In This Issue
Microbiomes of the Built Environment
September 15, 2022 Volume 52 Issue 3
The covid-19 pandemic suddenly directed awareness to potential health impacts of the built environment of everyday living – schools, dwellings, offices, public buildings, and other spaces. This issue explores the “microbiome” of the built environment in the postpandemic reality in terms of ventilation performance, filtration, understanding and quantification of transmission risk, protection of “benign” microbes, and the important role of equity, among others.

Active Air Interventions

Tuesday, September 20, 2022

Author: Jelena Srebric and Donald K. Milton

Ventilation, filtration, and germicidal UV air disinfection can be even more effective with investment, standards, research, development, and policies to support their use.

Active air interventions are an important mechanism for reducing risks of infection transmission due to inhalation of airborne microorganisms in public congregant settings. While use of masks and respirators as source control and personal protection are by now well-established means of communicable respiratory infection control, active air interventions have the advantages of not requiring individual behavior change and protecting building occupants before detection of a local epidemic.

Recent Lessons Learned

Experiences during the SARS-CoV-2 pandemic, as well as earlier studies of influenza, indicate that increased ventilation, filtration, and air disinfection with germicidal ultraviolet (GUV) light are effective active interventions (Fischer et al. 2022; Gettings et al. 2021; Parhizkar et al. 2022). But ventilation measurements during the pandemic showed that many heating, ventilation, and air conditioning (HVAC) systems, especially in public spaces, such as schools or buses, did not have sufficient capacity to safely accommodate their population (CDPH 2020; Zhu et al. 2022).

Administrative measures that allow risk management by reducing exposure to rebreathed air (Rudnick and Milton 2003) without ventilation system augmentation include decreases in both (i) population density (limiting occupancy rates and effectively increasing the ventilation rate per person) and (ii) time continuously spent in a space. Because of the constraints imposed by such measures on the core activities of building occupants, facilities need scalable active interventions that complement existing ventilation to relax administrative measures.

Active Interventions to Prevent Airborne Infection Transmission

Active interventions to prevent airborne infection transmission fall into the following categories:

  • central HVAC systems with improved filtration or in-duct GUV;
  • portable, in-room filtration or enclosed GUV -systems; and
  • upper-room and direct in-room GUV.

In this article we focus on these three control strategies. They have been well studied and have well-known benefits and risks.

Control of relative humidity (RH) with increased RH during winter has been proposed as an additional means of controlling airborne infection transmission. However, high indoor RH in cold climates presents risks of mold and allergen exposure and structural damage to buildings (Fisk et al. 2007, 2019; IOM 2004). Furthermore, control of RH is less well studied and its effectiveness is unknown.

Ventilation is frequently discussed in terms of air changes per hour (ACH); we discuss the three approaches noted above in terms of the equivalent air changes per hour (eACH) that each can provide.

Central HVAC Systems with Additional Filtration or In-Duct GUV

Central HVAC Systems

Central HVAC systems are tasked with the supply of clean air to occupied spaces by running outdoor air and recirculated building air through a filtration system. The air conditioning, filtration, and circulation subsystems of central HVAC account for roughly half of total building energy use (Heidarinejad et al. 2014). Active interventions with additional filtration can be an expensive operational upgrade because they require continuous or extended operation of the central HVAC fans rather than allowing cycling based on reduced demand for ventilation, cooling, or heating. If cycling takes place both the media filtration and/or induct GUV interventions would be ineffective during downtime periods, which could be a majority of the time.

For public buildings, such as typical educational facilities, MERV-13 filtration[1] was a recommended upgrade of central HVAC to reduce the risk of airborne covid-19 transmission (NYSERDA 2021). This recommendation considers operational trade-offs between filtration efficacy in removing microorganisms and required fan capacities to handle the greater pressure drop due to the additional filtration.

Active air interventions have the advantages of not requiring individual behavior change and protecting building occupants before detection of a local epidemic.

A typical classroom, for example, could see an increase from 7 to 9 eACH when upgrading from MERV-8 to MERV-13, but research is needed to examine the full effects of these operational recommendations. Public transportation vehicles, such as buses, could upgrade their filtration systems to MERV-10 filtration to achieve a similar air cleaning effect due to high airflow rates. These suggested upgrades provide significant air cleaning, as if the air change rate were roughly tripled (NIOSH 2003).

In-Duct GUV

In-duct GUV is an alternative to media filtration that allows for air disinfection without occupant exposure to GUV. Such systems typically use 254 nm germicidal UV light capable of inhibiting the reproduction of microorganism cells and thus inactivating them.

The GUV lamps need to be appropriately sized and operated to provide a sufficient dose of light to microorganisms that move through the ducts at fairly high velocities. They also require continuous energy input. The eACH achieved by in-duct GUV cannot exceed the ACH that would be achieved by operating the system without recirculation, typically from 2 to 4 ACH.

In-duct GUV systems are best used as a supplement to existing or upgraded filtration, rather than a standalone system, because they do not filter particulate matter.

Portable, In-Room Filtration or Enclosed GUV Systems

Localized air filtration can match or even exceed the risk reduction potential of upgraded centralized filtration systems. The CDC’s National Institute for Occupational Safety and Health has shown that local filtration supplementing a centralized system can reduce exposure by 65 percent; used in combination with even loose--fitting masking, two local filtration devices reduced exposure by up to 90 percent (Coyle et al. 2021). Another study demonstrated that combining increased ventilation with portable in-room filtration could reduce exposure by 80 to 90 percent (Blocken et al. 2021).

Combining increased ventilation with portable in-room filtration could reduce exposure by
80 to 90 percent.

Localized systems have to be properly sized, distributed, and maintained to be effective; typical installations provide approximately 3 eACH (EPA 2018). Care must also be exercised to avoid creating a tripping hazard (from electric cords), and noise from the devices may discourage their use (Olsiewski et al. 2021).

Unit distribution in a room should be uniform to promote mixing and facilitate the removal of pathogens close to their source. Importantly, a unit should be placed in areas with stagnant airflow—such as, in a classroom, an air recirculation zone near a wall with a blackboard—to limit the possibility of creating high aerosol concentrations where an individual might spend a significant amount of time (Srebric et al. 2008).

If properly used, localized air filtration systems can work really well and have been widely successfully deployed in schools during the pandemic.

Upper-Room and Direct In-Room GUV

Upper-room GUV using fixtures that emit 254 nm UV-C was first demonstrated for control of measles in elementary schools in the late 1930s and early 1940s (Wells et al. 1942).[2] It has been extensively deployed in hospitals dedicated to treatment of Mycobacterium tuberculosis infections over the last 60 to 70 years (Nardell 2021). Far-UVC air disinfection using KrCl (krypton chloride) excimer lamps, which emit primarily 222 nm UV-C, was introduced more recently; significant production capacity and commercial availability began in 2020.

Germicidal UV employs UV-C wavelengths in the range of 200 to 275 nm. Because high-dose 254 nm UV-C can cause severe eye irritation (although it is not a significant skin cancer risk compared with UV-B in sunlight), fixtures are designed and installed (up to 6½ feet above the floor) so that very little of the GUV enters the occupied part of the room.

The eACH achievable with in-room GUV depends on the wavelength, intensity, and distribution of GUV in the room in addition to the target organism’s sensitivity to inactivation at the wavelength used. Studies of conventional 254 nm GUV systems show a five- to tenfold risk reduction with use of both properly sized and distributed light sources and a ceiling fan that brings air upwards to the inactivation zone (First et al. 1999; Mphaphlele et al. 2015; Zhu et al. 2014). The equivalent ventilation rates achieved with these systems can range from 20 to hundreds of eACH in test chambers (McDevitt et al. 2008), and approximately 100 eACH have been reported for an installation in school classrooms (Wells et al. 1942).

Recent research on the 222 nm GUV systems, which do not need to be restricted to the upper room, suggests that they may substantially improve effectiveness compared with the older systems. Because higher doses of 222 nm GUV, when filtered to remove longer-wavelength near UV-C (Buonanno et al. 2021; Eadie et al. 2021; Narita et al. 2022), are safe for skin and eye exposure (Kaidzu et al. 2021; Welch et al. 2022a) when used within the recently revised exposure guidelines (ACGIH 2022), and because the GUV is not restricted to the upper room, it has great potential to protect against short-range (conversational distance) as well as longer-range exposures (across a room or hallway). A laboratory test recently demonstrated that a high--density installation of 222 nm GUV fixtures could achieve more than 180 eACH (Eadie et al. 2022).

Challenges to GUV Implementation

The primary challenges associated with implementing conventional GUV (254 nm UV-C) have been (i) widespread misunderstanding of its skin safety and (ii) the potential for severe eye irritation, which is due to improper installation. Because 254 nm UV-C does not penetrate to the basilar layer of the skin and because exposures in the occupied, lower room are limited by concerns about acute eye irritation, the risk of skin cancer from 24/7 exposure to conventional GUV indoors at the maximum recommended dose is equivalent to the skin cancer risk of being outside at noon on a sunny June day in North America for 10 minutes (CIE 2010; Forbes et al. 2021). But acute eye irritation is possible with exposures to 254 nm GUV well below those that result in acute or chronic effects on the skin. The fixtures must therefore be installed more than 7 feet from the floor and ceilings must be more than 8 feet high. When the devices are installed properly, actual GUV doses to room occupants are well below recommended limits and eye irritation from the lamps does not occur (First et al. 2005; Nardell et al. 2008).

The effectiveness of conventional GUV is dependent on air movement to ensure that contaminated air moves up and through the irradiated zone frequently, yet slowly enough to give aerosols sufficient GUV exposure to inactivate pathogens (Pichurov et al. 2015). Safe and effective installation of conventional GUV systems requires special skills and training, but formal training and certification programs are currently lacking. Newer 222 nm GUV systems are less dependent on air movement and easier to install safely because higher exposure levels are eye and skin safe (Blatchley et al. 2022; Kaidzu et al. 2019; Welch et al. 2022a). However, special skill and training are still needed and training and certification programs are lacking for this technology as well.


Media filtration, whether central or in-room, is well-understood technology. Distributed air cleaning solutions are an effective option for active air intervention especially in the short term. However, if not properly sized and operated, the increased energy demand could be significant and of concern for its climate impact (Heidarinejad et al. 2018). In comparison, active air interventions in the central HVAC system, whether by filtration or GUV, are more energy intensive. They may also be less effective at controlling airborne micro-organisms emitted by building occupants, compared to properly operated in-room active air interventions.

Safe and effective installation of GUV systems requires special skills and training, but training and certification programs are lacking.

Conventional upper-room GUV is also well understood, with over 80 years of research and field application experience. Its energy efficiency can be far greater than that of central HVAC-based technologies, with an order of magnitude lower cost per infection prevented (ASHRAE 2019, ch. 62; Ko et al. 2001). Far-UVC technologies hold great promise to improve on both the -energy efficiency and safety profiles of conventional GUV.

What’s Needed to Move Forward with GUV

There remain important questions about how to optimize this newer technology in terms of fixture design and placement, performance against short-range transmission, and optimal amounts of GUV to provide. As a new technology, the cost of equipment is expected to decline as more producers enter the market. Future developments in LED and other technologies will likely eventually supplant the current KrCl excimer lamp technology, with further increases in energy efficiency.

Current conventional upper-room GUV and far-UVC technologies are ready for widespread use in preventing superspreading events in large congregant settings, such as dining facilities, nursing homes, hospital waiting rooms, convention centers, entertainment venues, and religious meeting houses.

The newer 222 nm GUV has a large potential payoff due to both its ability to disinfect air in the -vicinity of occupants and virus sensitivity to -germicidal UV light (McDevitt et al. 2010; Welch et al. 2022b). But significant investment in developing this technology, including testing efficacy in laboratory and community studies, as well as standards and certification will be needed.

Importantly, the home is both an important site of respiratory infection transmission and residential buildings are generally poorly ventilated. Current residential building stock relies primarily on outdoor air infiltration, and the median home in the United States has an ACH of approximately 0.5 (Nazaroff 2021). Therefore, a focus of policy should be to improve ventilation in homes; the role of other active interventions in homes is beyond the scope of this paper.

Finally, research is needed on different interventions and occupant health outcomes, and to provide a systemic foundation to not only link risk reduction to specific active air interventions but also reduce uncertainties in quantifying these relationships in buildings and transportation vehicles.


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[1]  A minimum efficiency reporting value (MERV) rating measures a filter’s effectiveness in preventing the transmission of dust and other contaminants.

[2]  The ultraviolet light spectrum is divided into UV-A, UV-B, and UV-C with progressively shorter wavelengths. UV-A (315–99 nm) and UV-B (280–314 nm) from sunlight penetrate the atmosphere, whereas UV-C (100–279 nm) is absorbed by the ozone layer and does not. UV-C is further categorized as near (231–79 nm), far (200–30 nm), and vacuum (<200 nm).

About the Author:Jelena Srebric is the Margaret G. and Frederick H. Kohloss Chair Professor in Mechanical Engineering and Donald Milton is MPower Professor in Environmental Health, both at the University of Maryland.