- Open windows in a classroom may give a false, or incomplete, impression of good ventilation.
- Cold surfaces, such as windows, can pose an additional risk in enclosed spaces.
- The key to reducing transmission is to limit horizontal airflow at the breathing level.
- To reduce SARS-CoV-2 exposure indoors, it is crucial to space seating according to guidelines, wear masks, and keep windows open.
In the early days of the pandemic, researchers proposed that the main route of SARS-CoV-2 transmission was either via large droplets that people exhaled in a projectile motion through coughs and sneezes or via surface contamination. Recent work, however, has uncovered that virus particles can also infect others via long-range spread.
Behind this long-range spread are
Larger particles are of lesser concern in that aspect as they tend to sink to the ground and remain there.
The researchers behind a new study, which appears online in the journal Building and Environment, decided to explore this phenomenon further by applying it to classrooms to see whether having students and teachers present in the room would change the SARS-CoV-2 exposure risk.
To try to understand the risk of COVID-19 transmission at schools, the researchers — from the Massachusetts Institute of Technology (MIT) — simulated two different types of ventilation in a classroom: ceiling diffusers and open windows. They used computational fluid dynamics and results from several cases to explore the effects of both systems.
The researchers found that people at rest generate about 75 watts of body heat and that this creates rising plumes of warm air around them. These thermal plumes then entrain cold air (and any particles inside it) and continue to rise until they encounter a layer of air at the same temperature.
Ceiling vents and diffusers could further encourage his upward motion, the team hypothesized.
The investigators found that only the air that leaves the mouth at very low speeds can rise with the human plume.
Therefore, without a face mask, particles in coughs and forceful exhales from the mouth can easily escape and spread across the room by interacting with the surrounding air.
These particles are less likely to rise, the study showed, so they are also more likely to spend longer periods at breathing level or below, making for a more infectious environment.
To avoid that outcome, the study also explored the effects of windows.
The well-mixed assumption
The previously held assumption was that the concentration of aerosols is uniform throughout a room at a specific moment.
The new study discovered that this was far too conservative of an estimation, finding that the density of aerosol particles near individuals in a room can be 2.5 times more than experts had thought.
Therefore, it is only possible to assume that the air indoors is well-mixed, and hence presents a lower infection risk, in unoccupied spaces.
The researchers note that, in fact, the location of the individuals and the ventilation type largely influence the distribution of aerosols in a room.
The MIT team tested various scenarios in classrooms and found that the concentration of virus particles was 50–150% greater than the widely held assumption based on the air being well-mixed.
“Due to time constraints, we were unable to model all the possible combinations of ventilation strategies and location of infected students. Our current results identify several potential hazardous situations,” Leon Glicksman, professor of building technology and mechanical engineering at MIT, told Medical News Today.
The study identified two particularly concerning situations.
In one scenario, the team found that open windows could contribute to the horizontal spread of particles.
Although airing out the classroom by opening the windows is a natural instinct and the main method of ventilation in many classrooms, the study found a caveat.
If the windows are on the same level as students’ desks, simulations showed that the cold air that enters the room contributes to the horizontal spread of particles and droplets.
To improve this, the study suggested “displacement ventilation,” which people can achieve in a practical and simple way by placing a baffle inside the window to direct air to the floor.
“Aerosol from an infected person sitting in line with an open window can be carried to the students behind. The exposure is reduced if the airflow from the window is directed toward the floor using a simple deflector or fan,” Prof. Glicksman explained.
Another concerning scenario involved a student with COVID-19 sitting next to sealed and cold windows.
As expected, in this simulation, each cough and exhale caused the projection of droplets and aerosolized particles through the air. This proved particularly problematic because the cold air flowing down the window recirculated aerosols into the breathing zone.
Prof. Glicksman explained:
“In a classroom with closed windows and a heating, ventilation, and air conditioning (HVAC) system of ventilation, cold single glazed windows pose a hazard. Aerosols from a [person with the infection] sitting near a cold window initially rise but then are captured in the cold air flowing down near the window and return to the breathing level.”
The team suggested combatting this by using window covers, implementing better thermal insulation, or placing a portable heater below the window surface.
It is important to note that the study has not taken into account every possible additional arrangement that Prof. Glicksman mentioned. Chilled beams and the influence of students moving around the room, for example, still require investigation.
Prof. Glicksman said that the research underscores the important role that face masks play in limiting aerosol spread among groups of students.
“While we didn’t deal with the filtration effectiveness of masks, we modeled masks through a change in the area and velocity of the mouth. We found that aerosol particles escaping around a mask with low velocity are quickly carried upward within the warm air rising around the body. Aerosol from an unmasked individual escapes the warm air plume and remains in the breathing zone for a longer time.”
Many experts have expressed discontent about making masks optional at schools.
“Not only [should masks be] encouraged, but [they] should be mandatory in schools in any location where there is community transmission of the virus, which includes the entire U.S. right now,” said Jose-Luis Jimenez, professor of chemistry and fellow of CIRES at the University of Colorado.
He underscored that when choosing a face mask, it is crucial to make sure that it is of high quality with effective filtration and a good fit to the face.
Prof. Jimenez recommended that teachers and school staff wear N95 masks or elastomeric half-masks at school. For younger children, he said, Korean KF94 masks or elastomeric masks may be a better option.
He also stressed the importance of providing masks to low-income students.
Activities during which people cannot wear masks, such as lunch, should ideally take place outdoors, he added.
The current guidelines, which encourage people to wear face masks and keep at least
Open windows and good heating, as well as the installation of proper HVAC systems, can make classrooms much safer during the pandemic.
Older school buildings and those with insufficient funding will likely have to resort to opening windows as their only option besides mask-wearing and physical distancing.
According to Prof. Jimenez, the results of the study strongly depend on the geometry of the classroom and the airflow, but the study’s message for schools is clear:
“We need sufficient ventilation in all classrooms to control the spread of the Delta variant. By now, we know that aerosol transmission is the dominant mode of transmission, both in close proximity and in shared room air.”
He said that many schools need to take a serious look at their ventilation systems and put appropriate measures in place to prevent transmission.
“If ventilation is not sufficient to achieve at least six air changes per hour, then it needs to be supplemented by filtration, such as with portable HEPA (High Efficiency Particulate Air) filters.”
Aircraft currently use HEPA filters, which can capture more than 99% of airborne microbes, recirculating the air 20–30 times every hour.
Prof. Jimenez also warned against placing electronic air cleaners, such as those using ions, plasmas, photocatalysis, or hydroxyls, in classrooms. Not only do they often not work, he pointed out, but they also have the potential to harm people and create toxic pollutants.
“Unfortunately, many school districts have installed such electronic air cleaners, and it is urgent that they disable them and replace them with filters,” he added.
Pointing out that the risk of transmission will always be higher indoors than outdoors, even with physical distancing or mask-wearing, Prof. Jimenez told MNT:
“For indoor periods, it is critical to improve ventilation systems to operate with as much outdoor air as possible, upgrade filters in HVAC systems to MERV 13, measure CO2 in the classrooms to verify that the ventilation is sufficient and keep it below 700 [parts per million (ppm)] (or below 1,000 ppm if there is supplemental filtration), add filtration to the spaces where ventilation is not sufficient, and avoid electronic air cleaners.”
Acknowledging that it was a challenge of means and energy to move additional air around a school building, Prof. Glicksman said that schools could start with the officially recommended measures, which are to increase airflow from the outside when possible and improve the filtration of recirculated air.
The caveat to this will, unfortunately, be an increase in energy consumption, and that may prove even more challenging in winter, he said.
“[W]e are working with an evolving situation, and those decisions need to be flexible to adapt. We have largely remained proponents of natural ventilation, and simple measures — such as windows, fans aimed to the floor, and scheduling classrooms to have time to flush — can have measurable impacts. In the long run, the use of displacement ventilation systems that bring in clean air at floor level has the potential to provide safer classrooms.”