The role of far-UVC in dentistry

The COVID-19 pandemic has undoubtedly had a significant impact on all health provision sectors. Within dental care, service disruption has been particularly problematic due to the high level of patient contact and the potential for generation of aerosols during treatment sessions. The service, however, remains vulnerable to future coronavirus-type pandemics.

Role of ultraviolet light

It has been known for some time that the ultraviolet wavelength of 254 nm, in the UVC band, produced from mercury vapour discharge tubes, has the ability to decontaminate clinical environments,¹ and its effectiveness has also been demonstrated against the SARS-CoV-2 virus.² This has been the conventional basis of systems for rapid decontamination of clinical areas in, for example, critical care environments and accident and emergency departments. The UVC treatment method using 254 nm was typically only applied to individual room spaces owing to the risk of exposure of clinical staff in ‘open’ clinical areas.

Estimation of safe levels of ultraviolet exposure

Over time, the response of human skin/eye to ultraviolet radiation has been developing as measurement technologies mature. Separate guidelines have been issued by the International Commission on Non-Ionising Radiation Protection (ICNIRP) with headquarters near Munich, and the American Conference of Governmental Industrial Hygienists (ACGIH) with headquarters in Cincinnati, OH, USA.^3,4Figure 1 outlines the spectral response to ultraviolet radiation as indicated by the ICNIRP. This threshold is a level at which cellular changes become detectable and is structured to provide safe occupational exposure. Both organizations indicate a weighted exposure limit of 3 mJ/cm² within a notional 8-hour period.

Role of far UVC for neutralization of pathogens

The publication by Welch et al, prior to the COVID-19 pandemic, on the usefulness of wavelengths at 222 nm, in the so-called ‘far-UVC’ wavelength range, to neutralize pathogens was not widely taken up by the clinical community,⁵ although the attractive characteristics of far-UVC became evident to wider elements of the scientific community as a means of mitigating the effects of the COVID-19 pandemic.⁶

The realization that the far-UVC wavelength range could play a vital role in disinfection processes created a strong case for raising these safety limits.⁷ In due course, the spectral weighting factors between ICNIRP and ACGIH diverged after a revision of the ACGIH guidelines in 2022. The ACGIH spectral weighting values at 222 nm were then increased from 23 mJ/cm² (for both eye and skin) to 161 mJ/cm² for the eye and 479 mJ/cm² for the skin. A key factor in the selective ability of far UVC at 222 nm to provide a means to disable viruses, but not cause cellular damage, is that the radiation can penetrate the thin protective coatings of bacteria and viruses, but not reach within cell structures of the skin or of the eye.

Initially, Woods et al had investigated in some detail the effect of significant doses of far-UVC radiation in human subjects from a commercially available lamp system (Sterilray, NH, USA).⁸ A threshold level of 40–50 mJ/cm² for detection of skin erythema was identified, which was significantly lower than the value determined more recently by Eadie et al with a lamp with superior filtering above 240 nm.⁹ This demonstrated that no visible reaction was evident up to an exposure of 1500 mJ/cm² from a filtered Kr lamp. This is some 65 times greater than the present ICNIRP limit. A key comment in the study by Eadie et al is that there is an urgent need to prudently revise upwards the current ICNIRP exposure limits.

Far-UVC lamps and ozone production

A useful review of such hazards is provided by Claus where the peak of ozone production is described at around 160 nm and which falls rapidly by a factor of around 10⁷ to 240 nm.¹⁰ To reduce the level of any ozone production from far-UVC lamp source, it is important to reduce the lamp outputs below 200 nm although the contribution to ozone production of the main peak at 222 nm should be negligible.

Evaluation of far UVC in room disinfection

A description of the operation of such a disinfection system using far UVC at 222 nm is detailed by Eadie et al.¹¹ In this important study, significant reductions of airborne Staphylococcus aureus were achieved using far-UVC technology in a room-sized test environment and where the pathogen reduction was equivalent to 35 air changes per hour (ACH). This was also achieved at an appropriate output setting compliant with current ICNIRP 8-hour exposure levels. There is an obvious potential for widespread deployment of such technology within healthcare facilities, including dental practices, as a preventive measure against common airborne viruses, including SARS-CoV-2. Figure 2 indicates anticipated use of far-UVC radiation in diverse public spaces.

Discussion

There is a clear indication of the usefulness of far UVC in the decontamination of healthcare premises where, for wavelengths below 230 nm, the radiation is unable to penetrate human cellular structures. The group of healthcare professionals active in clarification of safety characteristics of far-UVC radiation is anxious, however, to provide greater clarity in assessing levels of safe exposure, and the situation is made more complex by a range of products already on the market claiming to be ‘safe for humans’.

There is an urgent need, however, for ‘experts' advising on the use of UVC radiation within dental facilities in the UK to take on board recent developments in providing safe and effective systems incorporating far-UVC radiation. Relevant professional bodies should include within their membership appropriate experts with relevant knowledge of far-UVC technology to provide appropriate guidance.

A useful state-of-the-art review of the basic science and technology of far-UVC applications for pathogen deactivation is provided by Blatchley et al, where specific details of the revision of exposure guidance by ACGIH is also provided.¹² In a reassessment of previous studies, O'Mahoney et al raises an element of caution regarding the revised ACGIH exposure limits for 222-nm lamp systems, which indicates that while the ICNIRP limits are likely to be an indication of safe levels of exposure, the revised ACGIH exposure limits should be treated with an element of caution.¹³