Adapted from “2019 Novel Coronavirus (COVID-19) Pandemic: Built Environment Considerations to Reduce Transmission,” mSystems, April 7, 2020. Excerpted and modified under the creative commons Attribution 4.0 International License. See the full text and complete references at msystems.asm.org/content/5/2/e00245-20.

In December 2019, SARS-CoV-2, a novel CoV that causes coronavirus disease 2019 (COVID-19), was identified in the city of Wuhan, a major transport hub of central China. The modes of transmission have been identified as host-to-human and human-to-human. While many of the precautions typical for halting the spread of respiratory viruses are being implemented, other less understood transmission pathways should also be considered and addressed to reduce further spread. Preliminary evidence suggests that environmentally mediated transmission may be possible—specifically, that COVID-19 patients could be acquiring the virus through contact with abiotic built environment surfaces.

Environmentally mediated pathways for infection by other pathogens have been a concern in buildings for decades, most notably in hospitals. Substantial research into the presence, abundance, diversity, function, and transmission of microorganisms in the built environment has taken place in recent years. More than a decade of microbiology research is reviewed here to provide knowledge into the control and mediation of common pathogen exchange pathways and mechanisms in the built environment with as much specificity to SARS-CoV-2 as possible.

COVID-19 Transmission and the Built Environment
Most humans spend 90% of their daily lives inside the built environment. Built environments serve as potential transmission vectors for the spread of COVID-19 by inducing close interactions between individuals, by containing fomites (objects or materials that are likely to carry infectious diseases), and through viral exchange and transfer through the air. The occupant density in buildings—influenced by building type and program, occupancy schedule, and indoor activity—facilitates the accrual of human-associated microorganisms.

The occupant density in buildings—influenced by building type and program, occupancy schedule, and indoor activity—facilitates the accrual of human-associated microorganisms.

Viral particles can be directly deposited and resuspended due to natural airflow patterns, mechanical airflow patterns, or other sources of turbulence in the indoor environment such as foot fall, walking, and thermal plumes from warm human bodies. These resuspended viral particles can then resettle back onto fomites. Evidence suggests that fomites can be contaminated with SARS-CoV-2 particles from infected individuals through bodily secretions, contact with soiled hands, and the settling of aerosolized viral particles and large droplets spread via talking, sneezing, coughing, and vomiting.

Research and preliminary data suggest that SARS-CoV-2 can potentially persist on fomites ranging from a couple of hours to five days depending on the material. The virus appears to survive longest at a relative humidity (RH) of 40% on plastic surfaces (with a half-life of 15.9 hours) and shortest in aerosol form (half-life of 2.74 hours); however, survival in aerosol was determined at a RH of 65%. Survival of SARS-CoV-2 at 40% RH on copper (half-life of 3.4 hours), cardboard (half-life of 8.45 hours), and steel (half-life of 13.1 hours) collectively fall between survival in the air and on plastic.

However, it should be noted that there are no documented cases thus far of a COVID-19 infection originating from a fomite. While transmission of COVID-19 has been documented only through respiratory droplet spread and not through deposition on fomites, steps should still be taken to clean and disinfect all potential sources of SARS-CoV-2. For a conceptualization of SARS-CoV-2 deposition, see Fig. 1.

Fig. 1: Conceptualization of SARS-CoV-2 Deposition(A) Once an individual has been infected with SARS-CoV-2, viral particles accumulate in the lungs and upper respiratory tract. (B) Droplets and aerosolized viral particles are expelled from the body through daily activities—such as coughing, sneezing, and talking—and nonroutine events—such as vomiting—and can spread to nearby surroundings and individuals. (C, D) Viral particles, excreted from the mouth and nose, are often found on the hands (C) and can be spread to commonly touched items (D) such as computers, glasses, faucets, and countertops. There are currently no confirmed cases of fomite-to-human transmission, but viral particles have been found on abiotic built environment surfaces.
University of Oregon Biology and the Built Environment Center Fig. 1: Conceptualization of SARS-CoV-2 Deposition
(A) Once an individual has been infected with SARS-CoV-2, viral particles accumulate in the lungs and upper respiratory tract. (B) Droplets and aerosolized viral particles are expelled from the body through daily activities—such as coughing, sneezing, and talking—and nonroutine events—such as vomiting—and can spread to nearby surroundings and individuals. (C, D) Viral particles, excreted from the mouth and nose, are often found on the hands (C) and can be spread to commonly touched items (D) such as computers, glasses, faucets, and countertops. There are currently no confirmed cases of fomite-to-human transmission, but viral particles have been found on abiotic built environment surfaces.

Based upon previous investigation into SARS, spread through aerosolization remains a potential secondary transmission method, especially within the built environment. Mitigation of viral transmission through built environment air delivery systems often relies on inline filtration media. Most residential and commercial buildings utilize a minimum efficiency reporting value of 5 to 11 (MERV-5 to MERV-11), and in critical health care settings, MERV-12 or higher and HEPA filters are used. MERV-13 filters have the potential to remove microbes and other particles ranging from 0.3 to 10.0 microns. Most viruses, including CoVs, range from 0.004 to 1.0 micron, and some have been observed in coagulated aerosols less than 1 micron in size, necessitating more effective filtration techniques to reduce transmission potential against pathogens such as SARS-CoV-2. Furthermore, no filter system is perfect. Gaps in the edges of filters have been identified by a hospital investigation to be a contributing factor of the failure of filtering systems to eliminate pathogens from the shared air environment.

Control and Mitigation
Even though viruses may be found in aerosols of a size that can penetrate high efficiency filters, ventilation and filtration are important in reducing the transmission potential of SARS-CoV-2. Proper filter installation and maintenance can help reduce the risk of airborne transmission. Higher outside air fractions and air exchange rates in buildings may help to dilute the indoor contaminants from air that is breathed within the built environment. This may be achieved by further opening outside air damper positions on air-handling units, thus exhausting a higher ratio of indoor air and any airborne viral particles present.

There are some cautions to consider relative to these modified building-operation parameters. First, increasing outside air fractions may come with increased energy consumption. In the short term, this is a worthwhile mitigation technique to support human health, but building operators are urged to revert to normal ratios after the period of risk has passed. Second, not all air-handling systems have the capacity to substantially increase outside air ratios, and those that do may require a more frequent filter-maintenance protocol. Third, increasing airflow rates that simply increase the delivery of recirculated indoor air, without raising the outside air fraction, could potentially increase the transmission potential.

Higher outside air fractions and air exchange rates in buildings may help to dilute the indoor contaminants from air that is breathed within the built environment.

Increasing evidence indicates that humidity can play a role in the survival of membrane-bound viruses, such as SARS-CoV-2. At typical indoor temperatures, an RH above 40% is detrimental to the survival of many viruses, including CoVs in general, and higher indoor RH has been shown to reduce infectious influenza virus in simulated coughs. Based upon studies of other viruses, including CoVs, higher RH also decreases airborne dispersal by maintaining larger droplets that contain viral particles, thus causing them to deposit onto room surfaces more quickly.

Although the current ventilation standard adopted by health care and residential care facilities, ASHRAE 170-2017, permits a wider range of RH from 20% to 60%, maintaining an RH between 40% and 60% indoors may help to limit the spread and survival of SARS-CoV-2 within the built environment, while minimizing the risk of mold growth and maintaining hydrated and intact mucosal barriers of human occupants. Indoor humidification is not common in most HVAC system designs, largely due to equipment cost and maintenance concerns related to the risk of overhumidification increasing the potential of mold growth. Implementing central humidification may be too time intensive to implement in response to a specific viral outbreak or episode. Therefore, targeted in-room humidification is another option to consider.

Building ventilation source and distribution path length can affect the composition of indoor microbial communities. Introducing air directly through the perimeter of buildings into adjacent spaces is a ventilation strategy that does not rely on the efficacy of whole-building filtration to prevent the network distribution of microorganisms. A similar approach can be accomplished through distributed HVAC units, such as packaged terminal air-conditioners, frequently found in hotels, senior housing facilities, and apartments, or through perimeter passive ventilation strategies such as perimeter dampered vents.

However, for most buildings, the easiest way to deliver outside air directly across the building envelope is to open a window. Window ventilation not only bypasses ductwork but also increases outside air fraction and total air-change rate as well. Care should be taken to avoid exposing occupants to extreme temperature profiles, and caution should be taken where close proximity would promote potential viral transfer from one residence to another.

However, for most buildings, the easiest way to deliver outside air directly across the building envelope is to open a window.

Light is another mitigation strategy for controlling the viability of some infectious agents indoors. In a study simulating sunlight on influenza virus aerosols, virus half-life was significantly reduced from 31.6 minutes in the dark control group to approximately 2.4 minutes in simulated sunlight. In buildings, much of the sunlight spectrum is filtered through architectural window glass, and the resulting transmitted ultraviolet (UV) light is largely absorbed by finishes and not reflected deeper into the space. Therefore, further research is needed to understand the impact of natural light on SARS-CoV-2 indoors; however, in the interim, daylight exists as a free, widely available resource to building occupants with little downside to its use. Administrators and building operators should encourage blinds and shades to be opened when they are not needed to actively manage glare, privacy, or other occupant comfort factors.

While daylight’s effect on indoor viruses and SARS-CoV-2 is still unexplored, spectrally tuned electric lighting is already implemented for disinfection indoors. UV light in the region of shorter wavelengths (254-nanometer UVC) is particularly germicidal, and fixtures tuned to this part of the light spectrum are effectively employed in clinical settings to inactivate infectious aerosols and can reduce the ability of some viruses to survive.

Airborne viruses that contain single-stranded RNA (ssRNA) are reduced by 90% with a low dose of UV light; the UV dose requirement increases for ssRNA viruses found on surfaces. A previous study demonstrated that 10 minutes of UVC light inactivated 99.999% of CoVs tested, including SARS-CoV and MERS-CoV. However, UV germicidal irradiation (UVGI) has potential safety concerns if the room occupants are exposed to high-energy light. For this reason, UVGI is safely installed in mechanical ventilation paths or in upper-room applications to indirectly treat air through convective air movement. More recently, far-UVC light in the 207- to 222-nanometer range has been demonstrated to effectively inactivate airborne aerosolized viruses. While preliminary findings appear favorable to not cause damage to human skin and eyes, further research must be conducted to verify the margin of safety before implementation. Implementing targeted UVC and UVGI treatment may be prudent in other space types where individuals that tested positive for COVID-19 were known occupants, but routine treatment may have unintended consequences and should be implemented with appropriate precaution.

Spatial configuration of buildings can encourage or discourage social interactions. In recent years, Western society has valued design that emphasizes a feeling of “spaciousness” indoors, whether at home through the use of open plan concepts or at workplaces that harness open office concepts that intentionally direct occupants to nodes of “chance encounters,” thought to enhance collaboration and innovation among employees. While these spatial configurations are culturally important, they may inadvertently enhance opportunities for transmission of viruses through designed human interaction.

Space syntax analysis demonstrates a relationship between spatial disposition and degrees of connectivity (Fig. 2) and has been shown to correlate with the abundance and diversity of microbes within a given space. Understanding these spatial concepts could be part of the decision-making process of whether to implement social-distancing measures, to what extent to limit occupant density, and for how long.

Fig. 2: Spatial Connectivity, Highlighting Betweenness and Connectance of Common Room and Door Configurations(A) Classic network representation uses circles and lines. (B) The architectural translation of networks uses rectangles. Shaded areas correspond to a measure of betweenness (the number of shortest paths between all pairs of spaces that pass through a given space over the sum of all shortest paths between all pairs of spaces in the building), degree (the number of connections a space has to other spaces between any two spaces), and connectance (the number of doors between any two spaces). (C) The arrows represent possible directions of microbial spread as determined by the layout of the built environment. (D) The circles represent the current knowledge of microbial spread based on microbial abundance through built environments as determined by layout. Darker colors represent higher microbial abundance, and lighter colors represent lower microbial abundance.
University of Oregon Biology and the Built Environment Center Fig. 2: Spatial Connectivity, Highlighting Betweenness and Connectance of Common Room and Door Configurations
(A) Classic network representation uses circles and lines. (B) The architectural translation of networks uses rectangles. Shaded areas correspond to a measure of betweenness (the number of shortest paths between all pairs of spaces that pass through a given space over the sum of all shortest paths between all pairs of spaces in the building), degree (the number of connections a space has to other spaces between any two spaces), and connectance (the number of doors between any two spaces). (C) The arrows represent possible directions of microbial spread as determined by the layout of the built environment. (D) The circles represent the current knowledge of microbial spread based on microbial abundance through built environments as determined by layout. Darker colors represent higher microbial abundance, and lighter colors represent lower microbial abundance.

Special Considerations for Health Care Settings
Health care and hospital facilities have limited options for social-distancing measures to prevent infectious spread, and they also often co-house patients with vastly different requirements from the built environment around them. For example, high-risk immunocompromised patients are often kept within protective environment (PE) rooms, positively pressured to limit outside airborne infectious agents from entering. However, this pressurization differential also increases the likelihood that aerosols in the patient room will migrate into the higher traffic corridor space when the door is open, involuntarily exposing health care workers, other patients, and visitors.

In comparison, airborne infection isolation (AII) rooms utilize a negative pressure differential relative to the corridor space and adjacent rooms, directly exhausting room air to the building exterior. The same negative pressure can involuntarily expose the room occupants to airborne pathogens from occupants of the corridor space. Both PE and AII rooms may be designed with an anteroom that is used as an additional buffer between common areas and protected spaces to prevent pathogen spread and provide a location for hospital staff to apply and remove personal protection equipment. However, anterooms are not required for PE or AII rooms and exist in only some facilities. They use significant additional floor area, therefore increasing costs, and increase the travel distance as well as the visual barrier between patients and the medical care team. These trade-offs might be reconsidered in future design and operational protocols given the high costs of pandemics and the critical role of health care environments during these times.

The majority of hospital patient rooms are not inherently designed with airborne respiratory viruses in mind.

The majority of hospital patient rooms are not inherently designed with airborne respiratory viruses in mind. Renewed consideration should be given to general facility design to fulfill various requirements for different patient conditions and operational requirements during both routine conditions and disease outbreaks. One such consideration includes separating the means of thermal space conditioning from ventilation provisions, a strategy that can also facilitate increased energy efficiency. Furthermore, future designs should reconsider the best way to triage and complete initial assessment of patients that present symptoms related to airborne viruses to minimize exposure to areas with other patient types.

In planning for the future, architects, designers, building operators, and health care administrators should aspire for hospital designs that can accommodate periods of enhanced social distancing and minimize connectance and flow between common areas, while also affording flexibility for efficient use of space during normal operating conditions.

Adapted from “2019 Novel Coronavirus (COVID-19) Pandemic: Built Environment Considerations to Reduce Transmission,” mSystems, April 7, 2020. Excerpted and modified under the creative commons Attribution 4.0 International License. See the full text and complete references at msystems.asm.org/content/5/2/e00245-20.