Faster Air Exchange in Buildings Is Not Always Beneficial for Coronavirus Levels

When the infected person in the office coughs to the left, respiratory droplets containing viral particles come out through the vent in the ceiling of the office. Some droplets leave the building, while others are sent into the building and several rooms via the air handling unit. A PNNL team found that a high ventilation rate can increase levels of viral particles downstream from a source chamber. Credit: Illustration / Animation: Cortland Johnson / Sara Levine, Pacific Northwest National Laboratory

Model study suggests that vigorous ventilation can cause a spike in viral concentrations.

Powerful and fast air exchanges may not always be a good thing when it comes to addressing levels of coronavirus particles in a multi-room building, according to a new model study.

The study suggests that, in a multi-room building, rapid air exchanges in high concentrations can quickly spread the virus from the source room to other rooms. Particle levels peak in adjacent chambers within 30 minutes and can remain high for up to about 90 minutes.

The findings, published in the journal Building and Environment, come from a team of researchers at the U.S. Department of Energy’s Pacific Northwest National Laboratory. The team includes construction and HVAC experts as well as experts in aerosol particles and viral materials.

“Most studies have looked at particle levels in just one room, and for a single-room building, more ventilation is always helpful in reducing their concentration,” said Leonard Pease, lead author of the study. “But for a building with more than one room, air exchanges can pose a risk in adjacent rooms by increasing virus concentrations faster than they would otherwise.

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“To understand what is happening, consider how passive smoking is spread throughout a building. Near the source, air exchange reduces the smoke near the person, but can spread the smoke at lower levels to nearby rooms, ”added Pease. “The risk is not zero for any respiratory disease.”

The team modeled the distribution of particles similar to SARS-CoV-2, the virus that causes COVID-19, through air conditioning systems. Scientists have modeled what happens after a person has a five-minute coughing fit in a room of a small three-room office building, running simulations with five-micron particles.

Researchers looked at the effects of three factors: different filtration levels, different rates of outside air included in the building’s air supply, and different ventilation rates or air changes per hour. For downstream rooms, they found an expected clear benefit of increasing outside air and improving filtration, but the effect of an increased ventilation rate was less clear.

Cleaner outside air reduces transmission

Scientists have studied the effects of adding varying amounts of outside air to the building’s air supply, from no outside air to 33 percent of the building’s air supply per hour. As expected, the addition of more clean outside air reduced the transmission risk in the connected rooms. Replacing a third of the air in a building per hour with clean outside air reduced the risk of infection in downstream rooms by about 20 percent compared to the lower levels of outside air typically found in buildings. The team noted that the model assumed the outside air was clean and virus-free.

“More outside air is clearly a good thing for transmission risk, as long as the air is virus-free,” said Pease.

Strong filtration reduces transmission

The second factor studied – strong filtration – was also very effective in reducing coronavirus transmission.

The team studied the effects of three filtration levels: MERV-8, MERV-11 and MERV-13, where MERV stands for minimum efficiency reporting value, a common measure of filtration. A higher number translates to a stronger filter.

Filtration significantly reduced the risk of infection in the connected chambers. A MERV-8 filter reduced the peak level of virus particles in connected chambers to only 20 percent of what it was without filtration. A MERV-13 filter reduced the peak concentration of virus particles in a connected chamber by 93 percent, to less than one-tenth of what it was with a MERV-8 filter. The researchers note that the stronger filters have become more common since the start of the pandemic.

More ventilation – a more complex picture

The most surprising finding of the study involved ventilation – the effect of what researchers call hourly air changes. What’s good for the source room – reducing the transmission risk within the room by 75 percent – isn’t great for connected rooms. The team found that a rapid air change, 12 air changes per hour, can cause a spike in viral particle levels in connected rooms within minutes. This increases the risk of infection in those chambers for a few minutes to more than 10 times as much as at lower air exchange rates. The higher transmission risk in connected rooms persists for about 20 minutes.

“More ventilation is clearly a good thing for the source space. But that air is going somewhere, ”said Pease. “Perhaps more ventilation is not always the solution.”

Interpret the data

“There are many factors to consider, and the risk assessment is different for each case,” said Pease. “How many people are there in the building and where are they located? How big is the building? How many rooms? There isn’t much data at this point on how viral particles move in multi-room buildings.

“These numbers are very specific to this model – this particular type of model, the amount of virus particles shed by a person. Every building is different, and more research needs to be done, ”added Pease.

Co-author Timothy Salsbury, a building management expert, notes that many of the trade-offs can be quantified and weighted depending on the circumstances.

“Stronger filtration translates into higher energy costs, as does the introduction of more outside air than would normally be used in normal operations. Under many conditions, the energy consumption for the increased fan power required for strong filtration is less than the energy consumption for heating or cooling additional outside air, ”said Salsbury.

“There are many factors to consider – filtration level, outside air levels, air exchange – to minimize transmission risk. Building managers have certainly taken their work off their hands, ”he added.

Additional experimental studies are underway

The team is already conducting a series of experimental studies in the same direction as the model study. Like the newly published study, the additional analyzes look at the effects of filtration, outside air intake, and air changes.

These ongoing studies involve real mucus particles (without the actual SARS-CoV-2 virus) and take into account differences between particles expelled from different parts of the airways, such as the oral cavity, larynx and lungs. Researchers use an aerosol machine that dispenses the virus-like particles as they would be spread by a cough, as well as fluorescent tracking technology to monitor where they are going. Other factors include varying particle sizes, how long viral particles are likely to be contagious, and what happens when they fall and decay.

Reference: “Investigation of possible aerosol transmission and infectivity of SARS-CoV-2 through central ventilation systems” by Leonard F. Pease, Na Wang, Timothy I. Salsbury, Ronald M. Underhill, Julia E. Flaherty, Alex Vlachokostas, Gourihar Kulkarni and Daniel P. James, January 29, 2021, Building and Environment.
DOI: 10.1016 / j.buildenv.2021.107633

In addition to Pease and Salsbury, the authors of the published study include Nora Wang, Ronald Underhill, Julia Flaherty, Alex Vlachokostas, Gourihar Kulkarni, and Daniel James.

The study, the latest in a series of PNNL findings on COVID-19, brings together PNNL’s strengths in building technologies and aerosol science. The work was funded by the National Virtual Biotechnology Laboratory, a consortium of all 17 national DOE laboratories focused on response to COVID-19, with funding from the Coronavirus Aid, Relief and Economic Security, or CARES, Act.