Airborne-mediated microbial diseases represent one of the major challenges to worldwide public health1. Common examples are influenza2, appearing in seasonal3 and pandemic4 forms, and bacterially-based airborne-mediated diseases such as tuberculosis5, increasingly emerging in multi-drug resistant form.
A direct approach to prevent the transmission of airborne-mediated disease is inactivation of the corresponding airborne pathogens, and in fact the airborne antimicrobial efficacy of ultraviolet (UV) light has long been established6,7,8. Germicidal UV light can also efficiently inactivate both drug-sensitive and multi-drug-resistant bacteria9, as well as differing strains of viruses10. However, the widespread use of germicidal ultraviolet light in public settings has been very limited because conventional UVC light sources are a human health hazard, being both carcinogenic and cataractogenic11,12.
By contrast, we have earlier shown that far-UVC light generated by filtered excimer lamps emitting in the 207 to 222 nm wavelength range, efficiently inactivates drug-resistant bacteria, without apparent harm to exposed mammalian skin13,14,15. The biophysical reason is that, due to its strong absorbance in biological materials, far-UVC light does not have sufficient range to penetrate through even the outer layer (stratum corneum) on the surface of human skin, nor the outer tear layer on the outer surface of the eye, neither of which contain living cells; however, because bacteria and viruses are typically of micron or smaller dimensions, far-UVC light can still efficiently traverse and inactivate them13,14,15.
The earlier studies on the germicidal efficacy of far UVC light13,15,16,17,18 were performed exposing bacteria irradiated on a surface or in suspension. In that a major pathway for the spread of influenza A is aerosol transmission3, we investigate for the first time the efficacy of far-UVC 222-nm light for inactivating airborne viruses carried by aerosols – with the goal of providing a potentially safe alternative to conventional 254-nm germicidal lamps to inactivate airborne microbes.
Figure 1 shows representative fluorescent 40× images of mammalian epithelial cells incubated with airborne viruses that had been exposed in aerosolized form to far-UVC doses (0, 0.8, 1.3 or 2.0 mJ/cm2) generated by filtered 222-nm excimer lamps. Blue fluorescence was used to identify the total number of cells in a particular field of view, while green fluorescence indicated the integration of live influenza A (H1N1) viruses into the cells. Results from the zero-dose control studies (Fig. 1, top left) confirmed that the aerosol irradiation chamber efficiently transmitted the aerosolized viruses through the system, after which the live virus efficiently infected the test mammalian epithelial cells.
Figure 1 :
Figure 2 shows the surviving fraction, as a function of the incident 222-nm far-UVC dose, of exposed H1N1 aerosolized viruses, as measured by the number of focus forming units in incubated epithelial cells relative to unexposed controls. Linear regressions (see below) showed that the survival results were consistent with a classical exponential UV disinfection model with rate constant k = 1.8 cm2/mJ (95% confidence intervals 1.5–2.1 cm2/mJ). The overall model fit was good, with a coefficient of determination, R2 = 0.95, which suggests that most of the variability in virus survival was explained by the exponential model. The rate constant of 1.8 cm2/mJ corresponds to an inactivation cross-section (dose required to inactivate 95% of the exposed viruses) of D95 = 1.6 mJ/cm2 (95% confidence intervals 1.4–1.9 mJ/cm2).
We have developed an approach to UV-based sterilization using single-wavelength far-UVC light generated by filtered excilamps, which selectively inactivate microorganisms, but does not produce biological damage to exposed mammalian cells and tissues13,14,15. The approach is based on biophysical principles in that far-UVC light can traverse and therefore inactivate bacteria and viruses which are typically micrometer dimensions or smaller, whereas due to its strong absorbance in biological materials, far-UVC light cannot penetrate even the outer dead-cell layers of human skin, nor the outer tear layer on the surface of the eye.
Here we applied this approach to test the efficacy of the 222-nm far-UVC light to inactivate influenza A virus (H1N1) carried by aerosols in a benchtop aerosol UV irradiation chamber, which generated aerosol droplets of sizes similar to those generated by human coughing and breathing. Aerosolized viruses flowing through the irradiation chamber were exposed to UVC emitting lamps placed in front of the chamber window.
As shown in Fig. 2, inactivation of influenza A virus (H1N1) by 222-nm far-UVC light follows a typical exponential disinfection model, with an inactivation cross-section of D95 = 1.6 mJ/cm2 (95% CI: 1.4–1.9). For comparison, using a similar experimental arrangement, but using a conventional 254 nm germicidal UVC lamp, McDevitt et al.19 found a D95 value of 1.1 mJ/cm2 (95% CI: 1.0–1.2) for H1N1 virus. Thus as we13,15 and others16,17,18 reported in earlier studies for bacterial inactivation, 222-nm far-UVC light and 254-nm broad-spectrum germicidal light are also comparable in their efficiencies for aerosolized viral inactivation. Other recent work comparing viral inactivation across the UVC spectrum has shown variations in efficiency are expected, but in general both regions of the spectrum are effective in inactivation, though the precise cause of inactivation may differ20,21. However as discussed above, based on biophysical considerations and in contrast to the known human health safety issues associated with conventional germicidal 254-nm broad-spectrum UVC light, far-UVC light does not appear to be cytotoxic to exposed human cells and tissues in vitro or in vivo13,14,15.
If these results are confirmed in other scenarios, it follows that the use of overhead low-level far-UVC light in public locations may represent a safe and efficient methodology for limiting the transmission and spread of airborne-mediated microbial diseases such as influenza and tuberculosis. In fact the potential use of ultraviolet light for airborne disinfection is by no means new, and was first demonstrated more than 80 years ago8,22. As applied more recently, airborne ultraviolet germicidal irradiation (UVGI) utilizes conventional germicidal UVC light in the upper part of the room, with louvers to prevent direct exposure of potentially occupied room areas23. This results in blocking more than 95% of the UV radiation exiting the UVGI fixture, with substantial decrease in effectiveness24. By contrast, use of low-level far-UVC fixtures, which are potentially safe for human exposure, could provide the desired antimicrobial benefits without the accompanying human health concerns of conventional germicidal lamp UVGI.
A key advantage of the UVC based approach, which is in clear contrast to vaccination approaches, is that UVC light is likely to be effective against all airborne microbes. For example, while there will almost certainly be variations in UVC inactivation efficiency as different influenza strains appear, they are unlikely to be large7,10. Likewise, as multi-drug-resistant variants of bacteria emerge, their UVC inactivation efficiencies are also unlikely to change greatly9.
In conclusion, we have shown for the first time that very low doses of far-UVC light efficiently inactivate airborne viruses carried by aerosols. For example, a very low dose of 2 mJ/cm2 of 222-nm light inactivates >95% of airborne H1N1 virus. Our results indicate that far-UVC light is a powerful and inexpensive approach for prevention and reduction of airborne viral infections without the human health hazards inherent with conventional germicidal UVC lamps. If these results are confirmed in other scenarios, it follows that the use of overhead very low level far-UVC light in public locations may represent a safe and efficient methodology for limiting the transmission and spread of airborne-mediated microbial diseases. Public locations such as hospitals, doctors’ offices, schools, airports and airplanes might be considered here. This approach may help limit seasonal influenza epidemics, transmission of tuberculosis, as well as major pandemics.