STUDY: Germicidal Efficacy and Mammalian Skin Safety of 222nm UVC Light

Our studies suggest that far-UVC light (200–225 nm), unlike conventional UV germicidal lamps, has considerable promise to be a safe and inexpensive modality for SSI reduction, while being cytotoxic to both drug-resistant and drug-sensitive microbes.

STUDY: Germicidal Efficacy and Mammalian Skin Safety of 222nm UVC Light

Reprinted from: Columbia University APRIL 2017

Authors: Manuela Buonanno, Brian Ponnaiya, David Welch, Milda Stanislauskas, Gerhard Randers-Pehrsona, Lubomir Smilenov, Franklin D. Lowy, David M. Owensb and David J. Brenner


The use of ultraviolet (UV) light for inactivating bacteria and viruses is well established (1, 2). However, UV radiations emitted by typical germicidal lamps with a peak emission at 254 nm represent a human health hazard, causing skin cancer (3, 4) and cataracts (5, 6).

We have developed an approach to kill bacteria without harming human cells in skin tissue models (7) and mouse skin in vivo (8) that employs single-wavelength UVC light generated by inexpensive filtered excilamps (9). The approach is based on the limited penetration distance of UVC light in the wavelength range of 200–222 nm in biological samples. Specifically, while far-UVC light has enough range to traverse microbes that are much smaller in size than human cells [less than 1 μm in diameter (10, 11), compared to the diameter of typical human cells ranging from about 10–25 μm (10)], it is strongly absorbed by the proteins in the cytoplasm of human cells (12, 13) and is drastically attenuated before reaching the human cell nucleus. It follows that far-UVC light is not able to penetrate the stratum corneum of skin and reach the underlying critical basal cells or melanocytes (4).

Another organ especially sensitive to UV damage is the lens; however, the lens is positioned distal to the cornea that is sufficiently thick [~500 μm (14))]. Therefore, penetration of farUVC ~200-nm light through the cornea to the lens is predicted to be essentially zero (15).

The potential use of UVC light for microbe sterilization purposes in the presence of humans paves the way to numerous clinical applications, including reduction of surgical site infections (SSI) that are the second most common healthcare-associated infections resulting in read-missions, prolonged hospital stays, increased morbidity and mortality, and an overall higher medical cost (16, 17). A key factor contributing to the severity of SSI is the incidence of drug-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) (18, 19).

To address the issue of reducing SSI, we have developed an approach that involves the use of inexpensive excimer lamps that, appropriately filtered, emit monoenergetic wavelengths in the far-UVC range. A crucial property of UVC-mediated germicidal killing is that it is essentially independent of acquired drug resistance (20, 21).

We have previously shown that 207-nm light emitted by a filtered krypton-bromine (Kr-Br) excilamp has bactericidal efficacy while being minimally cytotoxic to human cells in a 3D skin tissue model in vitro (7) and in a hairless mouse skin model in vivo (8). Thus, continuous exposure of the wound to far-UVC light during surgery may inactivate the microbes alighting directly onto the surgical wound from the air. If proven to be safe to eyes as well as skin, continuous operation of far-UVC light would not require the use of cumbersome protective clothing, hoods and eye shields for the surgical staff and the patient (22, 23).

Here we extended those studies to a filtered krypton-chlorine (Kr-Cl) excimer lamp that produces essentially monoenergetic UV light at 222 nm and established that a wavelength window exist in the far-UVC region (from 200–222 nm) that inactivates bacteria efficiently but is not cytotoxic or mutagenic to mammalian cells. Buonanno et al. Page 2 Radiat Res. Author manuscript; available in PMC 2017 August 10. Author Manuscript Author Manuscript Author Manuscript Author Manuscript We describe measurements of MRSA survival and of typical UV-induced premutagenic DNA lesions in a 3D human skin model immediately after exposure to different fluences of 222-nm light. We compared the results to those measured in samples exposed to similar fluences from a typical germicidal lamp emitting at 254 nm. We further tested eight cellular and molecular endpoints relevant to skin damage in vivo in dorsal skin of hairless mice 48 h after exposure to 222-nm or 254-nm light used as positive control (8), at a fluence at which the 254-nm light is predicted to produce a significant decrease in SSI rates (22).

In agreement with our previous results using 207-nm light (7, 8), here we showed that 222- nm light has similar antimicrobial properties as a conventional germicidal lamp but without causing mammalian skin damage. The finding that a far-UVC wavelength window (200–225 nm) is differentially cytotoxic to bacteria relative to mammalian cells is novel and can be used for various applications that would require the presence of humans, including room sterilization and reduction of surgical site infections, without the need of additional personal protective equipment. MATERIALS AND METHODS UV Lamps We used an excimer lamp based on a krypton-chlorine (Kr-Cl) gas mixture that emits principally at 222 nm. The lamp (High Current Electronics Institute, Tomsk, Russia) was air cooled with a 6,000-mm2 exit window (24). A custom bandpass filter (Omega Optical, Brattleboro, VT) was used to remove essentially all but the dominant 222-nm wavelength emission. A UV spectrometer (Photon Control, BC, Canada) sensitive in the wavelength range from 200–360 nm was used to characterize the wavelength spectra emitted by the excimer lamp, and a deuterium lamp standard with a NIST-traceable spectral irradiance (Newport Corp, Stratford, CT) was used to calibrate the UV spectrometer.

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