Health authorities agree that coronavirus transmission occurs mostly from person to person, at distances below six feet. Large droplets with the virus are released when infected individuals sneeze or cough, or even when they yell or speak loudly. Infection can occur through direct contact with these droplets, or when they fall on surfaces of objects that are then touched by someone else. Smaller droplets or aerosols with the virus are more dangerous, since they can stay airborne more time, traveling longer distances.
HVAC systems cannot do much against short range transmission. In this case, infection occurs before SARS-CoV-2 can be handled by filtering, disinfection or exhaust systems. However, airborne transmission at longer distances has not been ruled out, and here is when HVAC measures become effective. ASHRAE published extensive guidance on how to make buildings resilient against COVID-19.
Redesign your HVAC system for COVID-19 prevention.
HVAC Design Strategies Against Coronavirus
HVAC engineers can use several design features to disrupt the transmission pathways of coronavirus. These strategies are not mutually exclusive, and they achieve the best effect when combined:
- Dilution with increased ventilation: This reduces concentration of pathogens in the air.
- Room pressurization: Controlling the direction of airflow patterns to minimize exposure.
- Humidity control: The transmission of bacteria and germs is minimized at 40% - 60% relative humidity.
- Thermal comfort: Indoor temperatures should be controlled effectively and kept within a comfortable range. It is believed that thermal stress makes building occupants more vulnerable to infection.
- Filtration: High-performance filters such as MERV 13-16 and HEPA can capture many airborne germs, and also non-biological pollutant particles.
- Ultraviolet germicidal irradiation: UV light can inactivate pathogens like coronavirus by destroying their DNA or RNA.
Disabling HVAC systems is not recommended under any circumstances, even when a building is still empty. When left unchecked, air humidity and temperature variation can damage several materials and devices. All facilities should follow the latest versions of design standards and building codes, and also the published guidance and best practices to deal with COVID-19.
Preventing Coronavirus Infection by Small Airborne Droplets
Aerosols smaller than 10 microns are of special interest for ASHRAE, since they can stay airborne for hours or even days. This gives them plenty of time to spread with indoor air movement, and possibly infect occupants who are far from the source. Larger droplets can also shrink by evaporation, becoming airborne aerosols as well. Since aerosols are so small and numerous, the best strategy against them is controlling the bulk movement of air, combined with filtering and disinfection measures.
As mentioned above, HVAC systems have little or no effect against short-distance coronavirus infection. These prevention measures are intended as a complement for social distancing and personal protection equipment (PPE), not a replacement. The effectiveness of HVAC measures against COVID-19 depends on several factors:
- Source location and density.
- Droplet size.
- Type of air distribution system.
- Air temperature.
- Relative humidity.
The first two factors cannot be controlled by HVAC engineers. However, the rest can be optimized to reduce the risk of coronavirus infection in buildings. The air distribution system should remove the air in rooms that may contain coronavirus sources, before it can spread to other building areas.
Portable air purifiers with HEPA filters and UV disinfection are recommended, as long as they don’t release ozone. These devices can often capture germs and other air pollutants at the source. Particles with a size of around 0.3 microns are the most penetrating, and a HEPA filter can capture 99.97% of them, with a higher efficiency for smaller and larger particles. Consider that filtration effectiveness also depends on the filter’s location, and the airflow through the unit.
Ideally, a building should be ventilated with 100% outdoor air, combined with high-efficiency filtration and ultraviolet disinfection. There are other viable measures to improve indoor air quality, but these three are the top 3 recommendations from ASHRAE. If the ventilation layout allows it, each building area should have its own air supply and exhaust, minimizing air movement between rooms.
Additional Measures for a COVID-19 Resilient Building
Companies can deploy all the latest technologies to fight coronavirus, but the most effective measure is keeping the virus away from the buildings in the first place. Business managers should analyze all job positions, and collaborators should be instructed to work from home unless their presence is completely necessary.
Prevention measures are required in all commercial occupancies, but especially in buildings where many people interact: healthcare, offices, retail and manufacturing are some examples. Coronavirus can also spread with mass transit, providing even more reasons to work from home as much as possible.
Maintenance personnel should wear personal protective equipment (PPE) when working with building systems that may contain SARS-CoV-2 or other germs. For example, PPE should be worn when changing filters or working with air-handling units (AHU). ASHRAE recommends using the same PPE as a healthcare worker: N95 respirators, eye protection, disposable gloves, disposable coveralls and shoe covers. After the task is completed, the used PPE should be disposed of and the personnel involved should shower and change clothes.
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