How Interior Office Plants can increase Relative Humidity levels to 40-60% within your office workplace reducing the survival levels of the Coronavirus

2019 Novel Coronavirus (COVID-19) Pandemic: Built
Environment Considerations To Reduce Transmission
Leslie Dietz, Patrick F. Horve, David A. Coil, Mark Fretz, Jonathan A. Eisen, Kevin Van Den Wymelenberg
Biology and the Built Environment Center, University of Oregon, Eugene, Oregon, USA
Genome Center, University of California—Davis, Davis, California, USA
Institute for Health and the Built Environment, University of Oregon, Portland, Oregon, USA< Department of Evolution and Ecology, University of California—Davis, Davis, California, USA< Department of Medical Microbiology and Immunology, University of California—Davis, Davis, California, USA Genome Center, University of California—Davis, Davis, California, USA Leslie Dietz and Patrick F. Horve contributed equally to this work. Their order in the byline was determined alphabetically by their last name. ABSTRACT With the rapid spread of severe acute respiratory syndrome coronavirus
2 (SARS-CoV-2) that results in coronavirus disease 2019 (COVID-19), corporate entities, federal, state, county, and city governments, universities, school districts, places of worship, prisons, health care facilities, assisted living organizations, daycares, homeowners, and other building owners and occupants have an opportunity to reduce the potential for transmission through built environment (BE)-mediated pathways. Over the last decade, substantial research into the presence, abundance, diversity, function, and transmission of microbes in the BE has taken place and revealed common pathogen exchange pathways and mechanisms. In this paper, we synthesize this microbiology of the BE research and the known information about SARSCoV- 2 to provide actionable and achievable guidance to BE decision makers, building >operators, and all indoor occupants attempting to minimize infectious disease transmission through environmentally mediated pathways. We believe this information is useful to corporate and public administrators and individuals responsible for building operations and environmental services in their decision-making process about the degree and duration of social-distancing measures during viral epidemics and pandemics.

KEYWORDS COVID-19, SARS-CoV-2, building operations, built environment, novel ncreased spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causing coronavirus disease 2019 (COVID-19) infections worldwide has brought increased attention and fears surrounding the prevention and control of SAR-CoV-2 from both the scientific community and the general public. 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. 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 (BE) has taken place in recent years. This work has revealed common pathogen exchange pathways and mechanisms that could lend insights into potential methods to mediate the spread of SARS-CoV-2 through BE-mediated pathways. Coronaviruses (CoVs) most commonly cause mild illness, but they have occasionally, in recent years, led to major outbreaks of human disease. Typically, mutations that Citation Dietz L, Horve PF, Coil DA, Fretz M, Eisen JA, Van Den Wymelenberg K. 2020. 2019 novel coronavirus (COVID-19) pandemic: built environment considerations to reduce< transmission. mSystems 5:e00245-20. https://doi.org/ Editor Jack A. Gilbert, University of California San Diego This article followed an open peer review process. The review history can be read Copyright © 2020 Dietz et al. This is an openaccess< article distributed under the terms of the Creative Commons Attribution 4.0 International license Address correspondence to Patrick F. Horve, pfh@uoregon.edu. Our review on #SARSCoV2 and #COVID19 in the built environment provides guidance for steps that can be taken in the built environment to potentially slow down spread of SARS-CoV-2 and #flattenthecurve during this global pandemic Published MINIREVIEW Applied and Environmental Science crossm March/April 2020 Volume 5 Issue 2 e00245-20 msystems.asm.org 1 7 April 2020 Downloaded from http://msystems.asm.org/ on April 20, 2020 by guest
cause structural changes in the coronavirus spike (S) glycoprotein enable binding to new receptor types and permit the jump from an animal host to a human host (1) (zoonotic transmission) and can increase the risk of large-scale outbreaks or epidemics. In 2002, a novel CoV, severe acute respiratory virus (SARS), was discovered in the Guangdong Province of China (3). SARS is a zoonotic CoV that originated in bats and resulted in symptoms of persistent fever, chills/rigor, myalgia, malaise, dry cough, headache, and dyspnea in humans (4). SARS had a mortality rate of 10% and was transmitted to 8,000 people during an 8-month outbreak in 2002 to 2003 (5). Approximately 10 years after SARS, another novel, highly pathogenic CoV, known as Middle

East respiratory syndrome coronavirus (MERS-CoV), emerged and is also believed to

have originated from bats, with camels as the reservoir host (6). MERS-CoV was first

characterized in the Arabian Peninsula and spread to 27 countries, having a 35.6%

mortality rate in 2,220 cases (7).

Coronavirus disease 2019 (COVID-19). In December 2019, SARS-CoV-2, a novel

CoV, was identified in the city of Wuhan, Hubei Province, a major transport hub of

central China. The earliest COVID-19 cases were linked to a large seafood market in

Wuhan, initially suggesting a direct food source transmission pathway (8). Since that

time, we have learned that person-to-person transmission is one of the main mechanisms

of COVID-19 spread (9). In the months since the identification of the initial cases,

COVID-19 has spread to 171 countries and territories, and there are approximately

215,546 confirmed cases (as of 18 March 2020). The modes of transmission have been

identified as host-to-human and human-to-human. There is preliminary evidence that

environmentally mediated transmission may be possible, specifically, that COVID-19

patients could be acquiring the virus through contact with abiotic BE surfaces (10, 11).

Epidemiology of SARS-CoV-2. The Betacoronavirus SARS-CoV-2 is a single-stranded

positive-sense enveloped RNA virus ( with a genome that is approximately 30 kb in

length (12, 13). Spike glycoproteins, the club-like extensions projecting from the cell

surface, facilitate the transfer of viral genetic material into a host cell by adhesion (14,

15) (Fig. 1). The viral genetic material is then replicated by the host cell. The infection

history of SARS-CoV-2 is believed to have begun in bats with a possible intermediate

host of pangolin (16). There are several other betacoronaviruses that occur in bats as a

primary reservoir, such as SARS-CoV and MERS-CoV (17). The manifestation of SARSCoV-

2 in a human population occurred late in December 2019, among persons known

to frequent a seafood market (18). The first symptoms observed clinically were fever,

fatigue, and dry cough, with symptoms ranging from mild to severe (19). Currently, the

FIG 1 Structure of SARS-CoV-2 virus. (a) Artistic rendering of the structure and cross section of the

SARS-CoV-2 virus (14, 15). (b) Transmission electron micrograph of a SARS-CoV-2 virus particle isolated

from a patient and imaged at the NIH, specifically, the National Institute of Allergy and Infectious

Diseases (NIAID) Integrated Research Facility (IRF) in Fort Detrick, Maryland (93).

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protocol developed by the Centers for Disease Control and Prevention (CDC) for

diagnosis (20) is a combination of clinical observation of symptoms and a positive result

for the presence of the virus using real-time PCR (rt-PCR) (21).

COVID-19 and the impact of the BE in transmission. The built environment (BE)

is the collection of environments that humans have constructed, including buildings,

cars, roads, public transport, and other human-built spaces (22). Since most humans

spend 90% of their daily lives inside the BE, it is essential to understand the potential

transmission dynamics of COVID-19 within the BE ecosystem and the human behavior,

spatial dynamics, and building operational factors that potentially promote and mitigate

the spread and transmission of COVID-19. BEs 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 (23, 24). The occupant density

in buildings, influenced by building type and program, occupancy schedule, and indoor

activity, facilitates the accrual of human-associated microorganisms (22). Higher occupant

density and increased indoor activity level typically increase social interaction and

connectivity through direct contact between individuals (25) as well as environmentally

mediated contact with abiotic surfaces (i.e., fomites). The original cluster of patients

were hospitalized in Wuhan, China, with respiratory distress (December 2019), and

approximately 10 days later, the same hospital facility was diagnosing patients outside

the original cohort with COVID-19. It is presumed that the number of infected patients

increased because of transmissions that potentially occurred within the hospital BE (10).

The increased exposure risk associated with high occupant density and consistent

contact was demonstrated with the COVID-19 outbreak that occurred on the Diamond

Princess cruise ship in January 2020 (26). Current estimates of the contagiousness

(known as the R0) of SARS-CoV-2, have been estimated from 1.5 to 3 (27, 28). R0 is

defined as the average number of people who will contract a disease from one

contagious person (29). For reference, measles has a famously high R0 of approximately

12 to 18 (30), and influenza (flu) has an R0 of 2 (31). However, within the confined

spaces of the BE, the R0 of SARS-CoV-2 has been estimated to be significantly higher

(estimates ranging from 5 to 14), with 700 of the 3,711 passengers on board the

Diamond Princess (19%) contracting COVID-19 during their 2-week quarantine on the

ship (26, 32). These incidents demonstrate the high transmissibility of COVID-19 as a

result of confined spaces found within the BE (33). With consideration to the spatial

layout of the cruise ship, the proximity of infected passengers to others likely had a

major role in the spread of COVID-19 (33).

As individuals move through the BE, there is direct and indirect contact with the

surfaces around them. 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 (22, 34). These resuspended viral particles can then resettle back onto

fomites. When an individual makes contact with a surface, there is an exchange of

microbial life (35), including a transfer of viruses from the individual to the surface and

vice versa (36). Once infected, individuals with COVID-19 shed viral particles before,

during, and after developing symptoms (37, 38). These viral particles can then settle

onto abiotic objects in the BE and potentially serve as reservoirs for viral transmission

(18, 34, 39). Evidence suggests that fomites can potentially be contaminated with

SARS-CoV-2 particles from infected individuals through bodily secretions such as saliva

and nasal fluid, contact with soiled hands, and the settling of aerosolized viral particles

and large droplets spread via talking, sneezing, coughing, and vomiting (34, 40). A

study on environmental contamination from MERS-CoV demonstrated that nearly every

touchable surface in a hospital housing MERS-CoV patients had been contaminated

with the virus (41), and a survey of a hospital room with a quarantined COVID-19

patient demonstrated extensive environmental contamination (18, 34). Knowledge of

the transmission dynamics of COVID-19 is currently developing, but based upon studies

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of SARS and MERS-CoV, preliminary data on SARS-CoV-2, and CDC recommendations,

it seems likely that SARS-CoV-2 can potentially persist on fomites ranging from a couple

of hours to 5 days (39, 42, 43) depending on the material (43). Based upon preliminary

studies of SARS-CoV-2 survival, the virus survives longest at a relative humidity of 40%

on plastic surfaces (half-life median15.9 h) and shortest in aerosol form (half-life

median2.74 h) (43); however, survival in aerosol was determined at a relative humidity

of 65%. Based on data related to SARS and MERS, we predict that the viability

of SARS-CoV-2 in aerosol is likely longer at lower relative humidity levels. Survival of

SARS-CoV-2 at 40% relative humidity on copper (half-life median3.4 h), cardboard

(half-life median8.45 h), and steel (half-life median13.1 h) collectively fall between

survival in the air and on plastic (43). However, it should be noted that there are no

documented cases thus far of a COVID-19 infection originating from a fomite. There is

preliminary data demonstrating the presence of SARS-CoV-2 in stool, indicating that

transmission can potentially occur through the fecal-oral pathway (18, 29, 34, 44). 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 under the assumption that active virus

may be transmitted by contact with these abiotic surfaces (34, 39). With an abundance

of caution, it is important to consider the possibility that the virus is transmitted

through aerosols and surfaces (45). For a conceptualization of SARS-CoV-2 deposition,

see Fig. 2.

FIG 2 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 (34, 40). (c and 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 BE (built environment) surfaces (34, 39, 42).

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Previously, it has been confirmed that SARS can be, and is most often, transmitted

through droplets (46). Considering that SARS-CoV-2 is from a sister clade to the 2002

SARS virus (47) that is known to transmit from person-to-person, the high incidence of

observed person-to-person transmission and the rapid spread of COVID-19 throughout

the world and communities, it is accepted at this time that SARS-CoV-2 can also be

spread through droplets (13, 48). Based upon previous investigation into SARS (49),

spread through aerosolization remains a potential secondary transmission method,

especially within the BE. Mitigation of viral transmission through BE air delivery systems

is most often reliant on inline filtration media. Residential and commercial systems

typically require a minimum efficiency reporting value (MERV) of 8, which is rated to

capture 70 to 85% of particles ranging from 3.0 to 10.0 m, a strategy employed to

minimize debris and loss of efficiency impacts to cooling coils and other heating,

ventilation, and air conditioning (HVAC) components. Higher MERV ratings are required

to filter incoming outside air based on local outdoor particulate levels. Protective

environment (PE) rooms in hospitals require the most stringent minimum filtration

efficiency (50). A MERV of 7 (MERV-7) or greater is required as a first filter before heating

and cooling equipment, and a second high-efficiency particulate air (HEPA) filter is

placed downstream of cooling coils and fans. HEPA filters are rated to remove at least

99.97% of particles down to 0.3 m (51). Most residential and commercial buildings

utilize 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 m. Most viruses, including CoVs, range from 0.004

to 1.0 m, limiting the effectiveness of these filtration techniques against pathogens

such as SARS-CoV-2 (52). Furthermore, no filter system is perfect. Recently, it has been

found that gaps in the edges of filters in hospitals has been a contributing factor of the

failure of filtering systems to eliminate pathogens from the shared air environment (53).

In recent years, the sharing economy has created environments and added new

components to how multiple people share the same spaces. It is possible that infectious

disease transmission may be impacted by this shift to the sharing economy.

Shared workspaces such as cowork environments, rooms in homes, cars, bikes, and

other elements of the BE may increase the potential for environmentally mediated

pathways of exposure and add complexity to enacting social-distancing measures. For

example, in cases where alternate modes of transportation were previously single

occupancy vehicles, these trips are now often replaced with rideshare programs or

transportation network companies, the potential for exposure may increase.

Control and mitigation efforts in the BE. The spread of COVID-19 is a rapidly

developing situation, but there are steps that can be taken, inside and outside the BE,

to help prevent the spread of disease. On an individual level, proper handwashing is a

critical component of controlling the spread of SARS-CoV-2, other coronaviruses, and

many respiratory infections (54–56). Individuals should avoid contact and spatial proximity

with infected persons and wash hands frequently for at least 20 s with soap and

hot water (39). Furthermore, since it is difficult to know who is infected and who is not,

the best way to avoid spread in some situations is by avoiding large gatherings of

individuals, also known as “social distancing.” At this time, the Food and Drug Administration

(FDA) does not recommend that asymptomatic individuals wear masks during

their everyday lives to preserve masks and materials for individuals who have been

infected with COVID-19 and for health care workers and family that will be in consistent

contact with individuals infected with COVID-19 (57). Additionally, wearing a mask can

give a false sense of security when moving throughout potentially contaminated areas,

and the incorrect handling and use of masks can increase transmission (58). However,

as masks become available, and while prioritizing access to masks for health care

workers that are in a higher risk environment daily, wearing a mask would be prudent.

There is sufficient evidence to suggest that airborne transmission is possible (49)

through aerosolized particles beyond six feet and that a mask would aid in preventing

infection through this route.

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Since the end of January 2020, many countries have issued travel bans to prevent

person-to-person contact and particle-based transmission. These mobility restrictions

have been confirmed to help contain the spread of COVID-19 (59). Within local

communities, a variety of measures can also be taken to prevent further spread (60). As

a whole, these measures are known as non-health-care-setting social-distancing measures.

These measures include closing high-occupancy areas such as schools and

workplaces. These community-level measures act to prevent disease transmission

through the same mechanisms as the worldwide travel restrictions by reducing typical

person-to-person contact, decreasing the possibility of fomite contamination by those

that are shedding viral particles, and decreasing the possibility of airborne, particle

transmission between individuals in the same room or close proximity. These decisions

are made by individuals with administrative authority over large jurisdictions, communities,

or building stock and are weighed in balance with numerous factors, including

health risks and social and economic impacts. Furthermore, despite substantial socialdistancing

and quarantine practices in place, specific building types and space uses are

considered critical infrastructure and essential to maintaining communities, such as

health care facilities, housing, and groceries. Better understanding of BE mediating

variables can be helpful in decision-making about whether to implement socialdistancing

measures and for what duration, and to individuals responsible for building

operations and environmental services related to essential and critical infrastructure

during periods of social distancing, and all building types before and after socialdistancing

measures are enacted.

Within the BE, environmental precautions that can be taken to potentially prevent

the spread of SARS-CoV-2 include chemical deactivation of viral particles on surfaces

(39). It has been demonstrated that 62 to 71% ethanol is effective at eliminating MERS,

SARS (42), and SARS-CoV-2 (34). This ethanol concentration is typical of most alcoholbased

hand sanitizers, making properly applied hand sanitizer a valuable tool against

the spread of SARS-CoV-2 in the BE. Items should be removed from sink areas to ensure

aerosolized water droplets do not carry viral particles onto commonly used items, and

countertops around sinks should be cleaned using a 10% bleach solution or an

alcohol-based cleaner on a regular basis. Again, it is important to remember that the

main and much more common spread mechanism of previous CoVs has been identified

as droplets from talking, sneezing, coughing, and vomiting than by the fecal-oral

pathway (34, 38, 39). Administrators and building operators should post signage about

the effectiveness of handwashing for at least 20 s with soap and hot water, ensure soap

dispensers are full, provide access to alcohol-based hand sanitizer, and implement

routine surface cleaning protocols to high-touch surfaces where contamination risks

are high, such as around sinks and toilets (39). Most importantly, to prevent the

transmission of microbes and thus, undesirable pathogens, it is important to exercise

proper handwashing hygiene (39, 61).

Enacting enhanced building HVAC operational practices can also reduce the potential

for spread of SARS-CoV-2. Even though viral particles are too small to be contained

by even the best HEPA and MERV filters, ventilation precautions can be taken to ensure

the minimization of SARS-CoV-2 spread. Proper filter installation and maintenance can

help reduce the risk of airborne transmission, but it is important to understand that

filters should not be assumed to eliminate airborne transmission risk. Higher outside air

fractions and higher air exchange rates in buildings may help to dilute the indoor

contaminants, including viral particles, from air that is breathed within the BE. Higher

outside air fractions 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 (62). There are some cautions to consider relative to these building

operations 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

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filter maintenance protocol. Third, increasing airflow rates that simply increase the

delivery of recirculated indoor air, without increased outside air fraction, could potentially

increase the transmission potential. Higher airflow rates could increase resuspension

from fomites and increase the potential for contamination throughout the building

by distributing indoor air more quickly, at higher velocities and volumes, potentially

resuspending more ultrafine particles (62). Additionally, increasing the indoor air

circulation rate could increase the human exposure to viable airborne viral particles

shed from other building occupants. Administrators and building operators should

collaborate to determine whether increased outside air fractions are possible, what

limitations or secondary implications must be considered, and determine a plan around

managing the outside air fraction and air change rates.

Increasing evidence indicates that humidity can play a role in the survival of

membrane-bound viruses, such as SARS-CoV-2 (63–65). Previous research has found

that, at typical indoor temperatures, relative humidity (RH) above 40% is detrimental to

the survival of many viruses, including CoVs in general (63, 66, 67), and higher indoor

RH has been shown to reduce infectious influenza virus in simulated coughs (67). 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 (63, 68, 69). Higher humidity likely

negatively impacts lipid-enveloped viruses, like CoVs, through interactions with the

polar membrane heads that lead to conformational changes of the membrane, causing

disruption and inactivation of the virus (70, 71). Furthermore, changes in humidity can

impact how susceptible an individual is to infection by viral particles (72) and how far

into the respiratory tract viral particles are likely to deposit (68). Decreased RH has been

demonstrated to decrease mucociliary clearance of invading pathogens and weakened

innate immune response (72–74). However, RH above 80% may begin to promote mold

growth, inducing potentially detrimental health effects (75). 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 a RH between

40% and 60% indoors may help to limit the spread and survival of SARS-CoV-2 within

the BE, while minimizing the risk of mold growth and maintaining hydrated and intact

mucosal barriers of human occupants (50, 67). 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. While

administrators and building operators should consider the costs, merits, and risks of

implementing central humidification, especially during new construction or as a retrofit,

it may be too time intensive to implement in response to a specific viral outbreak

or episode. In addition, increased RH may lead to increased buildup on filters, decreasing

airflow. However, in pandemic situations, this practice likely increases the effectiveness

of capturing viral particles, and this benefit outweighs the increased filter

maintenance required. Therefore, targeted in-room humidification is another option to

consider, and this may reduce the likelihood of a maintenance oversight causing

overhumidification.

Building ventilation source and distribution path length can affect the composition

of indoor microbial communities. Ventilating a building by introducing air directly

through the perimeter of buildings into adjacent spaces is a strategy that does not rely

on the efficacy of whole-building filtration to prevent network distribution of microorganisms.

Delivering outside air directly through the envelope into an adjacent spatial

volume has been shown to increase the phylogenetic diversity of indoor bacterial and

fungal communities and create communities that are more similar to outdoorassociated

microbes than air delivered through a centralized HVAC system (76). In some

buildings, a similar approach can be accomplished through distributed HVAC units,

such as packaged terminal air-conditioners (PTAC) frequently found in hotels, motels,

senior housing facilities, condominium units, and apartments or through perimeter

passive ventilation strategies such as perimeter dampered vents (77, 78). However, for

most buildings, the easiest way to deliver outside air directly across the building

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envelope is to open a window. Window ventilation not only bypasses ductwork but

increases outside air fraction and increases total air change rate as well (79). Administrators

and building operators should discuss a plan for increasing perimeter, and

specifically window, ventilation when outdoor temperatures are adequate for this

practice. 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 (94, 95).

Light is another mitigation strategy for controlling the viability of some infectious

agents indoors. Daylight, a ubiquitous and defining element in architecture, has been

shown in microcosm studies to shape indoor bacterial communities in household dust

to be less human associated than in dark spaces (80). Moreover, daylight in both the UV

and visible spectral ranges reduced the viability of bacteria compared to dark controls

in these microcosm spaces (80). In a study simulating sunlight on influenza virus

aerosols, virus half-life was significantly reduced from 31.6 min in the dark control

group to approximately 2.4 min in simulated sunlight (81). In buildings, much of the

sunlight spectrum is filtered through architectural window glass, and the resulting

transmitted UV 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 and many documented

positive human health benefits (80–83). 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 to admit abundant daylight

and sunlight.

While daylight’s effect on indoor viruses and SARS-CoV-2 is still unexplored, spectrally

tuned electric lighting is already implemented as engineering controls for disinfection

indoors. UV light in the region of shorter wavelengths (254-nm UV C [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 (84). It is important to note that most UVC light

is eliminated in the atmosphere, while much of the UVA and UVB spectrum is eliminated

through building glass layers. Airborne viruses that contain single-stranded RNA

(ssRNA) are reduced by 90% with a low dose of UV light, and the UV dose requirement

increases for ssRNA viruses found on surfaces (85, 86). A previous study demonstrated

that 10 min of UVC light inactivated 99.999% of CoVs tested, SARS-CoV, and MERS-CoV

(87). 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 (88, 89). More recently, far-UVC light in the

207- to 222-nm range has been demonstrated to effectively inactivate airborne aerosolized

viruses. While preliminary findings from in vivo rodent models and in vitro

three-dimensional (3-D) human skin models appear favorable to not cause damage to

human skin and eyes (90, 91), further research must be conducted to verify the margin

of safety before implementation. If implemented safely, UVC and UVGI light offers a

range of potential disinfectant strategies for buildings and is a common strategy for

deep clean practices in health care settings. 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 visual transparency

and a feeling of “spaciousness” indoors, whether at home through the use of open plan

concepts or at workplaces that harness open office concepts with spatial layouts 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

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viruses through designed human interaction. For example, large, densely populated

open office spaces may increase connectivity while private offices may decrease

connectivity. Space syntax analysis demonstrates a relationship between spatial disposition

and degrees of connectivity (Fig. 3) and has been shown to correlate with the

abundance and diversity of microbes within a given space (92). 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

to implement the measures.

Special considerations for health care settings for current and future epidemics.

Hospitals present unique challenges during the process of mitigating and protecting

all inhabitants from an infectious disease outbreak. Not only do health care and

hospital facilities have limited options for social-distancing measures to prevent infectious

spread, but health care facilities also often cohouse patients with vastly different

requirements from the BE around them. For example, high-risk immunocompromised

patients are often kept within protective environment (PE) rooms, designed to limit

outside airborne infectious agents from entering into the room. To do this, these rooms

are positively pressurized, relative to the corridor space, with a minimum of HEPA

supply air (ASHRAE 170-2017 [50]). However, this pressurization differential also increases

the likelihood that aerosols in the patient room will migrate outside of the PE

room and into the higher traffic corridor space when the door is open. While PE rooms

typically function as intended for the occupant, if an immunocompromised patient is

also under treatment for an airborne infectious disease, the process of limiting pathogen

ingress into the room could potentially create involuntary exposure to health care

workers, other patients, and visitors via the corridor space. 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 exterior of the

building to contain aerosolized pathogens from spreading into circulation and shared

spaces. The same negative pressure that aids in preventing spread of aerosolized

pathogens from inside the room can involuntarily expose the room occupants to

airborne pathogens that are sourced 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

FIG 3 Spatial connectivity, highlighting betweenness and connectance of common room and door configurations. (a)

Circles and lines follow the classic network representation. (b) The rectangles follow the architectural translation of

networks. 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 BE. (d) The circles represent the current knowledge of microbial spread based on microbial abundance

through BEs as determined by layout. Darker colors represent higher microbial abundance, and lighter colors represent

lower microbial abundance.

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a location for hospital staff to apply and remove personal protection equipment (PPE).

However, anterooms are not required for PE or AII rooms and have drawbacks during

routine operation; therefore, they exist in only some facilities. They use significant

additional floor area, create more travel distance. and increase the visual barrier

between patient and rounding care team, therefore, increase costs. 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.

A discussion of PE and AII rooms does not adequately address the majority of

patient rooms within a hospital or health care facility that 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. Decoupling these functions permits decentralized mechanical or

passive ventilation systems integrated into multifunctional facades with heat recovery

and 100% outside air delivery. Mechanically delivering air through the facade would

permit all patient rooms to be operated in isolation and individually adjusted to be

positively or negatively pressurized, depending on patient requirements, with a higher

degree of operational resilience. 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 if

possible. 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.

Conclusion. The number of individuals who have contracted COVID-19 or have

been exposed to SARS-CoV-2 has been increasing dramatically. Over a decade of

microbiology of the BE research has been reviewed to provide the most up-to-date

knowledge into the control and mediation of common pathogen exchange pathways

and mechanisms in the BE with as much specificity to SARS-CoV-2 as possible. We hope

this information can help to inform the decisions and infection control mechanisms

that are implemented by corporate entities, federal, state, county, and city governments,

universities, school districts, places of worship, prisons, health care facilities,

assisted living organizations, daycares, homeowners, and other building owners and

occupants to reduce the potential for transmission through BE mediated pathways. This

information is useful to corporate and public administrators and individuals responsible

for building design and operation in their decision-making process about the degree

and duration of social-distancing measures during viral epidemics and pandemics.

ACKNOWLEDGMENTS

We thank Jason Stenson and Cassandra Moseley for comments on the manuscript.

We thank Paul Ward for his graphical contributions.

P.F.H., L.D., and K.V.D.W. conceived of the scope of the article. L.D. and P.F.H. wrote

the article, with significant writing contributions from D.A.C. and M.F. P.F.H. developed

and created Fig. 1. P.F.H., with outside help, created Fig. 2 and 3. K.V.D.W. and J.A.E.

provided significant edits. All authors reviewed the final manuscript.

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Joe Gallo

781.279.0032 x101

joe@pdiplants.com

www.pdiplants.com