Critical Scales and Settings of Aerosol Inhalation Transmission

The application and assessment of infectious aerosol controls indoors can be considered at three transfer scales: a) close-interactive, b) room, and c) building (between-room). Table 1 describes each scale and corresponding engineering controls. Administrative controls for all scales of transfer include pre-entry screening, face mask or respirator use as source control and PPE, activity limits (e.g., restricted loud speech, singing, or exercising), and frequent cleaning of common touch surfaces. Distancing tailored to setting-specific activities helps reduce close-interactive transfer. Limits to co-occupancy time, and cohort separation reduce room scale transfer. Distances 1–2 m have been commonly used to differentiate close- and long-range transmission along a continuum [1], related to likelihood of exposure to drops and aerosols. Features of close-interactive contact related to elevated airborne transmission have been described elsewhere [2, 3]. To differentiate between close-interactive—“garlic breath” scale of exposure [4]—and room scale exposure we define close-interactive aerosol transfer within 1–2 m during a duration consistent with the US CDC’s definition of contact, which is 15 min or more within a 24 h period [5]. A similar distinction in aerosol transfer exposure by proximity has been suggested by Nazaroff [6]. Aerosol transfer from infectious to susceptible individuals within rooms is relevant when transport occurs much faster than deposition, removal, or viral inactivation. A modelling study predicted that the predominant scale of respiratory infection transmission can switch between close-interactive and longer-range, within-room exposures as a function of variation in infectious aerosol size, interpersonal distance, and exposure time [7]. Increasing interpersonal distance within a room reduces the proportion of close-interactive transfer and increases the proportion of room scale transfer which is often more easily mitigated by ventilation, filtration, and GUV.

Table 2 below lists features of settings that pose elevated risks owing to higher emission activities, limits on the applicability or ability to enforce administrative controls, or the likelihood that existing facilities have the equipment needed for engineering controls. Dining and sporting activities are less amenable to face mask wearing. Settings with higher occupant density, greater magnitude of interaction, and activity level present greater challenges relative to those with lower density and stationary occupants. Studies have implicated specific settings for SARS-CoV-2 and other respiratory infection transmission. These include school lunchrooms, restaurants, and small, private gatherings [8, 9]; classroom learning [10]; a variety of occupational settings and worker dormitories [11]; meat processing [12]; religious services [13, 14]; public entertainment [15]; buses [16, 17]; nursing homes [18]; and fitness centers [19-23]. Residential settings, which typically foster extended exposure at close proximity, lend to transmission risk. A study of over 33,000 laboratory-confirmed influenza hospitalizations over two influenza seasons across the USA showed that having ≥5% of people living in crowded households—defined as more than one person per room—was associated with a 17% (95% CI 11-23%) increase in influenza hospitalizations, suggesting a possible role of within-household transmission in crowded environments [24]. A meta-analysis of household contacts estimated that spouses had a SARS-CoV-2 transmission rate of 38% (95% CI 26–51%) compared with 18% (12–25%) for other household contacts [25]. Overall, the meta-analysis estimated within-household SARS-CoV-2 transmission risk of 16.6% (95% CI 14.0–19.3%) [25]. Settings with immunologically susceptible populations and extended contact times include senior housing and assisted living communities, detention centers, homeless shelters, and healthcare settings. These settings may commonly have limited ventilation and air cleaning. Healthcare settings represent specialized environments with higher risks of exposure among and between patients and staff owing to increased sources of infectious aerosol [26-28]. Protection of immunologically susceptible patients is a priority and risk among staff is elevated with inadequate PPE and engineering controls [29-34]. Because of their unique exposures and elevated ventilation and infection control standards, healthcare settings are not a focus of this paper; however, many of the same controls applied in healthcare could be applied to other environments.

Community-level settings are likely to contribute to population transmission risk in unequal ways and this should be considered when assessing differential population exposure burdens in various settings. A study of US county level data showed that the effect of distancing policies on reducing COVID-19 cases and mortality was lower among communities of color, those with lower incomes, and those with higher levels of household crowding [35], potentially related to cumulative exposure burdens related to built environment factors in occupational and residential settings. This is despite data that shows increased adherence to masking and physical distancing among Black and Hispanic communities in a national US survey, after controlling for socioeconomic status [36]. Comprehensive, community-level prioritization of venues by risk level can be supported by risk estimation tools. For the purpose of minimizing overall spread and health impacts, priority intervention settings are those that serve as hubs for community transmission and those with populations that have higher risk of severe disease and adverse outcomes when infected.

Table 1. Scales of infectious aerosol transfer in buildings and associated engineering controls

	Close-interactive scale	Room scale	Building scale
Scale description	Interactions within 1-2 m, occurring for at least 15 min per 24 h. Interactions include conversing, eating, laughing, working, and other. Risk increases with source emission and susceptible person inhalation rate, both related to activity. Risk increases with duration.	Exposure within-room, between individuals not having close-interactive transfer. Poses the greatest risk for superspreading given potential large numbers of exposed and high concentrations of infectious aerosol when uncontrolled.	Exposure between rooms or floors in a building with air transfer occurring through forced air heating, ventilating, and air conditioning (HVAC) systems or through pressure-induced airflow via connections including internal doorways and halls, plumbing, or common exhaust fan inlets.
Ventilation and airflow management	Workstation airflow management. Room-scale directional airflow.	Increase clean air delivery. Directional airflow and clean air supply to the breathing zone. Multi-zone HVAC systems (increase mixing volume and decrease high concentration transfer when an infected person is in the room).	Increase clean air delivery. Directional airflow, e.g., exhausting from rooms expected to have greater presence of infectious aerosol.
Filtration by HVAC system	Increase clean air delivery via in-duct filtration. Must mix with breathing zone air.	Increase in-room clean air delivery via in-duct filtration.	Increase clean air delivery with multi-zone sources via in-duct filtration.
Filtration by portable air cleaner	Applied near the breathing zone (could be part of workstation airflow management). Personal air cleaning devices.	Applied near the breathing zone and/or throughout the space.	Approaches from other scales.
Infectious aerosol inactivation by GUV	Far GUV (222nm wavelength) in the breathing zone.	Far GUV in the breathing zone and/or throughout room space. Upper-room GUV (best with induced mixing of air to upper-room). GUV in recirculating HVAC ducts.	Approaches from other scales.
Other strategies	Physical barriers (more helpful for drop sprays and larger aerosols compared with smaller aerosols).	Flushing between occupant cohorts	Sealing openings between spaces

Table 2. Settings with elevated risk factors for infectious aerosol transmission

Priority settings (examples)	Potential risk factors related to activities done in the setting leading to exposure at room scale and sometimes close-interactive and/or building scale
Dining (school cafeteria, restaurant, institutional residential, event)	Impractical to wear masks; close-interactive; conversation, laughter; moderate duration (0.5–1 h) per encounter and possibly multiple encounters during infectious period; people moving makes directional airflow control challenging; many restaurants intended for high density occupancy; break rooms and dining areas may not be easily separable from mixing with other occupied areas, e.g., in schools, offices, and institutional residential buildings.
Risky occupational settings (slaughterhouse, meat packing, factory, call center, agricultural worker housing)	Long duration of co-occupancy over multiple days; conversation and loud speech may be required; PPE may not be available, may be uncomfortable to use, or poorly fitted; masking adherence may be problematic, especially in warm environments; special risks associated with some settings (e.g., talking at call centers, workstations that cannot be easily distanced, low temperature and relative humidity in meat processing plants); low ventilation; workers often limited to raise concerns; co-workers with elevated risk from living in high density homes with others also working in high risk occupational settings; limited/no paid leave discourages self-isolation/quarantine.
Schools, meetings (preschool, K-12, university, conference, classroom, dining hall, gym, auditorium)	Classrooms commonly have high occupant density, long periods of co-occupancy over consecutive days; loud speech often required for communication; conversations and small group interactions between classes and in halls are common; many facilities are older, with low ventilation or filtration in HVAC systems; students may have higher-risk living situations, e.g., sharing high-density housing with parents who work in high risk jobs; limited adherence to distancing and masking; limited or no access to respirators.
Local & regional public transit (train, bus, car sharing)	Economic viability of local and regional transit often depends on high rider densities during peak commutes; difficult to enforce mask rules and masks may not be correctly fitted or worn; screening is infeasible; co-occupancy with potentially large groups of strangers with unknown exposure for 0.5–1 h or more; limited or no access to respirators.
Institutional residential (dormitory, nursing home, long-term care, prison, shelter, military barrack)	Constant occupancy or for many hours per day; masking of residents is often infeasible; frequency and volume of conversation often cannot be controlled; close interactions between staff and residents needing care; many older facilities with inadequate ventilation and filtration; shared bedrooms; airflow connections between bedrooms and between dining areas and other common areas; natural ventilation often not practical; staff may work at multiple institutions and introduce infection between settings; residents may have comorbidities; worse conditions for underserved communities; limited or no access to respirators.
Grocery, retail	Employees exposed to large population (potentially infectious), close-interactive (e.g., cashier), long duration of workday exposure; infeasible to screen customers; some stores have small volume, narrow aisles, low ventilation; employees may not have paid sick leave; small businesses may not have enough staff to encourage staying home for infection control; limited or no access to respirators.
Oratory, singing, wind instrument, performance (rehearsal room, performance hall, religious gathering)	Many people present (potential for superspreading) and often high density; close-interactive; performers may be less likely to wear masks (e.g., film production), and are likely to use loud speech, shouting, or singing.
Offices	Conference rooms or similar space could host room-scale exposure for many people over a prolonged period of time (hours per day, over months); close interactions; limited or no access to respirators.
Athletics performance (fitness facility, playing field/court, spectator area)	Many people present; typically, without face masks; close-interactive (among players, among spectators), loud speech, shouting, increased heavy breathing; building scale transfer from strong source generation.
Private residential (single- or multi-unit housing)	Constant occupancy or exposure for many hours each day; close-interactive; masks typically not worn; inadequate ventilation in many homes, especially when hot or cold outside; often higher density in underserved communities; toilet-generated aerosols; potential airflow connections between apartments, spanning multiple floors (e.g., shared HVAC, stack-driven flows in converted single-family residence, exhaust ventilation in multi-story building).
There are variations possible in each setting, and risk of transmission is related to multiple elements, including: Number of potentially infectious people present Number of people present with elevated immune susceptibility and risk of severe illness Distancing and density of people in the indoor space Use of face mask for emissions control Use of PPE (including face masks, but especially N95 or equivalent filtering facepiece respirators) Duration of exposure Magnitude of respiration (speaking, speaking loudly or shouting, singing, breathing fast and/or heavy) Presence of mechanical ventilation Presence of filtration or GUV in forced air HVAC system Differential burdens of exposures that increase risk for underserved socioeconomic and demographic groups

Scenarios where engineering controls might be most helpful

Until effective vaccines are widely administered, current and future epidemics for which there are no or poorly effective vaccines, must rely on other layers of protection to reduce aerosol transmission risk in indoor environments where human interaction/proximity cannot be avoided. When masks cannot be worn, during meal times, for example, reliance on the other infection control considerations increases. This is of special concern in schools. Healthcare personnel who are at higher risk for influenza viral or SARS-CoV-2 exposure could wear well-fitting respirators as PPE, but respirators have not always been readily available to these workers. Risk of SARS-CoV-2 transmission is likely elevated for healthcare workers without well-fitting respirators despite use of surgical masks (which are most protective against larger aerosols) and eye protection [31, 32].

The extent to which engineering controls on their own, can reduce likelihood of exposure and transmission is uncertain for many exposure scenarios, which spurs an approach of robust precaution. As a means to control the COVID-19 pandemic, experts have encouraged the use of ventilation, filtration, and air disinfection by germicidal UV (GUV) in buildings [37, 38]. Improving characterization of the effect of control measures on infection risk supports strategies to ensure satisfactory infection control while reducing energy demands and other potential societal burdens associated with their deployment. Engineering controls to reduce airborne viral exposure can simultaneously improve indoor air quality associated with overall occupant health, wellness, satisfaction, and cognitive performance [39].

Engineering controls represent an important layer of respiratory infection control

Engineering controls like ventilation, filtration, and germicidal irradiation can be effective when the isolation of infectious cases is not possible or when precautions may not be considered due to the absence of symptoms among infectious individuals [40]. Asymptomatic SARS-2 infections have been shown to shed more virus than those exhibiting symptoms [41]. Epidemiologic-based evidence showing that more than 75% of transmission events are likely to occur by the day after onset of symptoms [42]. When infectious disease transmission threats become prevalent, on-demand systems that can enhance existing engineering controls (e.g., increase ventilation rate) offer a layer of protection. Reducing respiratory exposure to aerosol transmitted infections like rhinovirus can have numerous health effects such as decreasing the prevalence of severe asthma in children [43]. Elevated ventilation and filtration may also offer other respiratory health benefits by reducing exposure to dampness or mold [44].

The hierarchy of controls for environmental health exposures provides a helpful organizing structure for evaluating effectiveness [45]. It places elimination of the hazard as the most effective approach, followed by engineering controls that lessen or remove the hazard. Next in the effectiveness hierarchy are administrative controls that involve management of people, followed by personal protective equipment to be used when exposure cannot otherwise be eliminated or sufficiently reduced. One of the most effective ways to eliminate the source of an imminent infectious aerosol threat (e.g., during a pandemic) is for the public to adhere to face mask use as source control [46-50]. This should be done independent of symptom presentation since people can be infectious without any symptoms and may be most infectious leading up to the onset of symptoms [40, 42]. Reducing occupancy also contributes to source control.

The Health Impact Pyramid builds on the hierarchy of controls by explicitly weighing population impact (similar to “effectiveness” in the hierarchy of controls) against the effort required by individuals [51]. Administrative controls (such as physical distancing), and use of face masking in public (as source control and PPE) rely greatly on the cooperation of individuals and continual vigilance to assure adherence. In the United States, vaccination has become an issue requiring individual effort and buy-in, ensnared in a fictitious debate about unproven adverse health risks [52]. The challenges of achieving individual or population-level adaptation to address infection control (e.g., face mask wearing in public) also affects the implementation of engineering controls (e.g., motivating the implementation of built environment changes), yet once engineering controls are implemented, they can provide a layer of benefit over an entire population with little to no behavior change required.

Because of their population-level utility and efficacy, engineering controls such as air ventilation, filtration, and disinfection can be viewed as important layers of protection. Even if controls are not able to eliminate exposure to a respiratory virus or bacteria, they may be able to reduce exposure to a level below the infectious dose, or to a level resulting in attenuated infection. The effects of engineering controls are generally additive, but may be influenced by air flow.

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