A generation of research and experience has proven that when properly maintained
and operated, heating, ventilation and air-conditioning systems (HVAC) can reduce
the spread of viruses. These critical building systems not only provide thermal
comfort but, according to the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE), may also improve resistance to infection.
The American Society for Microbiology (ASM) has recently addressed the issue of COVID-19 transmission in the “built environment” (BE), defined as the buildings, automobiles and other indoor settings in which most humans spend more than 90 percent of their daily lives.
There are several major transmission vectors that promote infection in these built
environments, the report says, including occupant density, the amount of social activity and interaction, and human
contact with abiotic surfaces. The cruise ship industry, nursing homes and prisons have taught us about the risk of
transmission from settings where these vectors intersect. However, we also have learned that proper hand-washing
and social distancing work to reduce transmission.
Alongside these primary mitigants, HVAC systems work in a built environment to supply comfortable, clean,
recaptured air, mix in healthy levels of fresh air, and contain or exhaust contaminants. Air delivery systems can reduce the transmission of viruses through inline filtration, something HVAC professionals are capable of assessing.
Air-conditioning systems are also critical in maintaining healthy humidity levels. “Maintaining a RH (relative
humidity) between 40% and 60% indoors may help to limit the spread and survival of SARS-CoV-2 within the BE,”
the ASM suggests, “while minimizing the risk of mold growth and maintaining hydrated and intact mucosal barriers
of human occupants.”
The Centers for Disease Control (CDC) echoes these findings, saying that employers can decrease the spread of
COVID-19 by maintaining a healthy work environment. “Consider improving and engineering controls using the building ventilation system,” the CDC suggests, including increased ventilation rates and increased percentage of outdoor air circulating through the system.
Well before COVID-19, the Healthy Building Movement had begun to measure and improve air quality in the built
environment to improve productivity and health. Of the nine foundations for a healthy building, five relate to HVAC,
including air quality, ventilation, thermal health, moisture, dust and pests. “There’s just no reason anymore to
economize on airflow and filtration,” Harvard Business School’s John Macomber says. “It’s a cheap way to help people be healthier.”
A Restaurant Story
Modern, professionally maintained air conditioning can play a positive role in the
control of COVID-19 by ensuring a healthy built environment during and after
the pandemic. But news reports about an incident in a restaurant in China have
attributed the spread of the virus to the restaurant’s air-conditioning system.
Technically, none of this reporting was incorrect, but a careful look at the
underlying details reveals a very different story.
By February 10, 2020, 10 people from three families who had eaten at the same air-conditioned restaurant in
Guangzhou were infected with COVID-19. Researchers at the Guangzhou Center for Disease Control and Prevention
believe that the virus was transmitted from an asymptomatic 63-year-old woman in one family to at least one
member of each of two nearby families seated at neighboring tables about 1 meter apart. Because immunologists
are confident that COVID-19 can be transmitted via large infected droplets caused by talking, sneezing or coughing,
the researchers believe that this diner’s infected droplets — normally heavy enough to fall to the floor before
reaching a table 1 meter away — were boosted by airflow from the restaurant’s air conditioning.
Seventy-three other restaurant customers were identified as having close contact with members of those three
families, but none developed COVID-19 symptoms. Neither did the eight restaurant workers serving those guests.
Six smear samples from the air conditioner’s air outlet and air inlet also tested negative for the virus.
In other words, the restaurant’s air-conditioning system was virus-free and operating as intended. “The key
factor was the direction of the airflow,” researchers surmised.
Proper airflow management is essential. Without knowing all the details in this case, it is likely that improper air
distribution, combined with a lack of social distancing, may have contributed to the transmission in this restaurant.
It is important to manage airflow and airflow velocity in an occupied space. Research and ASHRAE guidelines
point to an upper limit of air velocity in an occupied space of 40 fpm. To achieve this condition, the air needs to be
properly blown by the HVAC system into the room, and properly distributed in the occupied space. It is unclear if the
restaurant in this case met these criteria, but, based on the researchers’ conclusions, it appears unlikely.
“To prevent spread of COVID-19 in restaurants,” the report concludes, “we recommend strengthening temperature monitoring surveillance, increasing the distance between tables, and improving ventilation.”
Nowhere in the report is there any suggestion of turning off the air conditioning as a mitigating action.
HVAC Best Practices
As previously mentioned, HVAC systems and the built environment can play an
important role in preventing the spread of viruses. To ensure the proper indoor air
purity, a good HVAC system should include some or all of the following:
1. (Demand Controlled) Ventilation: When outside air is not provided via separate devices, the HVAC system should
provide outside air based on the size/use of the space. Where possible, the HVAC system should include a sensor for
carbon dioxide or other pollutants to calculate and correct in real time the amount of ventilation needed. It is important to be aware that the increase of the ventilation rate may cause an increase of load, and the HVAC unit, if not properly sized, may not be able to provide sufficient cooling capacity. In such situations, it may be appropriate to consider Direct Outdoor Air Supply (DOAS) units, which are specifically designed for large amounts of outside air.
2. Filtration: Filters are rated on their ability to capture and retain particles of different sizes. The industry standard
is a Minimum Efficiency Reporting Value (MERV) rating. Filters with MERV>13 have a significant ability to capture
particulate matter (PM) and smaller particles. HEPA filters are even more efficient and are able to capture bacteria and viruses. Note that there are important tradeoffs to consider: the higher the filtration requirements, the greater the air pressure drop and the size of the filter. For this reason, the air management system of the HVAC needs to be carefully sized based on the filtration requirements.
3. Other Indoor Air Quality Devices: Numerous technologies are available to reduce the presence of contaminants.
Ultraviolet lights, ultraviolet photocatalytic oxidation, ionization, plasma, electrostatic active, active carbon and other components can be installed to specifically target volatile organic compounds (VOC), bacteria and viruses. Some of these options can be available as integral parts of the HVAC system.
Air Distribution:
1. The airflow rate, air velocity and direction of the air discharged by the air-conditioning unit need to be carefully
controlled. The goal is to have uniform distribution of temperature in the room and to avoid air velocities above 40
fpm in the occupied space, thus avoiding draft and risk of carrying particles from one part of the room to the other.
2. The total amount of airflow needs to be properly calibrated to the cooling capacity of the unit (a best practice
in North America of 200-400 cfm/ton is often quoted). In addition, the cooling capacity of the unit should not be
oversized or undersized compared with the cooling load of the space.
3. The location of the air outlet, the orientation of the air and the intensity of the air velocity at the discharge tend to
determine the airflow in the room and need to be optimized. The more the air is blown directly to an occupied area,
the more we will have a “spot cooling” effect and the worse the air distribution will be. On the other hand, an ideal
distribution is achieved by: (1) locating the air outlet in a position that ensures good airflow, but does not directly
blow air into the occupied space; (2) ensuring that air has the possibility to travel and expand before reaching the
occupied space.
Air Conditioning Facts
Air conditioning is defined as the process of controlling temperature, humidity,
purity and motion of air in an enclosed space. The main goal is to provide
comfort to the occupants or needed precision temperature and humidity
control.
In addition to comfort, good air conditioning improves health by reducing discomfort and thermal stress
and associated susceptibility to viruses.8
It is also proven that proper air conditioning in buildings increases productivity in schools and offices.9
In general, the primary parameters of indoor comfort/health are:
Temperature: It is the primary element of comfort. The ideal temperature (typically set using a thermostat)
varies depending on numerous conditions (season, location, clothes, etc.). ASHRAE and CDC recommend10 a
range of 68.5-75 F in the winter, 75-80.5 F in the summer.
Humidity: Excessively high or low humidity leads to discomfort. A target range of 40%-60% relative humidity is
normally used for comfort. ASHRAE recommends relative humidity below 60%.
Air Purity: In general, the presence of particulate, gases (carbon dioxide (CO2), radon, volatile organic
compounds), as well as viruses and bacteria cause poor air quality, with negative consequences for the
occupants. Air conditioning helps improve air quality with various techniques, of which the most widely used are
outdoor ventilation and filtration. ASHRAE prescribes specific ventilation rates depending on the application.11 For
instance, a conference room should see an outdoor ventilation rate of 15 cfm/person.
Air Velocity/Air Distribution: It is important that no sensation of draft (unwanted local cooling of the body
caused by air movement) is caused by the air conditioning or other elements of air movement in the occupied
space. Research and ASHRAE guidelines point to an upper limit of air velocity in the occupied space of 40 fpm.12
To achieve this condition, the air needs to be properly blown by the HVAC system into the room, and properly
distributed in the occupied space.
References
1 “Pandemic COVID-19 and Airborne Transmission,” ASHRAE Environmental Health Committee, approved April 17, 2020, Web April 23, 2020, https://
www.ashrae.org/file%20library/technical%20resources/covid-19/eiband-airbornetransmission.pdf.
2 Leslie Dietz et al., “2019 Novel Coronavirus (COVID-19) Pandemic: Built Environment Considerations To Reduce Transmission,” mSystems, Volume
5, Issue 2, March/April 2020, April 23, 2020, https://msystems.asm.org/content/5/2/e00245-20.
3 Leslie Dietz et al., “2019 Novel Coronavirus.”
4 “Interim Guidance for Businesses and Employers to Plan and Respond to Coronavirus Disease 2019 (COVID-19),” Centers for Disease Control and
Prevention, March 21, 2020, Web April 23, 2020, https://www.cdc.gov/coronavirus/2019-ncov/community/guidance-business-response.html.
5 Kristen Senz, “Why COVID-19 Raises the Stakes for Healthy Buildings,” Harvard Business School Working Knowledge, April 20, 2020, Web April 23,
2020, https://hbswk.hbs.edu/item/why-covid-19-raises-the-stakes-for-building-health.
6 Jianyun Lu et al., “COVID-19 Outbreak Associated with Air Conditioning in Restaurant, Guangzhou, China, 2020,” April 2, 2020, Web April 23, 2020,
https://wwwnc.cdc.gov/eid/article/26/7/20-0764_article.
7 Jianyun Lu et al., “COVID-19.”
8 ASHRAE Statement April 20, 2020: https://www.ashrae.org/about/news/2020/ashrae-issues-statements-on-relationship-between-covid-19-andhvac-in-buildings.
9 Joseph G. Allen and John D. Macomber, “Healthy Buildings – New Indoor Spaces Drive Performance and Productivity,” 2020.
10 ANSI/ASHRAE Standard 55-2013: Thermal Environmental Conditions for Human Occupancy.
11 ASHRAE Standard 62.1.
12 ANSI/ASHRAE Addendum b to ANSI/ASHRAE Standard 55-2013.
