Many organizations are beginning to implement plans to return to offices, schools, and other common areas. The COVID-19 pandemic has everyone reconsidering communal spaces to minimize risk, especially the all too common open-office space concept. Some employers and business owners are creating new safety-related jobs, such as safety managers and care coordinators, to keep employees safe, while some are simply re-arranging the desks to keep employees at a healthy distance. Many employers, however, are turning to simulations to examine how to best reconfigure office designs and HVAC to lessen the chances of transmission.

Some office of emergency management (OEM) organizations have recently proposed new layout ideas, such as implementing partition walls between desks, to create better separation. The above-mentioned solution is effective only if we assume that the affected person always stays in the same position with absolutely no movement. There is no measurable way to know where the employee will be sitting, standing, or walking at the time of the sneeze or cough and, furthermore, it is impossible to know where and in what direction the sneeze or cough would travel. Since COVID-19 can be carried as particles in the airflow, it’s crucial to understand the supply and return ventilation paths. 

Recently, the U.S. Centers for Disease Control and Prevention (CDC) updated the “Airborne transmission” of SARS-CoV-2 (the virus that causes COVID-19) to read as follows: Airborne transmission is infection spread through exposure to those virus-containing respiratory droplets comprised of smaller droplets and particles that can remain suspended in the air over long distances usually greater than 6 feet and time (typically hours)¹. 

This study presents comparisons of computational fluid dynamics (CFDs) modeling of different room airflow rates and patterns in order to find the optimized HVAC design helping to control COVID-19 and similar disease. A combination of appropriate design of higher air-change rates and uniform direction turns out to be promising. 

Standard HVAC Systems’ COVID-19 Limitations 

According to the CDC’s website, the principal mode by which people are infected with SARS-CoV-2 (the virus that causes COVID-19) is through exposure to respiratory droplets carrying infectious viruses.

Respiratory droplets are produced during exhalation (e.g., breathing, speaking, singing, coughing, sneezing, etc.) and span a wide spectrum of sizes that may be divided into two basic categories based on how long they can remain suspended in the air:

• Larger droplets, some of which are visible and that fall out of the air rapidly within seconds to minutes while close to the source; and
   
• Smaller droplets and particles (formed when small droplets dry very quickly in the airstream) that can remain suspended for many minutes to hours and travel far from the source on air currents.

Once respiratory droplets are exhaled, and as they move outward from the source, their concentration decreases through fallout from the air (largest droplets first, smaller later) combined with dilution of the remaining smaller droplets and particles into the growing volume of air they encounter.¹ 

Current standard air-handling units (AHUs) in HVAC systems circulate up to 80%-90% of the air in regular systems during peak-load conditions, such as winter and summer, when outdoor ventilation airflow is set at the minimum percentage to save energy. Current standard filtration units in HVAC systems cannot remove all droplets carrying infectious viruses within an airstream effectively. In some extreme cases, the HVAC system could become the “hub” to spread the virus by recirculating contaminated air into all the space it serves.

With limited outdoor fresh air to mix with the recirculated room air, most of the contaminates are not removed or well diluted, which means it could travel inside the building areas and spread from one room to another room(s) all the time as long as the HVAC system is running.

The minimum infectious dose of SARS-CoV-2 is unknown so far, but researchers suspect it is low. 

“The virus is spread through very, very casual interpersonal contact,” said W. David Hardy, a professor of infectious disease at Johns Hopkins University School of Medicine, told STAT. 

So, even with the best HVAC system, the first priority is still that people follow CDC guidelines, such as wearing masks, maintaining social distance, covering up coughs or sneezes, washing hands, etc. 

Target of this Study

The target of this study is to minimize the spread/concentration of containment with optimized HVAC system air distribution through comparison of CFD models of air distributions. The modeling featured here will use a simple setup classroom with six sitting students and one standing teacher, and one of the six students is infected by COVID-19. The duration of the class is about one hour. (This modeling will only consider stable conditions without moving people.)

CFD Modeling Setup 

• CFD Software: 6SigmaRoom by Future Facilities.
   
• Sample Classroom Room Configuration:
  • 1. Classroom has 6 feet of clearance around each seat. The room includes six students and one teacher. 
  • 2. Room dimension: 
    • a. Ceiling height: 10 feet;
    • b. Length: 38 feet (with a small portion reaching 42 feet);
    • c. Width: 26 feet; and
    • d. Total Area: 1,012 square feet.
  • 3. Occupants:
    • a. Six students (Numbered from one to six (see Figure 1). No. 4 is the sick person;
    • b. One teacher (individual No. 7, [see Figure 1]); and
    • c. Keep six feet of distance from each other’s seats.
  • 4. Results:
    • a. In order to obtain the contamination concentration data at each person’s breath zone, three “Profile Lines” are setup (see Figure 2). Two lines across the students and one line at the teacher, profile line will indicate the concentration difference through the locations.
       
    • b. As there is no minimum infectious dose of SARS-CoV-2, the virus that causes Covid-19, we use 0 ppm of concentration as the bottom value, any level above 0 ppm is considered to be dangerous. There has been no discernible evidence on the minimum infectious viral load for COVID-19 pandemic, but many researchers speculate that a few hundreds of SARS-CoV-2 viruses would be enough to cause the disease among susceptible hosts. [Reference 2]
• Assumption:
  • 1. To simplify design condition, assuming the sample classroom is located in middle of an air-conditioned building with no exterior wall, window or roof. So, there is no cooling or heating load change throughout the day due to outdoor temperature or solar load changes. The starting supply airflow rate is 1080 CFM, which is approximately 1 CFM/Square foot. 
     
  • 2. HVAC system provides either all fresh air (100% outdoor air) without any virus  or the contamination from room return air stream are removed by high efficiency filtration and/or killed by UV lamp or other devices. No re-circulation of contamination in the supply air.
     
  • 3. One sick student carrying COVID-19 virus in the classroom.(circled in red in Figure 1).  The “sick person” may not be aware of his/her condition and does not wear a face mask. All occupants stay in room in a relative steady status (no abruptly movement.)
     
• Modeling Goal: Find the optimized airflow rate and pattern to limit the spread of contamination from sick person into the minimal volume/area before it leaves the room. 

Section 1 Traditional Air Distribution with Modification

In this section, we setup Model 1.0 through Model 5.0, as summarized below.  

• Model 1.0 is a baseline model with a regular HVAC air distribution pattern: Six ceiling-mounted supply diffusers and four ceiling-mounted return registers. 
   
• Model 2.0 keeps everything the same as model 1.0, except it relocates the return register as a low wall return (LWR).  
   
• Model 3.0 is a type of “Displacement Ventilation,” which has six low wall supply registers and eight ceiling-mounted return registers (more evenly return airflow).
   
• Model 3a is similar to Model 3.0, except it has more and larger low wall supply registers, increases the total airflow accordingly. 
   
• Model 3b is similar to Model 3a, except it has more low wall supply registers and larger ceiling-mounted return registers, increasing the total airflow accordingly. 
   
• Model 4.0 keeps everything the same as model 3.0, except it relocates the supply register to be floor-mounted.  
   
• Model 5.0 keeps everything the same as model 4.0 except it adds three more supply registers (the total supply airflow rate stays the same).  

Following are detail configuration of model 1.0 through model 5.0.

1.0 Baseline Model

Model Input: 

1.1. HVAC System:

1.1.1. Supply: Six 2-by-2-foot, 180-cfm, rectangular ceiling diffusers (CD) (approximately 1 cfm/square foot). The total airflow is 180 x 6 = 1,080 cfm. 

Supply Air Temperature: 59°F (15°C).

1.1.2. Return: Four 1-by-1-foot, rectangular, ceiling-mounted, slotted return registers, designed to evenly take the return airflow.

1.2. Occupants:

1.2.1. Heat Rejection:

• Use the software library of seated and standing people.

• Seated Students: 80 W.

• Standing Teacher: 100 W.

1.2.2. One sick person

• Breath Airflow rate: 0.32 cfm

• Breath Air Temperature: 86°F (30°C).

• Contamination: As the exhale gas from human body is slightly heavier than air, chose formaldehyde from software containment library.

o Density: 1.25 kg/m3 ( slightly larger than air density of 1.225 kg/m3 at sea level) 

o Molecular Weight: 30 kg/kmol 

Model Output

See Figure 3a to 3c below and Table 3. The contamination is displayed based on concentration in parts per million (ppm), 1 ppm = 0.0001%. 

The results indicate that the contamination spread into most parts of the room (80%-90% of the floor area), making the entire room extremely dangerous. Each seated person is exposed to high risk of contamination. The contamination ppm at breath zone of four out of five healthy students are above 100 ppm. The contamination at the breath zone of the fifth student (No. 1) is 80 ppm. The area around the standing teacher is relatively safe, with 66 ppm of contamination at the breath zone. The walkway between the sick person and the north wall has a very high level of contamination.

2.0 Baseline Model with Low wall Return

Model Input: 

2.1 HVAC System:

2.1.1 Supply: Same as 1.0 Baseline Model

2.1.2 Return: Four 1-by-1-foot, rectangular, low wall mounted, slotted return registers, evenly taking return airflow.

2.2 Occupants:

Same as 1.0 Baseline Model

Model Output

The results indicate that the high contamination concentration area around the sick student is smaller than 1.0 Baseline model (see Figure 4a to 4c and Table 3).

The contamination ppm at breath zone of four out of five healthy students are above 100 ppm. The contamination at the breath zone of the fifth student (No. 1) is 72 ppm. The area around the standing teacher is relatively safe. The walkway between the sick person and the north wall has a very high level of contamination.

3.0 Displacement Ventilation (Low wall Supply, Ceiling Return)

Model Input: 

3.1 HVAC System:

3.1.1 Supply: Low wall mounted, six 24-by-24-inch, rectangular grille (uniform flow), 180 cfm each along two longer dimension walls. The total airflow is 180 x 6 = 1,080 cfm. 

Supply Air Temperature: 59°F (15°C).

Supply air is at extremely low velocity (50-70 fpm) per displacement ventilation guideline.

3.1.2 Return: Eight 1-by-1-foot, rectangular, ceiling-mounted slotted return registers, evenly taking return airflow.

3.2 Occupants: Same as 1.0 Baseline Model

Model Output

The results indicate that the contamination is mostly confined within a 6 foot diameter above the sick person’s breath zone (3 feet or so from floor), and the footprint increases when it raises up towards the ceiling, this significantly reduced the risk (see Figures 5a to 5c and Table 3). 

The contamination ppm at breath zone of all of the five healthy students are below 100 ppm. The area around the standing teacher is relatively safe with 13 ppm of contamination at the breath zone. The walkway between the sick person and the north wall still has a high level of contamination.

3a. Displacement Ventilation (More and Larger Wall Supply Diffusers Ceiling Return)

Model Input:  

3.3 HVAC System:

3.3.1 Supply: Low wall mounted, 11 24-by-60-inch, rectangular displacement diffuser (uniform flow), 180 cfm each, four on each of the two longer dimension walls (north and south) as well as one on east wall and two on the west wall. The total airflow is 180 x 11 = 1,980 cfm. 

Supply Air Temperature: 59°F (15°C).

Supply air velocity is lower than model 3.0.

3.3.2 Return: Eight 2-by-1-foot, rectangular, ceiling-mounted, slotted return registers, evenly taking return airflow.

3.4 Occupants: Same as 1.0 Baseline model

Model Output

The results are similar to model 3.0, with better (smaller) restraint of the contamination, specially at the seat directly behind the sick student (see Figures 6a to 6c and Table 3). 

The contamination ppm at the breath zone of four of the five healthy students are below 10 ppm; the fifth student (No. 2) has 47 ppm contamination at the breath zone. The area around the standing teacher is relatively safe with 0.10 ppm of contamination at the breath zone. The walkway between the sick person and the north wall still has a higher level of contamination.

This model provides large improvements with larger quantities and sizes of rectangular displacement diffusers, which distribute the airflow from four sides of the walls, creating uniform and slow air displacement ventilation. 

3b. Displacement Ventilation (More Low Wall, Large, Supply Diffusers, Ceiling Return)

Model Input:  

3.1 HVAC System:

3.1.1 Supply: 17 low wall mounted, 24-by-60-inch, rectangular displacement diffusers (uniform flow), seven 180 cfm on each of the two longer dimension walls (north and south) as well as one on the east wall and two on the west wall.

Total airflow: 180 x 17 = 3,060 cfm. 

Supply Air Temperature: 59°F (15°C).

More diffusers than model 3a.

3.1.2 Return: Eight 2-by-1-foot rectangular, ceiling-mounted, slotted return registers, evenly taking return airflow.

3.2 Occupants: Same as 1.0 Baseline Model

Model Output

The results are similar to model 3.0, with better (smaller) restraint of the contamination, especially at the seat directly behind the sick student (see Figures 7a to 7c and Table 3). 

The contamination ppm at breath zone of all of the five healthy students is below 10 ppm. The contamination at breath zone of the two students (Nos. 5 and 6) at the seats behind the sick person is less than 1.0 ppm. The area around the standing teacher is relatively safe with 0.05 ppm of contamination at the breath zone. The walkway between the sick person and the north wall still has a higher level of contamination.

This model provides improvement (lower ppm of contamination) with a larger quantity of rectangular displacement diffusers than model 3a. The room is relatively safe, except the north walkway.  

4.0 Displacement Ventilation (Floor-Mounted Supply, Ceiling Return)

Model Input: 

4.1 HVAC System:

4.1.1 Supply: Six, floor-mounted, 24-by-24-inch, 180-cfm, rectangular grille. The total airflow: 180 x 6 = 1,080 cfm. (Only two rows of floor supply grilles at the front and rear of the room; no supply grilles are located in the middle row of seats)

4.1.2 Supply Air Temperature: 59°F (15°C).

4.1.3 Supply air is at extremely low velocity (50-70 fpm) per displacement ventilation guideline.

4.1.4 Return: Eight 1-by-1-foot, rectangular, ceiling-mounted, slotted return registers, evenly taking return airflow.

4.2 Occupants: Same as 1.0 Baseline Model

Model Output

The results indicate is similar to model 3.0 for restraint of the contamination (See Figure 8a to 8c below and Table 3).

The contamination ppm at the breath zone of all five healthy students is below 100 ppm. The area around the standing teacher is relatively safe with 16 ppm of contamination at the breath zone. The walkway between the sick person and the north wall still has higher level of contamination.

This model result is similar to model 3.0; while results from model 3a and 3b, which are enhanced types of model 3.0, are much better than model 4.0. 

5.0 Displacement Ventilation (More Floor-Mounted Supply, Ceiling Return) 

Model Input: 

5.1 HVAC System:

5.1.1 Supply: Nine floor-mounted, 24-inch-by-24-inch, rectangular grilles with one row (three SA grilles) in the middle seats, 180 cfm each. The total airflow: 180 x 9 = 1,620 cfm. The room air change per hour (ACH) is about 10 ACH. The supply air is more than enough for normal cooling, and the excessive airflow rate is designed to control contamination. 

5.1.2 Supply Air Temperature: 59°F (15°C).

5.1.3 Supply air is at extremely low velocity (50-70 fpm) per displacement ventilation guideline.

5.1.4 Return: Eight 1-by-1-foot, rectangular, ceiling-mounted, slotted return registers, evenly taking return airflow.

5.2 Occupants: Same as 1.0 Baseline Model

Model Output

The results indicate is similar to model 4.0 for restraint of the contamination (See Figures 9a to 9c and Table 3).

The contamination ppm at the breath zone of all five healthy students is below 100 ppm. The area around the standing teacher is relatively safe with 38 ppm of contamination at the breath zone. The walkway between the sick person and the north wall still has a higher level of contamination

This model result is similar as model 4.0. The increased supply airflow does not provide a significant improvement. 

Section 2. Cleanroom-Type Air Distribution with Modifications

In this section, we setup Model 6.0 through Model 8.0 as summarized below.  

• Model 6.0 through Model 8.0 simulates a cleanroom airflow pattern with laminar flow supply ceiling diffusers and low wall returns. The airflow rate is much higher than cooling/heating load needs, which helps control the particles in room. The airflow rate increases from model 6.0 to 8.0, and the associated air change per hour (ACH) is 15, 27, and 45.
   
• Model 8.0, which has the highest ACH of 45, presents the best capability to control the contamination in the minimized region within a small region close to his/her body and downward to the low wall return. It is the best air distribution pattern from all eight models we tested.
   
• There is no significant difference between Model 7.0 and 8.0 for the seated persons except that in Model 7.0. The walkways (in the middle of the room or along the side wall) are not covered by a laminar flow diffuser. When the sick person is in those zones, risk is presented to surrounding area; while Model 8.0 shows well control of the contamination in all areas.
   
• In real life, the cost of Model 8.0 will be higher than the currently accepted industry standard design before the COVID-19 pandemic. Also, people will feel the “draft” due to the high flowrate, so occupants need to dress accordingly; however, when people are anxious to go back to workplace, safety is more important when compared to comfort and cost.
   
• Models 7a and 7b simulate the sick student standing at the middle and north walkways, respectively, using same configuration of Model 7.0.
   
• Models 8a and 8b simulate the sick student standing at the middle and north walkways, respectively, using the same configuration of Model 8.0.

6.0 Super Clean Building Model (CNC Cleanroom Ventilation)

Model Input: 

6.1 HVAC System:

6.1.1 Supply: 15 200-cfm, 24-by-24-inch, ceiling-mounted, laminar (uniform) flow diffusers. The total airflow: 200 x 15 = 3,000 cfm. The room air change per hour (ACH) is about 18. The supply air is more than enough for normal cooling, and the excessive airflow rate is designed to control contamination. 

6.1.2 Supply Air Temperature: 67°F (19.4°C).

6.1.3 Return: 15 1-by-1-foot, rectangular, low wall, mounted, slotted return registers, evenly taking return airflow.

6.2 Occupants: Same as 1.0 Baseline Model

Model Output: 

The results indicate that the contamination is confined within 4 feet or so above the sick person’s breath zone (3 feet or so from floor), the footprint stays relatively constant when it raises up toward the ceiling. The walk space near the sick person has less contamination concentration than Models 3.0, 3a, and 3b (see Figures 10a through 10c and Table 6). 

The contamination ppm at the breath zone of four of the five healthy students is below 40 ppm, the fifth student (No. 2) has 75 ppm contamination at the breath zone. The area around the standing teacher is relatively safe with 15 ppm of contamination at the breath zone. The walkway between the sick person and the north wall still has a lower level of contamination compared to Models 3.0 to 5.0.

This model result is relatively safer than Models 1.0, 2.0, 4.0, and 5.0 with higher airflows (18 ACH). However, with a similar supply air flowrate, Model 3b has much better (lower) contamination ppm at the breath zones of all occupants.

7.0 Super Clean Building Model (Cleanroom Ventilation), Increase the Airflow Rate

Model Input: 

7.1 HVAC System: Increase the airflow rate

7.1.1 Supply: 27 250-cfm, 36-by-24-inch, ceiling-mounted, laminar (uniform) flow diffusers. The total airflow is 250 x 27 = 6,750 cfm. 

The room ACH is 40. The supply air is more than enough for normal cooling, and the excessive airflow rate is designed to control contamination. 

7.1.2 Supply Air Temperature: 67°F (19.4°C).

7.1.3 Return: 15 1-by-1-foot, rectangular, low wall mounted, slotted return registers, evenly taking return airflow.

7.2 Occupants: Same as 1.0 Baseline Model

Model Output: 

Compared to Model 6.0, contamination in Model 7.0 is better constrained, as most portions of contamination are forced toward the floor, and the overall footprint/volume of contamination is smaller (see Figures 11a through 11c and Table 6); however, the two walk spaces between the seats and the north and south walls are not covered by laminate airflow. 

The contamination ppm at the breath zone of three of the five healthy students is below 0.2 ppm, the fourth student’s (No. 5) is 1.7 ppm, and the fifth student’s (No. 6) is 31 ppm. The area around the standing teacher is relatively safe with 0.001 ppm of contamination at the breath zone.

• Model 7a simulates the sick student standing at the middle walkway using the same configuration of Model 7.0.

Figures 11d through 11f and Table 7 demonstrate the scenario when the sick person is standing in the walk space. The large contamination area has been generated surrounding him (going down toward the floor, as there is a diffuser covering him), which is dangerous. 

The contamination ppm at the breath zone of four of the five healthy students is below 25 ppm; the fifth student (No. 1) has 0.3 ppm contamination at the breath zone. The area around the standing teacher is relatively safe with 0.08 ppm of contamination at the breath zone.

• Model 7b simulates the sick student standing at the north walkway using the same configuration of Model 7.0.

Figures 11g through 11i and Table 7 demonstrate the scenario when the sick person is standing in the walk space. The large contamination area has been generated surrounding him (going upward toward the ceiling as there is no ceiling diffuser covering him), which is more dangerous than 7a if someone walks by. 

The contamination ppm at the breath zone of four of the five healthy students are below 0.5 ppm, the fifth student (No. 6) has 7 ppm contamination at the breath zone. The area around the standing teacher is relatively safe with 0.01 ppm of contamination at the breath zone.

To get safer results, the entire floor area shall be covered with laminar flow.

8.0 Super Clean Building Model (Cleanroom Ventilation) – an Increased Number of Diffusers Covering Entire Room

Model Input: 

8.1 HVAC System:  

8.1.1 Supply: 45 250-cfm, 48-by-24-inch, ceiling-mounted, laminar (uniform) flow diffusers (covering entire floor, including the two walkways). The total airflow is 250 x 45 = 11,250 cfm. The room ACH is about 67. The supply air is more than enough for normal cooling, and the excessive airflow rate is designed to control contamination. 

8.1.2 Supply Air Temperature: 67°F (19.4°C).

8.1.3 Return: 15 2-by-1-foot, rectangular, low wall mounted, slotted return registers, evenly taking return airflow.

8.2 Occupants: Same as 1.0 Baseline Model

Model Output: 

Compared to Model 7.0, there is a similar distribution of contamination when the sick person is seated; though, with the additional diffuser, the walkways besides the seats are safer with laminar flow covered (see Figures 12a through 12c and Table 6).

The contamination ppm at the breath zone of three of the five healthy students is below 0.04 ppm, the fourth student’s (No. 5) is 2.3 ppm, and the fifth student’s (No. 6) is 21 ppm. The area around the standing teacher is relatively safe with 1e-7 ppm of contamination at the breath zone, which is close to zero.

• Model 8a simulates the sick student standing at the middle walkway using the same configuration of Model 8.0.

Figures 12d through 12f and Table 7 demonstrate the scenario when the sick person is standing in the walk space. Compared to Model 7a, a smaller contamination area has been generated surrounding him (going down toward the floor, as there is a diffuser covering him), which is safer than Model 7a. 

The contamination ppm at the breath zone of three of the five healthy students is below 0.01 ppm, the fourth student’s (No. 5) is 1 ppm, and the fifth student’s (No. 3) is 9 ppm. The area around the standing teacher is relatively safe with 2e-7 ppm of contamination at breath zone, which is close to zero.

• Model 8b simulates the sick student standing at the north walkway using the same configuration of Model 8.0.

Figures 12g through 12i and Table 7 demonstrate the scenario when the sick person is standing in the walk space: A large contamination area has been generated surrounding him (going down toward the floor, as there is a diffuser covering him), which is safer than 7b if someone walks by. 

The contamination ppm at breath zone of all of the five healthy students is below 2e-7 ppm. The area around the standing teacher is relatively safe with 9e-9 ppm of contamination at the breath zone, which is close to zero.

Conclusion

• From the comparison of different HVAC air distributions, the traditional ceiling-mounted supply and return pattern has the highest risk to spread the virus, which has to be revised one way or another.
   
• Displacement Ventilation (model 3.0) with low-velocity, low-supply, and ceiling-mounted returns provides better control of the virus, which has a relatively lower initial cost and high energy efficiency and is relatively easy to apply in a green field new building, though not for a retrofit project.
   
• Enhanced Displacement Ventilation (Model 3a and 3b) with larger surface diffusers and increased airflow provides good results in reducing the overall contamination in all occupancies. The result is compatible with cleanroom-type ventilation, which has a much higher airflow rate.
   
• Cleanroom-Type Ventilation (Models 6.0, 7.0, and 8.0) has significant improvements when it comes to virus controllability. With the ACH increase, the controllability gets better; however, the initial cost and operational costs may be too high to be able to apply.

Recommendation

• Enhanced Displacement Ventilation shall be taken into immediate consideration of new construction with relatively low cost.
   
• Cleanroom-Type Ventilation can be further analyzed and modified to lower the cost (both initial cost and operation cost); it can be partially applied to building, especially to high people traffic areas (such as waiting rooms of a hospital, auditorium, theater lobby, public transportation hub, classroom, casino, high-end hotel lobby, club, etc.)
   
• 100% Outside Air System may be combined with cleanroom-type ventilation to avoid recirculation of the virus, assuming the business owner is willing to pay for the initial and operation costs to gain advantage in improving tenant safety.  

Reference:

1. Scientific Brief: SARS-CoV-2 and Potential Airborne Transmission: US Centers for Disease Control and Presentation website 

2. Transmission of COVID-19 virus by droplets and aerosols: A critical review on the unresolved dichotomy.   US National Library of Medicine National Institutes of Health(PMC)