Decades of research and practical experience show that properly maintained and operated heating, ventilation, and air-conditioning (HVAC) systems can reduce the spread of viruses. These critical building systems not only provide thermal comfort but also, according to the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE), may improve resistance to infection.
The Built Environment and Virus Transmission
The American Society for Microbiology (ASM) has recently addressed COVID-19 transmission in the “built environment” (BE), which includes buildings, automobiles, and other indoor spaces where most people spend over 90% of their daily lives.
Several major transmission vectors contribute to infection in these environments:
- Occupant density
- Social activity and interaction
- Contact with abiotic (non-living) surfaces
Settings like cruise ships, nursing homes, and prisons highlight risks where these factors intersect. However, proven mitigants such as proper hand-washing and social distancing remain effective.
HVAC Systems as a Mitigating Factor
In addition to primary mitigation strategies, HVAC systems help:
- Supply comfortable, clean, and recirculated air
- Introduce healthy levels of fresh outdoor air
- Contain or exhaust contaminants
Air delivery systems can reduce virus transmission through inline filtration, which HVAC professionals can assess and optimize.
Humidity Control
Maintaining indoor relative humidity (RH) between 40% and 60% can:
- Limit the spread and survival of SARS-CoV-2
- Minimize mold growth
- Keep mucosal barriers of human occupants hydrated and intact (ASM recommendation)
CDC Guidance
The Centers for Disease Control and Prevention (CDC) recommends employers:
- Improve ventilation rates
- Increase outdoor air circulation through HVAC systems
to help reduce COVID-19 spread in workplaces.
The Healthy Building Movement
Before COVID-19, efforts to measure and improve air quality in buildings focused on health and productivity. Five of the nine foundations of a healthy building are HVAC-related:
- Air quality
- Ventilation
- Thermal health
- Moisture control
- Dust and pests management
John Macomber of Harvard Business School notes that investing in airflow and filtration is a cost-effective way to enhance occupant health.
Case Study: COVID-19 Transmission in a Restaurant
A notable COVID-19 outbreak in a Guangzhou, China restaurant was initially linked to the air-conditioning system. However, detailed analysis reveals a more nuanced picture:
- By February 10, 2020, 10 people from three families who dined in the same air-conditioned restaurant contracted COVID-19.
- The virus likely spread from an asymptomatic diner to others at nearby tables about 1 meter apart, aided by airflow from the air conditioner boosting the reach of infected droplets.
- Notably, 73 other customers and eight staff with close contact did not develop symptoms.
- Air samples from the air conditioner tested negative for the virus, indicating the system itself was not contaminated.
The key factor was airflow direction, not the HVAC system’s failure. Improper air distribution combined with lack of social distancing likely contributed to transmission. ASHRAE recommends controlling air velocity to a maximum of 40 feet per minute (fpm) in occupied spaces to reduce risk.
The restaurant case highlights the need to:
- Strengthen temperature monitoring
- Increase distance between tables
- Improve ventilation
Importantly, turning off air conditioning is not recommended.
HVAC Best Practices for Virus Mitigation
To ensure healthy indoor air quality, HVAC systems should include:
1. Demand-Controlled Ventilation
- Supply outdoor air based on space size and use
- Use sensors (e.g., carbon dioxide) to adjust ventilation in real time
- Consider Direct Outdoor Air Supply (DOAS) units for large fresh air volumes
2. Filtration
- Use filters rated by Minimum Efficiency Reporting Value (MERV)
- MERV 13+ filters capture finer particles; HEPA filters capture bacteria and viruses
- Balance filtration efficiency with air pressure drop and system capacity
3. Additional Indoor Air Quality Technologies
- Ultraviolet (UV) lights
- Photocatalytic oxidation
- Ionization and plasma
- Electrostatic filters
- Activated carbon
Some technologies may be integrated into HVAC systems for targeting VOCs, bacteria, and viruses.
4. Air Distribution
- Control airflow rate, velocity, and direction to ensure uniform temperature
- Avoid air velocities above 40 fpm to prevent drafts and unwanted particle transport
- Calibrate airflow to the unit’s cooling capacity (typically 200-400 cfm per ton)
- Optimize air outlet location and orientation to avoid “spot cooling” and ensure even air mixing
Air Conditioning Fundamentals
Air conditioning involves controlling temperature, humidity, purity, and air motion in enclosed spaces to provide occupant comfort and health benefits.
Key Indoor Comfort and Health Parameters
- Temperature: ASHRAE and CDC recommend indoor temperatures of 68.5–75°F in winter and 75–80.5°F in summer.
- Humidity: Target relative humidity of 40%-60% for comfort and health, keeping it below 60% to avoid mold growth.
- Air Purity: Reduce particulates, gases (CO2, VOCs), viruses, and bacteria with ventilation and filtration. Ventilation rates vary by space usage (e.g., 15 cfm/person in conference rooms).
- Air Velocity: Maintain airflow velocity below 40 fpm in occupied areas to prevent drafts and discomfort.
References
- ASHRAE Environmental Health Committee, “Pandemic COVID-19 and Airborne Transmission,” April 17, 2020.
- Leslie Dietz et al., “2019 Novel Coronavirus (COVID-19) Pandemic: Built Environment Considerations To Reduce Transmission,” mSystems, 2020.
- Centers for Disease Control and Prevention, “Interim Guidance for Businesses and Employers,” March 21, 2020.
- Kristen Senz, “Why COVID-19 Raises the Stakes for Healthy Buildings,” Harvard Business School, 2020.
- Jianyun Lu et al., “COVID-19 Outbreak Associated with Air Conditioning in Restaurant, Guangzhou, China,” 2020.
- ASHRAE Statements and Standards, 2020.
- Joseph G. Allen and John D. Macomber, “Healthy Buildings – New Indoor Spaces Drive Performance and Productivity,” 2020.
- ANSI/ASHRAE Standards for Thermal Conditions and Ventilation.