SCIENTIFIC RESPONSES TO CONCERNS WITH USE OF UV-C AS AN INDOOR DISINFECTION PROCESS

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.

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.

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.

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.

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.