The current Covid-19 outbreak has prompted a growing interest into novel disinfection technology. Notably, “no-touch” automated room disinfection technologies received a lot of attention: in addition to our own ozone-based STERISAFE PRO unit, hydrogen peroxide foggers and UV light robots are amongst the lead actors in the sector. This short review focuses on the UV-based solutions, and aims to provide a basic understanding of the technology and a head-to-head comparison with the STERISAFE PRO.
What is UV?
Ultraviolet (UV) light is a part of the light spectrum invisible to the human eye. It is typically categorized in three types depending on the wavelength range: UVA, UVB, and UVC. The only type used for biocidal purposes is the UVC light (200-280 nm), which is known to cause cellular damage at the DNA or RNA level, causing the inactivation of microorganisms. Additionally, UVC is known to be able to oxidize other cellular targets such as cell walls. Producing UVC light can be done through various means and types of light bulb, which differ in their intensity, emission type (e.g. continuous vs pulsed), and ultimately, in their efficacy. The light-based action of UV technology causes an evident shortcoming: it cannot disinfect areas it cannot “see”. This is called the “shadow effect”, where in opposition to gas-based solutions such as ozonation, a UV device will never be able to disinfect every nook and cranny of a room. While this is the main and most obvious disadvantage of the technology, other aspects exist which could be of valid concern. Those concerns are mainly tied to the mode of action of UV light, which efficacy is highly dependent on (1) intensity and dose of the UV light, (2) distance between the light source and the surface to decontaminate, and (3) exposure time.
Those three factors are inevitably tied, as a shorter distance between the light source and the target surface will provide higher intensity, but will also treat a smaller surface area; and the needed exposure time will consequently have to be considered for all treated surfaces. It is thus of utmost importance to make sure that UV light-based disinfection systems are used properly to ensure adequate disinfection. To do so, proper monitoring of the important parameters (distance to target surfaces, the exposure time for all surfaces) is crucial at all times during the process – with STERISAFE PRO, this constant, real-time monitoring is provided by the integrated ozone and humidity detectors. Most serious companies will of course have conducted third-party efficacy tests to validate their claims, but this is not a legal requirement at the European level, as under most circumstances UV light does not fall under the Biocidal Product Regulation (BPR). And the controlled conditions of such tests are near impossible to replicate in real-life scenarios.
UV as a surface disinfectant
More particularly, UV efficacy claims (if any) are based on test results made from a fixed distance (typically 30cm, 50cm, or 1m). But as light obeys an inverse square law, the intensity that UV lights will drop exponentially with increasing distance. This means that as distance doubles, the remaining intensity to reach the target surface is only a quarter. This can be seen in Figure 1: from 1.2m away from its source, UV intensity falls to only 6.3% of the initial intensity. Because of this particularity, the intensity loss equals a loss of biocidal efficacy; as such, manufacturers typically advertise log reduction ranging between 2 and 4, a far cry from a log 6 reduction that can be seen in other technologies (link to our efficacy table). Additionally, to be efficient a static UV light system has to be placed at 3 or 4 different positions within a room, as a way to ensure a proper covering of the surfaces. While a single cycle time is indeed fast for UV-light and is usually set around 15min, the total cycle time for a single room has to be multiplied by the number of different positions.
Figure 1. UV light intensity drops with increasing distance from the source, obeying an inverse square law (I correspond to Intensity, d to distance from the source)
Automation of the UV process was developed to circumvent this issue. However, while more practical in terms of autonomy and general coverage, the inherent limitations of UV-based systems cannot be bypassed and ensuring full intensity on all surfaces is still close to infeasible. Figure 2 illustrates this problem, where a comparison is made between a passive UV system, an automated process, and a STERISAFE PRO cycle. Shadow effects are well-illustrated, as is the difference in exposure levels between different surfaces. As lower exposure levels could lead to lower inactivation rate, it is important to note that some organisms also possess mechanisms to deal with UV-damaged DNA, the most important being called photoreactivation. As with all types of biocides, partial disinfection can be dangerous, leading to increased resistance. As such, a proper standard operating procedure (SOP) is essential when using UV-based technology. Automated UV solutions typically rely on artificial intelligence (AI) for this, while passive solutions require a properly trained operator.
Figure 2. Exposure comparison between (1) passive UV-based technology (left); (2) automated UV-based technology; and (3) a STERISAFE PRO cycle.
In addition to the shortcomings associated with being a light source, UV-based technology share some issues with other types of disinfection. Manufacturers of UV light devices often claim that it does not generate direct by-products; however, by-products will still be formed upon reacting with UVC light, depending on the type of compounds already present in the room to be treated. Some organic compounds partially oxidize in the presence of UV light producing what are known as Disinfection By-Products (DBPs). This includes gaseous compounds such as formaldehyde, benzaldehyde, acetic acid, acetaldehyde, formic acid, and carbon monoxide. Fine particulate, similar to the smog pollution seen in dense urban areas, will also be formed. Particulate and gaseous species are responsible for what is known as Sick Building Syndrome (SBS) which is reliable for headaches, sluggish work performance, lack of concentration, and increased sick days for workers.
The production of such compounds is most obvious by the residual smell that is sometimes present after a UV-based treatment – burned particles of hair or skin will be present in the air, for example. Depending on the initial conditions, the intensity of the residual smell can be strong enough to incur some discomfort from the workers. It is important to note that virtually all disinfectants, ozone included, possess the potential to react with organic compounds initially present on a treated surface or in the air. But while STERISAFE PRO is equipped with an electrostatic precipitator (ESP) to filter out those particle residues to reduce them along with some gaseous species to safe levels, to our knowledge no UV-based has this feature (see our white paper on the subject at https://sterisafe.eu/direct-handling-of-disinfection-products-dangers-and-solution/). Moreover, as the case is with virtually every biocide, what is harmful to microorganisms will also be harmful to humans. The UVC light can penetrate the skin and thus, can cause damage to people exposed to it; damage to the skin and to the retina are potentially listed dangers. Different types of UVC can have different consequences or variable dangers, so it is important to know the safety measures associated with the technology.
STERISAFE PRO vs. UV technology
To recap, as with ozone or hydrogen peroxide, UV-based disinfection technology presents both its own advantages and disadvantages, and when it comes to choosing a solution, it is important to be aware of the particularities of each technology. Compared to its competitors, UV light will have an edge on treatment time and ease of use concerning room preparation. However, the STERISAFE PRO and its ozone-based technology will always boast much more reliable results due to its constant, real-time monitoring of its biocidal parameters and a guarantee that virtually every surface is treated equally within a treated room.
Comparison chart between technologies
REFERENCES
1. Byrns et al. (2017). The uses and limitations of a hand-held germicidal ultraviolet wand for surface Disinfection. Journal of Occupational and Environmental Hygiene 14 (19): 749-757. doi: 10.1080/15459624.2017.1328106
2. Lindblad et al. (in press). Ultraviolet-C decontamination of a hospital room: Amount of UV light needed. Burns. doi: https://doi.org/10.1016/j.burns.2019.10.004
3. Ng et al. (2007). Secondary organic aerosol formation from m-xylene, toluene, and benzene. A tmospheric Chemistry and Physics Discussions 7 (2): 4085-4126
4. NHS Scotland (2019). Literature Review and Practice Recommendations: Existing and emerging technologies used for decontamination of the healthcare environment – Ultraviolet Light. Health Protection Scotland, https://www.hps.scot.nhs.uk/web-resources-container/literature-review-and-practice-recommendations-existing-and-emerging-technologies-used-for-decontamination-of-the-healthcare-environment-uv-light/
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6. Yang et al. (2019). Effectiveness of an ultraviolet-C disinfection system for reduction of healthcare-associated pathogens. Journal of Microbiology, Immunology and Infection 52 (3): 487-493. doi: 10.1016/j.jmii.2017.08.017
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