Last fall and winter, more time indoors led to a predictable surge in COVID-19 cases in many parts of the U.S. and Canada. As we head towards another fall and winter – this time with the more contagious Delta variant still circulating – it’s time to make sure we’re doing all we can to reduce exposure risk indoors.
Facial coverings, physical distancing and handwashing continue to be important to reduce the spread of COVID-19. However, occupational safety and health principles tell us to prioritize controls that don’t rely on human behavior to work, and one such area is improving building ventilation.
“Risk for COVID-19 transmission is always going to be higher indoors than outdoors,” says LIUNA General President Terry O’Sullivan. “By following these ventilation best practices, building owners and employers can reduce risk for LIUNA members and all building occupants this fall and winter.”
Improving ventilation is centered around the goal of reducing the concentration of virus particles in the air. The lower the concentration, the less likely it is that virus particles will be inhaled into the lungs or contact the eyes, nose or mouth. The joint ACGIH/ASHRAE whitepaper, Ventilation for Industrial Settings During the COVID-19 Pandemic, provides many ventilation guidelines and practical suggestions that building owners and employers can use to reduce worker exposure to droplets and aerosols that may contain SARS-CoV-2.
Understanding Different Types of Ventilation
As a rule of thumb, more airflow is better, and the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) generally recommends against reducing or eliminating ventilation regardless of its source. However, when multiple ventilation options are available, some are preferable to others.
Dilution ventilation mixes contaminated air with clean air to lower the concentration of virus particles in a room. Examples include overhead fans or open windows. Because we still don’t know how many virus particles it takes to transmit COVID-19, dilution ventilation is not the preferred option.
Displacement ventilation pushes contaminated air away by replacing it with clean air, such as in an HVAC system. Replacing contaminated air with clean air is the preferred option to control occupant exposure levels.
Key Ventilation Principles and Best Practices
While no two buildings are exactly the same and ventilation may need to be adjusted based on heating and cooling requirements, humidity control and other factors, the following general principles should be followed as much as possible.
Maximize clean outside air. Increase the outdoor air supply to 100 percent, or the maximum allowed by the ventilation system or environmental conditions. ASHRAE recommends maintaining between six and twelve air changes per hour. This schedule will purge 99 percent of air every 30-60 minutes. However, ASHRAE notes that the “concept of purging does not apply if a source [i.e., an infected person] is still present in the workspace.”
Maximize filtration of recirculated air. It often won’t be feasible to use 100 percent outdoor air. In these cases, the practical solution is to use as much outdoor air as possible combined with recirculated air that has been adequately filtered. Increase the filtration efficiency of the system to Minimum Efficiency Reporting Value (MERV) 13, or as high as the system will allow.
Monitor airflow patterns. While more airflow is generally better, the way air flows should also be monitored to avoid carrying contaminated air to workers. For example, large overhead ceiling fans will cause air to flow downwards, potentially returning viral particles back toward workers. (Air exhaled by our lungs tends to rise because it’s warmer and more humid than the surrounding air.) Similarly, avoid having pedestal fans arranged in a way that blows air from one worker to another. It’s important to remember that in the case of COVID-19, every person is a potential source of transmission. In general, employers should try to place workers closer to cleaner air, not downstream where “dirty” air is being carried.
Monitor exhaust locations. Potentially contaminated air being displaced has to go somewhere. Ensure that exhaust systems operate continuously and vent air directly outdoors. Temporarily disable restroom fans that don’t exhaust outdoors. (Hand dryers can also be temporarily disabled and replaced with disposable paper towels.)
Considerations for Multiple Rooms
A Building and Environment journal study found that in buildings with multiple rooms, increasing outdoor air and improved filtration were more important for reducing viral load than air exchanges alone. When only air exchange was used, viral particles traveled to the room next door.
“Most studies have looked at particle levels in just one room, and for a one-room building, increased ventilation is always useful,” said Leonard Pease, the study’s lead author. “But for a building with more than one room, air exchanges can pose a risk in the adjacent rooms by elevating virus concentrations more quickly than would otherwise occur.”
The study showed the clear benefits of outdoor air and air filtration. Replacing a third of a building’s air per hour with outdoor air significantly reduced virus particles in adjacent rooms. Use of MERV-13 filtration reduced peak concentration of viral particles in connected rooms by 93 percent. On the other hand, the use of only rapid air exchanges reduced virus particles in the contaminated room by 75 percent, but also caused them to spike in adjacent rooms and remain elevated for approximately 90 minutes.
These results show the clear need for a combination of a large amount of outdoor air, minimum MERV-13 air filtration and monitored air exchanges.