<p dir="ltr">Prevention of the spread of diseases caused by airborne biological pathogens is of great concern. This was highlighted by the Covid-19 Pandemic, which was caused by the SARS-CoV-2 virus. One method for preventing the transmission of airborne pathogens is UV-C irradiation, which has been proven to effectively inactivate a wide range of airborne pathogens, including influenza viruses and coronaviruses. Disinfection of air using UV-C is usually employed through upper-room Ultraviolet Germicidal Irradiation (UVGI), UV-C based air cleaners in HVAC ducts, or stationary cabinets. In the research presented herein, near-field applications of UV-C disinfection are explored through implementation in personal protective equipment.</p><p dir="ltr">A series of personal protective devices were developed for real-time inactivation of airborne pathogens in military or civilian applications. The devices use UV-C radiation from low-pressure mercury lamps (254 nm) and UV-C LEDs (nominally 277 or 282 nm) to inactivate airborne pathogens that are inhaled or exhaled by users, thereby reducing the risk of disease transmission. The devices employ Porex PMR20, a material highly reflective of UV-C radiation, to promote photon recycling and improve the fluence rate fields inside the reactors. To quantify the efficacy of these UV-C devices, testing procedures were developed and applied. A two-part procedure was employed: (1) measurement of the fluence rate using the Micro Fluorescent Silica Device (MFSD) together with a positioning device and (2) biological experiments using T1 bacteriophage as an aerosolized challenge agent to quantify virus inactivation.</p><p dir="ltr">The fluence rate measurements were completed by fixing the location of the MFSD probe and moving the reactors to precise locations using the positioning device. The MFSD measurements were converted to fluence rate using measurements from an NIST-calibrated radiometer and collimated UV-C sources. When comparing the measurements with and without the PMR20, the local fluence rate for the LED reactors was found to be amplified by up to 10 times the value without the PMR20. A central peak was also found for both LED reactors, which was not present in measurements without the PMR20. Of the two LED reactors, the Nichia Reactor was found to have higher peaks in fluence rate, probably due to the higher output from the LEDs when compared to the JLED LEDs. The LP Hg Pod was found to have less significant amplification from the PMR20, with the maximum amplification being only 3 times the value without PMR20. The fluence rate near the walls lined with PMR20 was found to have the highest amplification for the Pod reactor, due to the lamp being located at the center of the reactor (unlike the LED reactors, where UV-C sources were positioned along the reactor walls).</p><p dir="ltr">The biological experiments were conducted using aerosolized T1 bacteriophage as a challenge agent. T1 was selected for these experiments because it has been shown to be more resistant to inactivation at the wavelengths of interest than most airborne pathogens; as such, T1 is a conservative surrogate for airborne pathogens in these applications. Experiments were conducted first at one flow rate and subsequently at a range of flow rates to quantify the effectiveness of the reactors and the impact the PMR20 on the inactivation response. When tested at the lowest flow rate of 2.5 L/min, all three reactors were found to provide inactivation at least as effective as the nominal removal provided by an N95 mask (95% or 1.3 log₁₀ units). The LP Hg Pod reactor was found to provide at least this level of inactivation (1.3 log₁₀ units) at flow rates up to 52.5 L/min when PMR20 lined the reactor. When tested without the PMR20, the loss in inactivation was substantial at 52.5 L/min but not at the lower flow rates.</p><p dir="ltr">The testing protocol developed and applied in this project could be applied to a range of other reactors intended for disinfection of air. A need exists to standardize testing and validation methods for UV-C based reactors and devices that are used to disinfect air. As such, the methods described herein may allow translation to other UV-C based devices.</p>

No / With the general increase in the worldwide incidence of tuberculosis there is increasing interest in the use of upper room ultraviolet germicidal irradiation (UVGI) systems to disinfect air. A number of researchers have demonstrated experimentally the ability of such systems to inactivate airborne microorganisms. However, relatively little theoretical work has been done to explain the results observed and few models exist to describe the performance of upper room UVGI systems. This paper presents a new model, which can be used both to design such systems and to evaluate their germicidal effectiveness. A theoretical study is undertaken, which indicates that although upper room UVGI systems work well at lower ventilation rates, they are of limited benefit in highly ventilated applications. The paper also demonstrates and quantifies the relationship between inter-zonal air velocity and room ventilation rate. In particular, the paper shows that under steady-state conditions the number of passes made by bioaerosol particles through an upper room UV field is independent of the ventilation rate.

<p dir="ltr">The COVID-19 pandemic raised concern of indoor air quality especially as related to airborne pathogens and spread of diseases. With this concern came a resurgence of interest in germicidal ultraviolet radiation (GUV) as mechanism for reducing the risk of disease transmission in indoor settings. Though GUV devices have been used in the past, there are still concerns and barriers to their implementation. These concerns arise from a lack of public knowledge and concerns about the safety of these fixtures. To address these concerns, a series of experiments and numerical simulations was conducted to address the efficacy and safety of GUV-based devices.</p><p dir="ltr">The first experiments involved use of spherical actinometry to measure the fluence rate fields produced by Far-UVC (222 nm) fixtures and a low-pressure mercury lamp fixture (254 nm). These experiments were based on the use of a potassium iodide/iodate solution exposed to UVC radiation to photochemically form triiodide. This actinometer solution was contained within 1 cm diameter quartz spheres to allow measurement of fluence rate at a point in space. Using Beer-Lambert’s law the local fluence rate of the room is determined. The UVC dose based on an 8-hour exposure was then calculated and compared to threshold limit values (TLVs) set by the American Conference of Governmental Industrial Hygienists (ACGIH) to determine the safety of these fixtures in a room. It was found that that the dosage calculated for 222 nm exposure remained below the threshold limit value of 161 mJ/cm² at almost all locations with the exception being directly under the lamps. For the 254 nm fixture, all areas measured exceeded the threshold limit value of 6 mJ/cm², thereby indicating potential for human over-exposure to UVC for this configuration.</p><p dir="ltr">The second experiment conducted was an air quality experiment using two Far-UVC (222 nm) fixtures. There were five air quality metrics measured within this experiment: carbon dioxide concentration, temperature, relative humidity, ozone, and particulate matter concentrations. The air velocity in the supply vents was also measured. Ozone measurements were taken using a 3-channel 2B Technologies ozone monitor with measurements of room air, supply air, and outdoor air. The weeklong experiment demonstrated that ozone concentrations did slightly increase during nighttime in the room air when the UV fixtures were turned on, however, the increase was less than 5 ppb_v. Additionally, there was a correlation between the air dynamics through the room and the ozone and particle concentrations.</p><p dir="ltr">The final experiment involved developing effectiveness spectra for outdoor UV radiation. Simulations were conducted for Air Mass 1.0 and Air Mass 1.5 to determine the generic outdoor effectiveness of solar radiation on the earth at solar noon. Additionally, data from three different locations were collected using a spectroradiometer. Effectiveness spectra were developed for eye exposure, skin exposure, and for non-melanoma skin cancer. For each of these spectra, it was found that radiation in the UV-B range (280-320 nm) tended to be the most damaging. In addition to these spectra, the time to reach threshold limit values without sun protection was determined for each location and the Air Mass data. The Air Mass data showed that threshold limit values would be reached in less than an hour while out of the seven locations, some areas were reaching threshold limit values in less than 10 minutes. These time values were finally compared to the time to reach TLVs for GUV devices showing that in general, you would reach TLVs quicker outdoors than in the occupied area of a room outfitted with a GUV system.</p><p dir="ltr">These results provide basic information to frame the risks and benefits of GUV use in indoor settings; however, additional experiments and simulations are needed to fully address these questions. Some future research should include spherical actinometry experiments with UV LEDs which can have a range of wavelengths and air quality experiments with both UV LEDs and low-pressure mercury lamps. Additionally, effort should be put into creating media to educate the public on GUV fixtures to ease concerns about UV in an indoor setting.</p>