Indoor Environment Control Strategies: Air Disinfection

Germicidal UV (GUV)

Germicidal ultraviolet irradiation (GUV, sometimes abbreviated UVGI) has been used to successfully control airborne infection since the 1930’s [1]. It produces short wavelength light (or irradiance) within the 200-280nm spectrum of UVC, which interferes with the genetic material in viruses, bacteria, molds, and other organisms, thus, rendering them noninfectious. UVC is almost entirely absorbed in the stratum corneum (outermost layer) of the skin and is not likely to pose a cancer risk like longer wavelength UVA or UVB. A minor irritation (i.e., welder’s flash) can occur from direct eye exposure to the light and this is why care is taken to avoid direct ocular exposure. Most GUV lamps produce irradiance at ~254 nm wavelength, which is close to the peak wavelengths for microbial inactivation (260-270 nm) while short enough to reduce potential skin or eye exposure. More information about GUV is housed at the Harvard Center for Global Health Delivery (http://www.ghdcenter.hms.harvard.edu/GUV-lighting).

Early research showed the inactivation of airborne E. coli and tuberculosis by GUV [2]. Studies have also demonstrated an elimination in tuberculosis infection in guinea pigs housed in cages ventilated with GUV irradiated air, from tuberculosis wards [3, 4]. Since then, the ability of GUV to rapidly inactivate airborne pathogens, including measles, influenza, and tuberculosis, has been demonstrated through extensive study [1, 5, 6]. The effectiveness of GUV systems to treat air and surfaces resulting in protection of humans from infection transmission has been demonstrated and varies with the time and intensity of the exposure dose to the pathogen of interest, the susceptibility of the pathogen to the UV wavelength. Infectious aerosols may be more efficiently inactivated at lower versus higher relative humidity [6-9]. GUV could provide a level of helpful infection control in addition to other control measures in settings where risk may be elevated or ventilation may be limited. Germicidal UV has been used safely in a number of settings, with regard to human skin and eye ultraviolet exposure, and can achieve effective air sterilization while operating below OSHA exposure limits.

GUV can also be used to inactive pathogens on surfaces, however particles on surfaces, like dust, and crevices on surfaces can shield against GUV and render it less effective. Thorough cleaning of surfaces to access small cracks and remove such shielding particles may be warranted and if an effective cleaning solution is used for that purpose, then additional GUV disinfection may not be necessary.

GUV was shown as effective in curbing measles, mumps, and chickenpox outbreaks in Philadelphia suburb grade schools [9, 10]. Subsequent failures to replicate these findings in other schools are likely due to increased sources of transmission outside of schools in more urban settings, as summarized in a review of the history of GUV [1]. Nonetheless, GUV may represent an important layer of protection for reducing population spread in combination with efforts to control transmission within the community outside of school. There is more to be learned about the extent to which GUV or other effective engineering controls can mitigate outbreaks when varying levels of control measures are taken in other public settings. A modelling analysis in a simulated urban environment showed that homes, schools, and offices represent important places to interrupt transmission since they represent hubs of network connection [11].

Whole-room GUV

Whole-room GUV sterilizes indoor environments by flooding the occupancy zone with irradiance. People should not be present so as to avoid exposure to skin and eyes. This approach can be useful for situations where contaminants may have built up in an environment in order to reduce subsequent exposure, however most transmission is likely to occur while people are occupying an indoor space and sharing the air, during which whole-room GUV cannot be used. In these scenarios, upper room GUV and far-GUV can help by inactivating potentially infectious aerosols generated by infectious people before susceptible people in the same space get a chance to inhale them (see subsequent sections for more information on these approaches).

Upper-room GUV

Upper-room GUV is typically installed on a wall or ceiling to irradiate the upper air zone of an indoor space. It has the advantage of rapidly treating large volumes of indoor air, making it a highly efficient airborne infection control method. Human-generated thermal plumes carry air into the upper-room sterilization zone, and good air mixing in the room contributes to the movement of air into the sterilization zone. The addition of a ceiling fan that can draw air up into the upper room sterilization zone could double the air change per hour equivalents produced by the GUV system [12].

Several studies, starting in the 1930s, showed that upper-room GUV was effective in reducing measles, tuberculosis, and influenza [1]. Wells and colleagues demonstrated that upper-room air GUV substantially reduced transmission of measles, mumps, and chickenpox in grade schools outside of Philadelphia, USA [9, 10]. Upper-room GUV may be most effective when applied in crowded spaces and when aerosol transmission predominates as a mode of transmission, however has been estimated to contribute to meaningful community-level infection control when aerosol transmission is expected to account for 20% of infections [1, 11, 13, 14]. Studies of paired upper-room and surface GUV in military barracks suggest about a 20% reduction in respiratory infections with its use [15]. A study during an influenza epidemic found that rates of infection in a Veterans Administration Hospital building with upper-room GUV were almost 90% lower than rates of infection in nearby hospital buildings without GUV [16]. A more recent study of upper-room GUV in homeless shelters to reduce tuberculosis was unable to detect an effect due to low rates of tuberculosis infection overall [17]. The study did, however, show that upper room GUV can be safely applied with little occupant exposure to UV light, complementing evidence of theoretical safety given expected exposure levels. The body of evidence provides strong support for the effectiveness of upper-room GUV to efficiently clean air and mitigate aerosol transmission. This is despite the challenges in performing evaluation studies related to low infection rates, multiple sources of infection outside of the intervention setting, and the logistics of mounting comprehensive, prospective infection monitoring and exposure assessment.

Far-GUV

Compared with whole-room and upper-room GUV, far-GUV uses a shorter wavelength within the UVC spectrum (200-230nm, typically 222nm) and may offer a similar level of air sterilization, while also reducing concerns about human exposure [18, 19]. The lower wavelength cannot penetrate the eye or skin but safety remains under scrutiny at this writing [20]. Far-GUV is designed to bathe indoor environments with light while occupied, leading to direct inactivation of infectious aerosols close to the source. Both far-GUV and upper-room GUV can reduce exposure to infectious aerosols and prevent infection between indoor occupants while they are sharing air in an indoor space. Whole-room GUV, on the other hand, cannot be deployed during occupancy, thus reducing its potential to interrupt transmission that is most likely to occur while occupants are present together indoors. Should further safety and efficacy evaluation of far-GUV yield favorable results, far-GUV could contribute to infection control at close distance between infectious and susceptible individuals. This could represent an improvement over upper-room GUV that cannot sterilize air directly in the breathing zone. However, upper-room GUV currently provides a very efficient engineering control to mitigate infectious aerosols, capable of achieving hundreds of equivalent air changes per hour [21].

GUV use in ducts

Germicidal UV can also be installed in the ducts of heating, ventilating, and air conditioning (HVAC) systems and irradiate potentially infectious airborne particles as air flows through the ducts. This may be helpful when there is concern about infection transmission between different areas of a building through air ducts. Although SARS-CoV-2 has been detected in hospital ductwork, its dilution throughout a building and the use of filters in ducts minimize transmission risk by such means. In scenarios of prolonged exposure, such as in early TB experiments, exposure to duct air from TB patient rooms led to animal infection, while animals exposed to duct air with GUV irradiation remained uninfected [4]. While in-duct irradiation provides efficacious air disinfection and could be installed to clean recirculating HVAC systems that don’t have appropriate filters, they do not protect occupants from exposure to contaminated air generated by a source in a shared air space that is likely to circulate the room before reaching the outlet. Upper-room GUV (or far-GUV should it achieve greater acceptance) provide the double benefits of protecting occupants within the same room where airborne transmission risk is likely to be highest, as well as those downstream of exhaust ducts.

Often, GUV systems in ducts aim to reduce microbial replication at the cooling coils and drain pans of air conditioning systems [22-24]. Mold and bacteria that often grow on untreated cooling coils and drain pans can shed particles into the occupant space, posing infectious and non-infectious disease risks. Maintenance of coil cleanliness by UV may also improve HVAC system energy performance [24]. The extent to which GUV inactivates infectious agents depends of the intensity of the UV light, the duration of irradiation, humidity, the organism, and system design [24-27]. Effective destruction of molds and bacteria on surfaces of cooling coils and drain pans requires a less intense UV light source than effective GUV deactivation of particles in a flowing airstream because the surfaces are irradiated continuously while infectious aerosols in flowing air streams are irradiated transiently. Research demonstrated up to 99% reductions of molds and bacteria on irradiated surfaces [22-24], which may confer health benefits to indoor occupants. In a study within three office buildings, GUV – directed at cooling coils and drip pans in ventilation – was turned on and off multiple times without informing occupants. Self-reported respiratory and mucosal symptoms were reduced overall by 30 and 40% during periods of GUV operation, respectively. Work-related respiratory and musculoskeletal symptoms among never-smokers were reduced by 60 and 50%, respectively [23]. GUV irradiation of in-duct air can have health benefits even in the absence of coil and drain pan irradiation. One experiment installed GUV in HVAC ducts, to irradiate air but not coils or pans, in 17 homes with mold-sensitized allergic children [28]. During some periods, GUV was replaced with placebo lights. Compared with placebo, GUV treatment was associated with 30-50% reductions in asthma symptom scores, days with asthma symptoms, and measures of lung performance.

Triethylene glycol

Glycols have been explored for their infection control properties in the wake of the 1918 pandemic when chemical air cleaning was experimented with [29, 30]. In January 2021 the US EPA approved an emergency exemption for use of a triethylene glycol (TEG) product (Grignard Pure) in public settings where distancing may not feasible [31]. An un-peer-reviewed, third-party scientific evaluation of the product showed reductions in infectious aerosolized MS2 bacteriophage, a proxy viral species, by 1.7-2.8 log₁₀ PFU/m³ after 30 seconds of exposure, and 2.7-2.8 log₁₀ PFU/m³ after 60 minutes of exposure [32]. These results suggest disinfection at a level that could be meaningful for infection control should similar results be observed in pathogens of interest. At a non-visual or light haze level produced by a TEG fogger in the study, mass concentration levels of TEG were unlikely to exceed 10mg/m³, which is the ANSI (Standard E1.5) 8-hour time-weighted average for the chemical of the glycol class [33]. Another study that measured TEG mass concentration in air following deployment found that a “moderate” level of visible fog did not produce TEG mass concentrations in a test chamber above 10mg/m³. A 1990s field sampling study of glycol exposure from fogging devices used in Broadway productions found low levels of exposure to fogging as used on set [34]. Earlier work showed that TEG at a concentration of 5E-2mg/m³ eliminated streptococcus and pneumococcus culture colonies, and protected mice from airborne influenza infection with pulmonary pathology [35]. Nonetheless, careful attention should be given to dosage of triethylene glycol in indoor settings in order to minimize potential health effects caused by chemical exposure. Some minor health effects have been described with repeated exposure [36]. It is possible that TEG could react with other indoor chemicals leading to additional and perhaps unexpected adverse health effects. One study showed that TEG could react with a common disinfectant to exacerbate its toxic effects human airway epithelial cells and in animal models [37]. In addition to airborne infection control, another study showed TEG vapor <3ppm could increase the rate of influenza viral decay on surfaces by 16-fold [38]. Considering the numerous other environmental airborne infection controls with greater evidence of effectiveness, and the uncertainties about TEG dosing, chemical mixtures, and health risks, use of TEG could be of lower priority among other layers of protection.

Ionization

ASHRAE 62.1 prohibits use of air cleaning devices that generate ozone (must be <5 ppb) in commercial buildings. Advancements in ionizer technology over the years have reduced ozone generation but other ionizer byproducts such as NO_x, VOC, other oxidative species, and chemicals formed through subsequent indoor chemistry may confer harmful health effects [39]. Ionizers aim to control infectious aerosols by direct disruption of viral or microbial proteins, and by aerosol particle aggregation leading to more rapid deposition. A chamber study showed particle removal did not impact PM_2.5 mass concentration and may not be sufficient to effect transmission dynamics [40]. Studies evaluating ionizers to control airborne infection are lacking. There is little evidence for the direct disruption of pathogen structural integrity. An evaluation study of Needlepoint Bipolar Ionization showed some evidence of viral aerosol inactivation but pointed toward particle aggregation as the more predominant mechanism [41]. Given the uncertainty about a) their efficacy to remove and/or inactivate pathogens, and b) their potential to generate byproducts of human health concern, the numerous other engineering strategies with proven efficacy that do not pose such accompanying health concerns should be prioritized for airborne infection control.

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