Study of UV-C LED Technology on In-door Air Disinfection: An Effective Mercury-free and Ozone-free Method to Irradicate Airborne Microorganisms

Abstract

The aim of this study was to evaluate the efficacy of a novel air sterilization system on airborne microorganisms in both lab envi-ronment and office environment. Here we used a UV-C LED based technique to continuously purify the air. It was observed that in lab testing environment, the disinfection rate of the UV-C LED system is 99.94% against staphylococci albus (8032) within a 20 m3 space in 2 hours, while 99.6%, 99.02% and 98.65% against natural microorganisms in 2 hours within 100 m3, 150 m3 and 210 m3 space respectively. Additionally, for a 5-day onsite testing conducted in normally operating offices, the results show a disinfection rate up to 92% against natural airborne microorganisms. The results also show zero ozone emission from the UV-C LED based air sterilization de-vices. This study demonstrates a promising role for this UV-C LED based novel technology in infection control and prevention by de-creasing the spread of airborne pathogens effectively and efficiently without introducing hazardous mercury or any chemicals.

CCS Concepts

Hardware → Emerging technologies∼Emerging optical and pho-tonic technologies.

Keywords：UV-C LED, air sterilization, mercury-free, ozone-free, infection control and prevention

In this study, the air sterilizer used for testing is designed and manufactured by MASSPHOTON Limited, Hong Kong. Different from the mercury lamp-based UV air sterilizers used and reported in previous studies, MASSPHOTON® air sterilizer utilizes UV-C LED as its germicidal light source. Figure 1 shows a front view picture of the sterilizer (left) and an exploded view of its interior composition (right). As illustrated in orange arrows, the untreated room air is drawn by the fans (4) and then goes through the inlet (6) and pre-filtration (5) to filter out large particles and dust in the air. Subsequently, air would go through the most critical part of the air sterilizer, the high UV reflective air duct (2) and UV-C LED module (10). The air duct (2) is specially designed to be most efficient for UV-C LEDs with high UV-C reflective interior coating. We selected UV-C LED with narrow beam shape which helps to collimate the light along the air duct as shown in Figure 2 (a). Figure 2 (b) illustrates the ray tracing simulation that light emitted by UV-C LEDs is collimated by the reflector and travels straight into the air duct, while the high angle light is reflected back into the air duct by its reflective interior surface. Therefore, the germicidal UV-C light is efficiently utilized and uniformly distributed in the air duct for effective sterilization.

Thanks to the small form factor of UV-C LEDs, such air duct has a much lower profile compared to its counterpart mercury lamp-based chambers. The UV-C LED module (10) is composed of 45 UV-C LEDs with peak wavelength at 275nm. These UV-C LEDs have a long lifetime over 10,000 hours; therefore, no replacement is necessary through its lifespan for 3 years. As the air is going through the air duct (2), germicidal light from UV-C LED module (10) is emitted into the duct therefore air is sterilized. Lastly, steril-ized air (illustrated as green arrows) comes out through the outlet (9) back into the room. For this study, the particular model we used is equipped with a panel lighting (7). The sterilizer units are all ceiling mounted in this study.

Ozone-free is among one of the most attractive characteristics of the UV-C LED technology. In this study, we measured the ozone concentration for 1 hour at a 5-minute interval in a 30 m3 test chamber using 106-L ozone monitor during operation of the air sterilizer.

To evaluate efficacy of the air sterilizer against airborne microor-ganisms, we firstly tested the sterilizer in a lab environment on staphylococci albus in a 20 m3 test chamber. Fresh slanted cultures of 4th generation Staphylococcus aureus (8032) were cultured at a constant temperature of 36℃ ± 1℃ for 18 hours, and diluted to the required concentration using nutrient broth medium. Testing is conducted under 25℃ room temperature and 50∼70% relative humidity. In the test chamber of the control group and the ex-perimental group, staphylococci albus is aerosol sprayed into the chamber with a fan stirring at the same time. After spraying the bacteria and resting for 5 minutes, samples before disinfection were taken respectively for the control group and the experimental group, which were used as positive controls before the start of the control group and before the disinfection of the experimental group. The MASSPHOTON® air sterilizer is then turned on for 2 hours. After the sterilization cycle, samples are taken for both control group and the experimental groups. Air samples are collected and incubated in nutrient agar. The test was repeated three times.

Following efficacy testing against staphylococci albus, we tested the sterilizer against natural airborne microorganisms in larger lab test chambers, 100 m3, 150 m3 and 210 m3, respectively. Before sterilization, the air sample in selected test chamber is collected with a six-level sieve air impact sampler. The air sterilizer is on for 2 hours during the sterilization cycle. Post sterilization air samples are collected and incubated in nutrient agar. The test was also repeated three times.

As we collected the efficacy date from a controlled lab testing environment, it is important to understand how the UV-C LED air sterilizer performs under a more complex real-world environment, such as normally operating office with occupancy. Therefore, we carried out an onsite efficacy study in office environment. Four individual offices with room space approximately 100 m3 are se-lected for the test. The office room details and layout can be found in Table 1 and Figure 3. One MASSPHOTON® air sterilizer is installed in the ceiling for each room. During the course of the testing, no additional air filtration or disinfection devices other than MASSPHOTON® air sterilizer are used. No interruption to normal office routine is introduced, expect that any cleaning pro-cedure and disinfectant usage is paused during the test. Pre and post sterilization air samples are collected from each room with a six-level sieve air impact sampler. The sterilization cycle lasts 3 hours. Air samples are collected and incubated in nutrient agar.

Figure 2: (a) UV-C LED beam shape and (b) ray tracing simu-lation of the high UV reflective air duct.

Table 1: Offices for Onsite Air Sterilization Testing.

Where Nt is the natural decay rate of bacteria in the air, %. V0 , Vt is the bacterial content in the air at different time points be-fore and after the start of the control group experiment (during the experiment), measured in units of colony formation per cubic meter (CFU/m³). Kt is the disinfection rate of bacteria in the air,%. V1, V2 is the bacterial content in the air at different time points before and after disinfection treatment (during the process) in the experimental group, measured in units of colony formation per cu-bic meter (CFU/m³). The results show the UV-C LED air sterilizer significantly reduced the bacteria in the air, with an average disin-fection rate of 99.93% after 2 hours of sterilization in a 20 m3 test chamber. Similarly, for lab testing against natural airborne bacteria showed in Table 4, the UV-C LED air sterilizer demonstrated its highly efficient disinfection rate of 99.6%, 99.02% and 98.64% for 100 m3, 150 m3 and 210 m3 chamber sizes, respectively.

The onsite office efficacy testing as previously described are carried out in 4 individual offices for 5 days. Due to accessibility, data was not collected at Day 1/3/5 for Office 3, and at Day 1 for Office 4. It is worth-noting that since this test is conducted under normally operating office environment with varying occupancy and real-time traffic, the microbial content in the air before sterilization varies from office to office and from day to day, ranging from 77 CFU/m3 to 1343 CFU/m3. Table 5 shows the microbial content before and after sterilization. The measured disinfection rate varies from 21% up to 92%, with 95% confidence interval 54.9% to 79.3%. Figure 4 illustrates the change of microbial content before and after sterilization. The variation can be due to multiple factors, such as office traffic, occupancy, weather, air quality and etc. Though the disinfection rate varies compared to the lab testing result, we observe significant reduction in airborne microorganisms after sterilize the room for 3 hours.

Table 2: Ozone Concentration Measurement during Opera-tion of the UV-C LED Air Sterilizer.

Table 4: UV-C LED air sterilizer disinfection rate against natural airborne bacteria.

Table 5: UV-C LED indoor air sterilization results for operating offices.

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