Severe acute respiratory coronavirus 2 (SARS-CoV-2) can be transmitted from one person to one or more people through viral transmission in airborne particles.

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The high levels of SARS-CoV-2 transmission, as has been well documented, have overwhelmed national healthcare systems, resulting in millions of deaths and long-term health problems.

As a result, it is clear that reducing or eliminating SARS-CoV-2 transmission is a major and unprecedented global challenge. Transmission control measures have varying degrees of success and each has its own set of challenges. Although the evidence of vaccination’s efficacy in reducing disease transmission is unclear, it has been one of the most effective methods in minimizing death and serious illness.

Other than COVID-19, airborne transmission has been identified as a key mechanism for a variety of other viral infections, including measles, influenza, other human coronaviruses (Middle East Respiratory Syndrome MERS-CoV, SARS-CoV), and Respiratory Syncytial Virus (RSV), as well as bacterial infections such as tuberculosis and some pathogens responsible for hospital-acquired infections.

Germicidal ultraviolet (GUV) is a control measure that meets above the criteria and has a proven track record in the scientific community. Accidental human exposure to conventional 254 nm GUV, on the other hand, poses a significant challenge, as it can cause painful sunburn-like reactions in the skin and cornea.

“Far-UVC,” germicidal ultraviolet-C radiation with wavelengths ranging from 200 to 230 nm, is one possible solution, which are Krypton chloride (KrCl) excimer lamps. The primary emission wavelength is 222 nm. The low residual emission all through the ultraviolet region of the electromagnetic spectrum is a common source of Far-UVC.

KrCl excimer lamps have been shown to inactivate gram-negative and gram-positive bacteria, drug-resistant bacteria, human coronaviruses, including the SARS-CoV-2 virus, and influenza viruses in laboratory experiments.

While the laboratory results are promising, the inactivation of a pathogen in a restricted bench-scale laboratory environment does not always imply lowered disease transmission when the technology is used in the real world.

Experiments in huge, room-sized aerosol chambers are a translational step forward into real-world studies. This can also give insight into how to use technology in rooms where an infectious person is likely to be present for a long time.

The current research, for the first time, investigates the efficiency of Far-UVC for inactivating an airborne pathogen in a full-scale room-sized bioaerosol chamber under steady-state conditions.

Results

The ceiling of a room-sized bioaerosol chamber at the University of Leeds was secured with five Krypton Chloride excimer lamps, filtered to decrease ultraviolet emissions at wavelengths longer than 230 nm. The lamps were positioned in a quincunx pattern (Figure 1), with the emission aimed at the floor.

Figure 1. 3D schematics of the bioaerosol chamber configuration showing room dimensions, the position of the lamps, pathogen source, and collection point (top) with an illustrative example of the Far-UVC lamp emissions (bottom). Image Credit: Eadie, et al., 2022

A constant release of aerosolized Staphylococcus aureus has been introduced to the room at a height of 168 cm in a mechanically ventilated 32 m3 chamber. The rate of ventilation is three air changes per hour (ACH). Following a 60-minute stabilization period, 10 air samples were collected over 50 minutes. The sampling was then continued for another 50 minutes with either one or five Far-UVC sources turned on.

The evaluation was repeated three times with three distinct lamp exposure rates (Table 1). The International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines for optical radiation exposure were used to determine the exposure rates.

Table 1. Average percentage pathogen reduction, irradiance, and calculated 8-hour exposure dose for three different exposure conditions at two heights from the ground. Source: Eadie, et al., 2022

Peak values Average values Average % pathogen reduction (SD) Height = 1.7 m Height = 1 m Height = 1.7 m Height = 1 m Irradiance

(µWcm-2) 8-h dose

(mJcm-2) Irradiance

(µWcm-2) 8-h dose

(mJcm-2) Irradiance

(µWcm-2) 8-h dose

(mJcm-2) Irradiance

(µWcm-2) 8-h dose

(mJcm-2) High 1 lamp 14.4 415 1.93 56 0.57 16.5 0.45 12.9 93.7**** (1.0) 5 lamps 14.4 415 3.42 98 2.73 78 2.01 58 98.4**** (0.7) Medium 1 lamp 0.92 26.5 0.13 3.7 0.03 0.87 0.03 0.82 65.9**** (4.0) 5 lamps 0.92 26.5 0.22 6.3 0.14 4.1 0.13 3.67 92.0**** (0.9) Low 1 lamp 0.09 2.65 0.01 0.37 0.003 0.09 0.003 0.08 12.8 ns (3.8) 5 lamps 0.09 2.65 0.02 0.63 0.01 0.41 0.01 0.37 28.7** (3.4)

The bold, italicised 8-h exposure values are above the ICNIRP 222-nm exposure limit of 23 mJcm-2. No exposures exceeded the 2022 ACGIH threshold limit value for skin of 478 mJcm-2 at 222 nm. Statistical significance is represented by: ns = p > 0.05, * = p ≤ 0.05, ** = p ≤ 0.01, *** = p ≤ 0.001, and **** = p ≤ 0.0001.

Before and after the lamps were turned on, the concentration of viable S. aureus pathogens in the air at the collection location was serially sampled. Figure 2 and Table 1 show the results in colony-forming units per cubic meter (cfu m-3) for the 45 minutes prior to “lamp on,” as well as serially for the 50 minutes after “lamp on.”

Figure 2. Percentage of viable airborne S. aureus remaining plotted on a logarithmic y-axis against time after switch-on of the Far-UVC sources for three different exposure scenarios—high (top), medium (middle), and low (bottom). Note that the pathogen was continuously released into the room throughout the experiment: The studies were undertaken using either a single central lamp (green, square data points, dashed lines) or all five Far-UVC lamps (blue, circular data points, solid lines). Image Credit: Eadie, et al., 2022

The “High” exposure scenario, as expected, resulted in the greatest reduction in the steady-state airborne viable S. aureus load. Despite the fact that the single lamp did not irradiate the entire room, the chamber’s good air mixing is likely to have resulted in this significant effect.

Using all five lamps, the “Medium” exposure scenario showed a 92% decrease in the steady-state viable pathogen load, with a maximum 8-hour exposure dose driven by the present ICNIRP guideline exposure limit at 222 nm of 23 mJcm-2.

The “Low” exposure scenario, which used very low intensity Far-UVC exposure rates, reduced viable pathogen load by 13% (one lamp) and 29% (five lamps).

Discussion

The study, published in Scientific Reports, displayed the capability of Far-UVC to rapidly reduce airborne pathogens in a realistically sized room with normal ventilation and a constant source of airborne pathogens for the first time. A 98% reduction was demonstrated at the ACGIH threshold limit values in less than 5 minutes. Figure 3 depicts a comparison of the two scenarios described.

Figure 3. Percentage of viable airborne S. aureus remaining plotted on a linear y-axis for two of the exposure scenarios motivated by ICNIRP guideline exposure limits (5 lamps “Medium”) and ACGIH Threshold Limit Values (5 lamps “High”). Note that the pathogen was continuously released into the room throughout the experiment with a mechanical ventilation rate of 3 air changes per hour. Image Credit: Eadie, et al., 2022

Aerosolized S. aureus pathogen has been used as a surrogate for more relevant airborne viruses such as human coronaviruses and influenza viruses, despite the fact that SARS-CoV-2 was not used for safety reasons. The rationale for this can be seen in Figure 4, which compares Far-UVC inactivation rates of airborne influenza virus (H1N1), human coronaviruses (OC43 and 229E), and airborne S. aureus.

Figure 4. Inactivation of aerosolized human coronaviruses HCoV OC43 and HCoV 229E and H1N1 influenza virus at relevant low far-UVC doses, compared with aerosolized S. aureus with a Far-UVC lamp. Measurement taken at the Columbia University laboratory-based aerosolized pathogen irradiation system. HCoV OC43, HCoV 229E, and H1N1 influenza data were published previously and included for comparison. Image Credit: Eadie, et al., 2022

The findings show that airborne S. aureus is more resistant to Far-UVC inactivation than airborne influenza and human coronaviruses, implying that S. aureus is a conservative surrogate.

The peak lamp intensities in this study could have been five times higher than in the “Medium” scenario, improving inactivation while keeping the average 8-hour dose within ICNIRP guideline exposure limits.

This emphasizes the importance of proper Far-UVC installation to ensure that the designated space is properly and safely irradiated.

Methodology

Experiments were carried out in a bioaerosol chamber that was kept at a constant temperature. The chamber is mechanically ventilated and runs on negative pressure, with a HEPA-filtered fresh air system. It was important to avoid placing the bacteria release point and sample point directly under a Far-UVC source.

In practice, aerosolized level-3 pathogens such as SARS-CoV-2 could not be used in the bioaerosol chamber. S. aureus is also a pathogen of interest in and of itself because it is representative of Methicillin-resistant Staphylococcus aureus (MRSA), a pathogen that is common in hospital infections and is used as a standard for cleanliness.

It was also thought that assessing technology for pathogen inactivation well beyond the prevailing pandemic was crucial.

After irradiation on gelatin filters dissolved in 5 ml PBS, the airborne S. aureus was collected and inactivated using the colony-forming unit (CFU) assay on tryptone soy agar (TSA) plates.

Five commercially available Krypton Chloride excimer Far-UVC lamps were modified to include a diffusing material that broadened the Far-UVC distribution and increased the amount of irradiated volume.

A calibrated UVC radiometer was used to measure the irradiance (E) field in the chamber in the horizontal plane. Measurements were taken in 0.5 m intervals throughout the chamber at two heights (z) from the ground, 1.7 m and 1 m.

The production of aerosols was carried out in a controlled environment. Pathogen suspension in other materials (1% Foetal Bovine Serum) had no significant effect on the results, according to preliminary research.

An Anderson 6-stage impactor was used to collect the samples. After sampling, the agar plates were incubated for 24 hours at 37 °C. The number of colonies on each plate was then counted using the Gallenkamp colony counter.

Over the course of 60 minutes, the airborne Staphylococcus aureus was allowed to set up a steady-state within the chamber. The Far-UVC lamps were then turned on, and the sampling procedure was repeated.

Prior to turning on the Far-UVC devices, the concentrations of Staphylococcus aureus were normalized by equating them to the mean concentration of all samples to allow comparisons within and between experiments.

To compare viable pathogens before and 20 minutes after Far-UVC lamps were turned on, unpaired t-tests were used.

Conclusion

The findings provide some preliminary data for comparison with other technologies, such as portable air cleaners. Far-UVC may have an advantage over air cleaners and upper-room GUV in that it does not necessarily require “good” air mixing within the room. The researchers intend to work on this in the future.

More studies are needed to investigate the effects of variables such as humidity, temperature, proximity to the infectious source, and ventilation rates. However, the findings provide confidence that Far-UVC, when used appropriately and in accordance with the present (or future) safety regulatory limits, is likely to be an efficient, human behavior-independent control measure for inactivating key airborne pathogens such as human coronavirus and influenza.

Journal Reference

Eadie, E., Hiwar, W., Fletcher, L., Tidswell, E., O’Mahoney, P., Buonanno, M., Welch, D., Adamson, C. S., Brenner, D. J., Noakes, C., Wood, K. (2022) Far-UVC (222 nm) efficiently inactivates an airborne pathogen in a room-sized chamber. Scientific Reports, 12, p. 4373. Available Online: https://www.nature.com/articles/s41598-022-08462-z#citeas.

References and Further Reading