviolet source, to consider all wavelengths and plot the source
spectrum on a logarithmic scale (18).
A recently completed study by Fukui et al. found similar results to our self-exposure, with no
visible erythema at 24 hours following 500 mJcm-2 irradiation with a filtered far-UVC device
(19). That study also reported on higher cyclobutane pyrimidine dimers (CPD) in the irradiated
region compared with a control site, although the analysis used was not able to determine in
which section of the skin the CPDs occurred. Our MCRT computer modeling supports these
findings as we demonstrate all wavelengths, including 222 nm, can penetrate to the top and
middle of the epidermis (Figure 3). In addition, our previous study also demonstrated that CPD
can be induced by all wavelengths, including 222 nm, in the upper and mid-epidermis (15).
Therefore, we propose that the CPD observed by Fukui et al. are likely to have occurred in the
upper epidermis where it is thought that DNA damage will not lead to induction of skin
This single individual study does not provide a definitive answer to the question of skin safety.
Our study is the basis for future exploration above the current ICNIRP limit values, which
would allow quicker inactivation of the virus than is currently permitted in occupied spaces.
Furthermore, what this research and other published literature clearly highlight is that the
hazard of all wavelengths emitted must be appropriately assessed - it is too simplistic to state
that far-UVC devices are “safe for humans”.
ACKNOWLEDGMENTS: We would like to thank Tatsushi Igarashi and Ushio Inc. for
a loan of the filtered far-UVC source, SafeZoneUVC.
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