Vigorous and rapid air exchange may not always be a good thing to tackle coronavirus particle levels in a multiroom building, according to a new modeling study.
The study suggests that in a building with several rooms, rapid air exchange can quickly spread the virus from the source room to other rooms at high concentrations. Particle levels rise in adjacent rooms within 30 minutes and can remain high for around 90 minutes.
The results, published online in their final form on April 15 in the journal Building and environment, come from a team of researchers from the Pacific Northwest National Laboratory of the US Department of Energy. The team includes experts in building and HVAC as well as experts in aerosol particles and viral materials.
“Most of the studies have looked at the levels of particles in a single room, and for a one-room building, increased 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 exchange can pose a risk in adjacent rooms by increasing virus concentrations faster than would otherwise happen.
“To understand what is going on, consider how second-hand smoke is distributed in a building. Near the source, air exchange reduces the smoke near the person but can distribute the smoke to lower levels in adjoining rooms. “, added Pease. “The risk is not zero, for any respiratory disease.”
The team modeled the spread of particles similar to SARS-CoV-2, the virus that causes COVID-19, through air handling systems. Scientists modeled what happens after a person has a five-minute coughing fit in a room in a small, three-room office building, by running simulations with five-micron particles.
The researchers examined the effects of three factors: different levels of filtration, different rates of incorporation of outside air into the building’s air supply, and different rates of ventilation or air renewal per hour. For downstream rooms, they found a clear expected benefit of increasing outside air and improving filtering, but the effect of increasing the ventilation rate was less evident.
Cleaner outside air reduces drivetrain
Scientists studied the effects of adding varying amounts of outside air to the building’s air supply, from no outside air to 33% of the building’s air supply per hour. As expected, the incorporation of cleaner outside air has reduced the risk of transmission into connected rooms. Replacing one-third of a building’s air per hour with clean outdoor air in downstream rooms reduced the risk of infection by about 20% compared to lower levels of outdoor air typically included in buildings. The team noted that the model assumed the outside air to be clean and virus-free.
“More outside air is clearly a good thing for the risk of transmission, as long as the air is virus-free,” Pease said.
Strong filtration reduces transmission
The second factor studied – strong filtration – was also very effective in reducing the transmission of the coronavirus.
The team studied the effects of three levels of filtration: MERV-8, MERV-11, and MERV-13, where MERV represents the minimum efficiency ratio value, a common measure of filtration. A higher number results in a stronger filter.
Filtration has significantly reduced the risk of infection in connected rooms. A MERV-8 filter reduced the peak level of virus particles in connected rooms to just 20% compared to what it was without filtration. A MERV-13 filter lowered the maximum concentration of viral particles in a smart room by 93%, to less than a tenth of what it was with a MERV-8 filter. The researchers note that more powerful filters have become more common since the start of the pandemic.
Increased ventilation – a more complex picture
The most surprising finding in the study was about ventilation – the effect of what researchers call changes of air per hour. What is good for the source part – reducing the risk of transmission in the room by 75% – is not so good for connected parts. The team found that a rapid air exchange rate, 12 air changes per hour, can cause viral particle levels to increase within minutes in connected rooms. This increases the risk of infection in these rooms for a few minutes to more than 10 times what it was at lower air change rates. The higher risk of transmission in connected rooms persists for approximately 20 minutes.
“For the source room, it’s clear that more ventilation is a good thing. But that air is going somewhere,” Pease said. “Maybe more ventilation isn’t always the solution.”
Interpret the data
“There are many factors to consider and the risk calculation is different for each case,” Pease said. “How many people are there in the building and where are they located? How big is the building? How many rooms? There isn’t a lot of data at this point on how the virus particles move around in multi-room buildings.
“These numbers are very specific to this model – this particular type of model, the amount of virus particles released by a person. Every building is different and more research needs to be done,” Pease added.
Co-author Timothy Salsbury, a building control expert, notes that many tradeoffs can be quantified and weighted depending on the circumstances.
“Stronger filtration results in higher energy costs, as does the introduction of more outside air than would typically be used in normal operations. In many circumstances, the energy penalty for increased airflow. Fan power required for strong filtration is less than the energy penalty for heating or cooling additional outside air, ”Salsbury said.
“There are many factors to balance – filtration level, outside air levels, air exchange – to minimize the risk of transmission. Building managers certainly have their work cut out for them, ”he added.
Further experimental studies underway
The team is already carrying out a series of experimental studies along the same lines as the modeling study. Similar to the recently published study, additional analyzes focus on the effects of filtration, outside air intake, and air changes.
These ongoing studies involve real mucus particles (not incorporating the SARS-CoV-2 virus) and take into account the differences between particles expelled from various parts of the respiratory tract, such as the oral cavity, larynx, and lungs. . Investigators are deploying an aerosol machine that disperses virus-like particles as they are dispersed through a cough, as well as fluorescent tracking technology to monitor where they are going. Other factors include the varying particle size, how long viral particles are likely to be infectious, and what happens when they fall and disintegrate.
Besides Pease and Salsbury, the authors of the published study include Nora Wang, Ronald Underhill, Julia Flaherty, Alex Vlachokostas, Gourihar Kulkarni and Daniel James.
The research, the latest in PNNL’s series of findings on COVID-19, brings together PNNL’s strengths in building technology and aerosol science. The work was funded by the National Virtual Biotechnology Laboratory, a consortium of DOE’s 17 National Laboratories focused on responding to COVID-19, with funding provided by the Coronavirus Aid, Relief and Economic Security Act, or CARES.