Introduction
Far ultraviolet-C (UV-C) light (200 to 230 nm) has been proposed as a technology for continuous surface and air decontamination in occupied spaces. Reference Donskey1,Reference Blatchley, Brenner and Claus2 Far UV-C is purported to be safe for use while people are present because it is strongly absorbed by proteins and other biomolecules and therefore penetrates minimally into skin and eye tissues. Reference Blatchley, Brenner and Claus2,Reference Memic, Osborne, Cadnum and Donskey3 There is some evidence that 222 nm far UV-C doses within threshold limit values proposed by the American Conference of Governmental Industrial Hygienists (ACGIH) and the International Commission on Non-Ionizing Radiation Protection may be safe. Reference Eadie, Hiwar and Fletcher4–Reference Fukui, Niikura and Oda7 However, there is a need for more evidence regarding safety and efficacy in real-world settings. Reference Memic, Osborne, Cadnum and Donskey3
Dental procedures pose a risk for transmission of bacterial and viral pathogens. 8 The risk can be minimized by following standard precautions. 8 As an adjunctive measure, a private dental office installed far UV-C lamps in 5 dental procedure rooms in 2020 in response to the coronavirus disease 2019 pandemic. The lamps have been operated during care of patients for more than 3 years. Here, we interviewed dental staff regarding their experience with the technology and evaluated the efficacy of the device in a dental procedure room.
Methods
Description of the far UV-C technology
The 250-Watt GermBuster Channel (Sterilray, Inc.) far UV-C technology uses a 45.7 cm krypton-chloride excimer lamp that emits 222 nm light. A picture of the device is shown in the supplementary material. The device includes a high (240 Watts) and low (99 Watts) setting.
Use of the far UV-C technology in the dental office
GermBuster Channel far UV lamps (Sterilray, Inc., Somersworth, NH) were installed in 5 dental treatment rooms ranging from 16.6 to 24.9 m3. The lamps are positioned near the ceiling ∼1.52 meters from patient’s head with the patient facing away from the lamp and are operated on the low setting during patient care up to 7 hours per day. Each of the lamps had been operated for more than 2,500 hours by August 2023. One person interviewed the dentist and a dental hygienist regarding their experience using the technology, acceptance by patients, and adverse effects. The interview questions are shown in the supplementary material.
Evaluation of the far UV technology in a dental treatment room
Testing was conducted in a 31.2 m3 dental treatment room with ∼12 air changes per hour at the Cleveland VA Medical Center like the private dental office rooms but larger. The far UV-C device was positioned 2 m from the floor angled toward the patient’s chair. Far UV-C delivery to 20 test sites classified as unshaded (i.e., in direct line of UV-C exposure), partially shaded (i.e., not in direct line of exposure), or fully shaded was assessed using a radiometer (UIT2400 Handheld Light Meter for 222 nm (Ushio America, Cypress, CA) and 222UVC Dots colorimetric indicators (Intellego Technologies, AB Gothenburg, Sweden) detecting doses ranging from 5 to 150 mJ/cm2. For the 20 sites, efficacy against methicillin-resistant Staphylococcus aureus (MRSA) in 5% fetal calf serum was tested using a modification of the American Society for Testing and Materials (ASTM) standard quantitative disk carrier test method (ASTM E 2197-02). 9 The concentration of MRSA on the disks was ∼3.5 log10 colony-forming units. The exposure time was 2 hours. Experiments were completed in triplicate. A 3-log10 or greater reduction in comparison to untreated controls was considered effective. Reference Memic, Osborne, Cadnum and Donskey3 The supplementary material provides an illustration of the dental room with numbers indicating the test sites and pictures of the room and the colorimetric indicators.
Reduction in aerosolized bacteriophage MS2
The efficacy of the far UV-C technology in reducing aerosolized bacteriophage MS2 was tested in a 23.5 m Reference Memic, Osborne, Cadnum and Donskey3 room with ∼4 air changes per hour. Reference Memic, Osborne, Cadnum and Donskey3 After release of aerosol containing 108 plaque-forming units (PFU) of bacteriophage MS2, air samples were collected using National Institute for Occupational Safety and Health two-stage bio-aerosol samplers (Tisch Environmental) over 5-minute periods 0–5 (baseline) and 40–45 minutes after release. Log10 reductions were calculated in comparison to control experiments with no far UV-C exposure.
Results
Based on interviews with the dentist and dental hygienist, the technology has been received positively by patients and staff with no reports of adverse health effects, damage to equipment, or discoloration of surfaces. All patients receive an information sheet describing the technology. Staff wear standard dental protective equipment including eye protection.
As shown in Table 1, far UV-C delivery to sites in the treatment room varied considerably, with substantially higher irradiance readings in unshaded or partially shaded (range, .6–3.8 µW/cm2) versus shaded (0–.2 µW/cm2) locations. For the shaded sites, the colorimetric indicators read unexposed and reductions in MRSA were ≤.5 log10 after 2 hours of exposure. In contrast, all unshaded and partially shaded sites had positive colorimetric indicator readings and MRSA reductions of ≥1.7 log10.
Table 1. Far ultraviolet-C delivery to unshaded, partially shaded, and shaded sites in a dental treatment room and reductions in methicillin-resistant Staphylococcus aureus at the test sites
Note. SE, standard error; m, meters.
Dosages were calculated for 2-hour and 7-hour exposure times using irradiance readings obtained using a radiometer.
After release, >6 log10 PFU of bacteriophage MS2 was recovered from air at baseline and after 45 minutes for control samples (Figure 1). The far-UV-C exposure resulted in a >5 log10 PFU reduction after 45 minutes.
Figure 1. Reduction in aerosolized bacteriophage MS2 over 45 minutes in a test room with and without exposure to far ultraviolet-C light. PFU, plaque-forming units; nm, nanometers. Error bars show standard error.
Discussion
Far UV-C is a promising technology for continuous decontamination of air and surfaces. Reference Donskey1,Reference Blatchley, Brenner and Claus2 However, the lack of published evidence regarding efficacy and safety in real-world settings has been a major impediment. In the current study, we evaluated the efficacy of a far UV-C technology used in a dental clinic during patient care. In a dental procedure room, a single wall-mounted far UV-C lamp delivered substantial doses of far UV-C to unshaded areas 1.6–3.0 m from the lamp and reduced MRSA in these locations by ≥3.5 log10 within 2 hours. Far UV-C doses and reductions in MRSA were substantially decreased in fully shaded areas as has been demonstrated for 254 nm UV-C. Reference Boyce and Donskey10 In a separate test room, aerosolized bacteriophage MS2 was reduced by >5 log10 in 45 minutes.
Safety is a major concern for all UV-C technologies. We did not conduct a formal safety assessment, but dental staff reported no adverse effects during more than 3 years of operation. For those considering implementation of far UV-C technologies in occupied areas, the manufacturer should be asked to provide assurance that personnel and patients will not be exposed to doses exceeding the 8-hour threshold limit values proposed by the ACGIH (161 mJ/cm2 for eyes and 479 mJ/cm2 for skin). Reference Blatchley, Brenner and Claus2 In that regard, the calculated doses of far UV-C delivered to 20 sites in a dental office would not exceed the threshold limit value for eye exposure during a 7-hour workday even if personnel were continuously in proximity to the device (Table 1); eye protection would further reduce the risk. Manufacturers must also provide information on the potential for their devices to generate ozone and requirements for adequate ventilation to prevent ozone accumulation. Reference Blatchley, Brenner and Claus2
Our study has some limitations. Although there were no reports of adverse effects, there remains a need for additional studies to assess the safety of far UV-C for long-term use in occupied areas. The testing with aerosolized bacteriophage MS2 was conducted in a small test room (23.5 m3) with ∼4 air changes per hour and results might differ in a larger, well-ventilated room. Only one organism was tested in the treatment room. However, in preliminary experiments, vancomycin-resistant Enterococcus was reduced by >3 log10 in 45 minutes, and Clostridioides difficile spores were reduced by ∼1 log10 in 24 hours.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/ice.2024.109.
Acknowledgments
We thank Sterilray, Inc. for providing the far-UV-C device used for testing and the dental office staff for helpful discussions regarding their experience with the technology.
Financial support
Supported by the Department of Veterans Affairs.
Competing interests
C.J.D has received research grants from Clorox and Pfizer. All other authors report no conflicts of interest relevant to this article.
