Mobile UV disinfection units [Pros and cons]

Generation of UV and its dangers

Currently, four types of light sources are used to emit UV-C light, as indicated in . Mercury-vapour lamps are the conventional choice but they are being replaced by newer generations of UV-C instruments. These newer lamp types do not contain mercury, which makes them more environmentally friendly [20]. Apart from that, LEDs and pulsed-xenon lamps, unlike mercury-vapour lamps, have no warm-up time [20,21]. Lastly, the output of mercury-vapour lamps varies noticeably with temperature whereas this effect is much smaller for the other light sources [19,22]. Mercury-vapour lamps can be divided into three classes, namely low-pressure (LP), medium-pressure (MP), and high-pressure (HP) lamps, of which the LP lamps have the highest UV-C efficiency and are thus most used [23]. These classes have different emission spectra, as can be appreciated in [9,23,24]. The values for LP lamps indicate that almost all the light is emitted at 185 nm or 254 nm, with a peak emission of 254 nm. MP and HP lamps, on the other hand, emit a discontinuous spectrum. For LEDs, the light emission peak can be modified by the manufacturer to a value between 255 and 275 nm [21]. By using LEDs at different wavelengths in one system, a wavelength spectrum can be emitted. Pulsed-xenon lamps always generate a spectrum, ranging between 200 and 1000 nm, which often undergoes filtering to emit mainly UV-C light [21,25]. Lastly, there are different types of excimer lamps, all with their characteristic spectrum. For UV-C disinfection, krypton chloride (KrCl) lamps that emit light at 222 nm are the most common [21]. This is known as far UV-C technology and is a relatively new disinfection method with limited knowledge about its effectiveness. This makes it riskier to completely rely on this technology for disinfection in the hospital and it is therefore not widely used [21]. However, it also has benefits such as a reduced risk for use near people due to a lower penetration depth into the skin and eyes [26]. Although the FDA also mention this potential, they commented that currently there are not enough long-term data available to be sure of its long-term effects on people [25]. Furthermore, ozone will be produced due to the emittance of wavelengths <240 nm, resulting in other health risks [21,26].

Table II

Another difference between the light sources is the emittance of continuous or pulsed light. According to some investigators, pulsed light is preferred since it has a higher penetration depth and can produce higher light energy, increasing intensity [27]. In research, the results on pulsed versus continuous UV-C disinfection efficiency vary. When comparing pulsed and continuous light it is important to keep other variables such as wavelength and dose constant. Nyangaresi et al. and Sholtes et al. both found that pulsed or continuous light emitted by LEDs led to comparable log10 reductions [15,28]. Luo et al., on the other hand, found pulsed light, at certain frequencies, to be more bactericidal [29]. Further, Nyangaresi et al. found that pulsed and continuous irradiation led to comparable photoreactivation [15].

UV-C light and ozone may be associated with human health risks. UV-C light poses a risk to the skin and eyes, whereas ozone is mainly harmful to the respiratory tract. Both can be solved by using a UV-C disinfection system that does not require human intervention. Otherwise, protective clothing can be worn against UV-C light [16]. In case ozone is not required for disinfection, a modified lamp can be used. For mercury-vapour lamps, doped quartz glass or specialized soft glass can filter out short-wave UV-C light. For pulsed-xenon, doped quartz can be used as well [30].

Advantages and disadvantages

UV-C has promising features for disinfection such as automatic disinfection, being less time-consuming than widely used manual or chemical disinfections, leaving no harmful residuals, and being environmentally friendly (if no mercury-vapour lamps are used) [31,32]. However, it also has limitations which can be found in together with their proposed (partial) solutions.

Table III

The first difficulty is that UV-C light needs a direct path to an object to be able to disinfect it. Thus, additional dirt on the surface is inconvenient but it can often be solved by a water-based pre-cleaning. However, it is also possible that the light becomes obstructed by other objects or that it only irradiates one side of an object. This is known as shadowing and indicates the increased risk of active micro-organisms remaining in non-illuminated areas. A first solution is using a reflective chamber in which the object disinfection takes place or placing reflective surfaces in a room that needs disinfection. In this way, UV-C rays are reflected, resulting in additional pathways and thus reduced shadowing [32]. However, one should be aware of the negative effect on UV intensity of this extra distance caused by this reflection step. Another option is combining UV-C disinfection with ozone disinfection. As mentioned, ozone is produced when the emitted light spectrum contains wavelengths under 240 nm, and may be helpful since it can freely move into the shadows to ensure disinfection [33]. The surface topography can also influence disinfection performance. First, high surface roughness or irregularities can cause shadowing. Second, these irregularities lead to an increase in effective surface area and thus higher required energy to maintain the intensity. After all, intensity is defined as energy per effective area [13]. The dose, and thus the achieved inactivation, would decrease if the intensity were not preserved under unchanged exposure time.

Compared to other disinfection techniques, UV-C disinfection has a low penetration depth. It is speculated that this depth increases when using pulsed light [27]. However, there is still no consensus in the literature regarding the preference for pulsed or continuous light.

Other difficulties surrounding UV-C disinfection technology are the lack of a uniform standard applied for commercially available devices, the uncertainty about delivered doses, and the rather low quality of available research. This last drawback became clear while reviewing the literature but is also reported by others [[34], [35], [36]]. The quality is often low due to low sample sizes, bias, and conflict of interest. Further, it is also challenging to draw definite conclusions from literature data due to the heterogeneity across studies, rendering the merge of data difficult [[34], [35], [36]].

Standardization and efficacy

UV-C disinfection standardization is a new field, resulting in a variability of standards being used by providers to prove efficacy. However, not all standards have the same evidential value. In Europe, EN 14885 is often used, although it has been developed for chemical disinfectants. It discusses the test standards and required log10 reductions to claim bactericidal, virucidal, fungicidal, sporicidal, and mycobactericidal activity [37]. If this standard is followed entirely and properly it can be used to claim high-level disinfection [38]. However, it is unclear how this standard and its corresponding tests are adapted to UV-C disinfection by companies that use it. Therefore it is uncertain whether the evidential value of the standard remains as powerful for UV-C disinfection as for chemical disinfectants. For example, it does not take the risk of repair after UV-C disinfection into account, which can impact the final inactivation efficacy and thus the classification as a high-level, intermediate-level, or low-level disinfectant. However, there are superior test standards available that apply specifically to UV-C disinfection, namely the ASTM E3135-18 for general use and the ASTM E3179-18 for the influenza virus. They define how to test UV-C light against micro-organisms on carriers but they do not describe how to handle shadowing [39,40]. The results of these tests can be compared to the requirements set by the US Environmental Protection Agency and the US Food and Drug Administration (FDA) to evaluate disinfection performance. For low-level disinfection a 3-log10 reduction for viruses and a 5-log10 reduction for bacteria are dictated [41]. For high-level disinfection a 6-log10 reduction of mycobacteria is required [42]. One should be aware that manufacturers can select for themselves the micro-organisms to be tested from a list available in the E3135-18 standard. It is important to ask the manufacturer against the micro-organisms its UV-C device has passed compliance with the standard. This can make a difference for a specific healthcare institution aiming to remediate a micro-organism specific problem.

A recent standard EN 17272:2020, derived from the French standard NFT 72–281, considers tests to be used by suppliers of automated airborne disinfection systems to claim defined antimicrobial activity. The standard focuses on devices distributing chemicals by air diffusion and describes requirements such as uniformity of biocide distribution, total airborne disinfection contact time, the specific micro-organisms used and the test volumes of the enclosure. The objective of the described processes of distribution of chemicals by air diffusion is to disinfect the surfaces of the overall area including the external surfaces of the equipment contained in such rooms (distribution test) [43].

The most recent standard, BS 8628:2022, is specifically developed to cover the requirements and methodology for testing the efficacy of UV devices. The standard is based on EN 17272:2020 with some minor differences specific to UV devices. By contrast with EN 17272:2020, the method consists of only the efficacy test. The distribution test is not performed since UV devices are not intended to decontaminate the whole room but rather ‘close by’ surfaces [44].

The reported efficacy in literature varies, partially due to the heterogeneity that is present in UV-C research. After all, the large diversity in UV-C disinfection devices leads to different applications and parameter values. This makes it difficult to compare effectiveness across studies [34]. However, Alvarenga et al. state, in their systematic review, that most studies report a 1- to 2-log10 reduction for UV-C disinfection combined with manual precleaning for surfaces. Other review articles also recommend or describe UV-C disinfection rather as a supplementary method to standard cleaning than as a stand-alone disinfection procedure [35,45]. Further, Alvarenga et al. mention that in laboratory experiments disinfection between 2- and 6-log10 was obtained. However, in real life, the achieved reductions might be lower due to suboptimal conditions including the presence of shadows and larger distances between the surface and the lamp [34]. It is therefore recommended to verify whether the UV-C device performs as well for the intended application as under test conditions, since RH and temperature can vary [46].

Commercial qualitative colorimetric indicator cards are available to assess delivery of UV-C light on surfaces. Although they provide rapid and easy-to-interpret information in a rather cheap manner, they provide only rough estimates of UV-C delivery. They confirm that in-use devices are working correctly and are helpful during initial training and verification for determination of the ideal cycle time and positions of the devices in a specific setting. When looking for a reliable colorimetric indicator when using a pulsed xenon UV-C device, it is important to check their susceptibility to change colour by UV-A and UV-B light exposure [14,47].

Some disinfection devices make use of a built-in dose-monitoring system that continuously measures the UV-C dose given during the disinfection cycle [4]. This system is used to replace colorimetric indicator cards. An example of how this can be achieved is by using two photodiodes for monitoring and a third independent photodiode for validation of the disinfection cycle. However, no irrefutable proof of the proper functioning of these types of systems was found during the literature search.

For semi-critical devices, high-level disinfection is recommended [42]. Although some UV-C disinfection companies claim to offer this, no definite proof of high-level disinfection with UV-C systems was found in the literature. There are studies reporting up to 7-log10 reductions for artificially contaminated flexible endoscopes [48]. However, they often do not take the risk of genomic repair into account or have a conflict of interest. Furthermore, UV-C light is not FDA-cleared for high-level disinfection and the US Centers for Disease Control and Prevention only mentions it in non-critical applications [49,50]. It should be borne in mind that these latter sources have not been updated since 2019 and 2016, respectively.