Introduction
From January 26 through February 10, 2020, an outbreak of 2019 novel Coronavirus Disease (COVID-19) affected 10 persons from 3 families who had eaten at the same air-conditioned restaurant in Guangzhou, China. On January 23, 2020, a family had traveled from Wuhan and arrived in Guangzhou where they ate lunch in an air conditioned restaurant on January 24, 2020 (Figure 1, Table A). Two other families sat at neighboring tables (Figure 1, Tables B and C) at the same restaurant. Later in the day on January 24, one of the family members (A1) became symptomatic. By February 5, a total of 9 others, including members of the other two families had become ill with COVID-19. It was determined that the only known source of exposure was the first person who became symptomatic. The tables the respective families ate at were separated by a distance of 1 meter. The air outlet and the return air inlet of the central air conditioner were located above the table C in Figure 1.
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On January 24, a total of 91 persons (83 customers, 8 staff members) were in the restaurant at the time of exposure. Of these, a total of 83 had eaten lunch at fifteen tables located in the restaurant. Among the 83 customers, 10 became ill with COVID-19; the other 73 were identified as close contacts and quarantined for 14 days. During that period, no symptoms developed, and COVID-19 test results were negative (Lu J, Gu J, Li K, et al., 2020).
This indicates that, while larger respiratory droplets (>5µm) remain in the air for a short time and travel for only a short distance, generally < 1m, the distance between the infected person and the other infected person was > 1m. This indicates that virus-laden small (< 5 µm) aerosolized droplets can remain in the air and travel long distances. However, since none of the other diners or staff were infected, it would indicate that the droplets did not travel greater than 5 meters from the source. The risk from becoming infected from an unprotected person at a distance of >5m is 1.08 x 10 -1 in the absence of any protective measures.
In the absence of detailed scientific data to provide non-cursory results, one can look to yet another airborne pathogen which has a transmission mobility similar to that which is known of COVID-19. In the Center of Disease Control Morbidity and Mortality Weekly Report, Volume 55, Number RR-9, the subject of Prevention and Control of Tuberculosis is discussed. Tuberculosis is a well-researched and studied airborne pathogen with a similar mobility of transmission of that which is known of COVID-19. It is reasonable to assume that the containment and control methodologies used for Tuberculosis would likewise be applicable to COVID-19.

Engineering Controls 
Based on the information in the afore mentioned document, a key element in the prevention of spread of this pathogen in an enclosed populated area is adequate ventilation. The document suggests that inadequate Air Changes per Hour (ACH) can lead to the spread of these airborne pathogens. The ACH is the ratio of the volume of air entering the room per hour to the volume of that room. It equals the exhaust airflow (Q) in cubic feet per minute (cfm) divided by the volume of the room (V) in cubic feet (ft3) multiplied by 60 minutes per hour, as expressed thus:
ACH = (Q / V) x 60
The most common minimum total ACH recommended for environments similar to an office environment with an HVAC system installed is 6 (CDC, 2006).
This is further substantiated by the World Health Organization (WHO) in their document entitled, Ventilation and airborne diseases. According the WHO, “Poorly ventilated buildings affect air quality and can contribute to the spread of disease. Microorganisms, such as those causing tuberculosis and legionellosis, can be transmitted by air-conditioning systems, particularly when they are poorly maintained or when the number of air exchanges per hour in a room are insufficient” (WHO, 2020).

The Figure below demonstrates the risk reduction as it relates to an increase of ACH for healthcare workers. (Figure 2)
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While the acceptable level of risk is defined by the respective organization, one would struggle to make an affirmative argument, that ACH values of less than 6 are acceptable in environments supported by mechanical ventilation where there is known to be, or could potentially be, an active source of COVID-19 infection. 

Air Sanitization 
The concept of control layering dictates that to increase the probably of control success, redundancy is necessary. Therefore, in addition to adequate ACH, air sanitization must also be utilized to control the transference of novel COVID-19 though HVAC systems. The two predominate methods for sanitizing air in HVAC systems are: 1) Air Filtration using High-Efficiency Particulate Arrestance (HEPA) or Ultra-Low Penetration Air (ULPA) filters, and 2) Ultraviolet Germicidal Irradiation systems. 

Air Filtration 
Fluid droplets from the cough or sneeze of an infected person are typically 5 microns (5×10-6 m) or larger. HEPA filters reliably capture 99.97% of particles that are 0.3 microns in diameter with efficiency increasing for both smaller and larger particles. 
The smallest particle one might be concerns with a single virion (unattached to any fluid droplet), having a diameter of approximately 0.12 microns. While these are conceivably filterable by a HEPA filter, Ultra-Low Penetration Air (ULPA) filters are even better, catching 99.99% of particles 0.12 microns and above. In theory all COVID-19 virions could be filtered and captured, assuming they can be that they can be brought into contact with an air filter (Blake, 2020). Both HEPA and ULPA type filters can be purchased for most industrial and residential HVAC systems and is more than adequate to capture and contain COVID-19 virus-laden particles. When selecting one should pay attention to the Minimum Efficiency Reporting Value (MERV) assigned to the respective filter. The MERV rating indicates, on a scale of 1-16, how effectively a filter traps small particles (Figure 3).

Ultraviolent (UV) Germicidal Irradiation Systems 
Unlike a standard air filer or an electrostatic air cleaner, a UV air treatment system uses concentrated UV light to destroy a wide array of indoor air pollutants. These systems may also be equipped with an enhanced filter (HEPA) to provide additional protection against airborne dust, microorganisms, and other particulates. Ultraviolet light can harm several types of microorganisms, including mold, mildew, fungi, bacteria, and viruses, by breaking down molecular bonds in their DNA. The exposure to the UV light either kills the bio-contaminants or renders them unable to reproduce. The UV air treatment system works in tandem with the HVAC system and is mounted inside of the systems ductwork. When the air circulates through the ducts, the pollutants in the air passes through the UV rays with destroy them. 

Conclusion 
In summation, given the information discussed and data available, the best course of action to prevent the spread of COVID-19 via an HVAC unit is to: 1) Ensure potentially infectious individuals are not within 5 meters of the HVAC inlet duct; 2) Measure the ACH of each affected space and take necessary steps to achieve an ACH value of 6 or greater; 3) Install air sanitation filters, UV light systems (or both) on the inlet side of HVAC systems which are recirculating air from inside the building. These tertiary controls should prove effective in preventing the transference of COVID-19 through HVAC Systems.  

​Sources:
Lu J, Gu J, Li K, Xu C, Su W, Lai Z, et al. COVID-19 outbreak associated with air conditioning in restaurant, Guangzhou, China, 2020. https://doi.org/10.3201/eid2607.200764 Accessed: April 23, 2020.

Abbott, Geoff. Council for Scientific and Industrial Research. Ventilation and airborne diseases, World Health Organization, N.D. https://www.who.int/sustainable-development/health-sector/health-risks/airborne-diseases/en/ Accessed: April 23, 2020

Centers for Disease Control and Prevention. Prevention and Control of Tuberculosis in Correctional and Detention Facilities: Recommendations from the CDC. MMWR 2006; 55(No. RR-9). July 7, 2006. Accessed April 23, 2020. https://www.cdc.gov/mmwr/PDF/rr/rr5509.pdf Accessed April 23, 2020.
Blake Elias and Yaneer Bar-Yam, Could air filtration reduce COVID-19 severity and spread?, New England Complex Systems Institute (March 9, 2020).