One of the reasons the Coronavirus spreads with such merciless vengeance is that it is transmitted through the air. If you’re in the wrong place at the wrong time you might become infected simply by breathing. Knowing where concentrations of the Coronavirus may be lurking unseen in the air and avoiding those places may help you stay safe. If you are responsible for the safety of the occupants of a building, knowing where there may be invisible spikes in the airborne infection risk will help you to protect those in the building from unknowingly absorbing an infectious dose of virus particles.
But how can you know where invisible particles are located in your building?
To better understand how virus particles flow and accumulate inside buildings, we have developed an agent-based simulator that imports the 3D structural design of a building and models how Coronavirus particles are emitted, spread, and accumulate inside it. Our simulator is capable of generating thousands of scenarios representing different conditions, mitigating measures, and many other parameters. It runs many simulations that produce huge amounts of data which we then analyze. It can also create a real-time visual representation of the space in question for any given scenario, much like a computer game.
In the series of articles that follow, I will discuss:
- The physics of virus emission: why do some exhaled virus particles stay airborne (become aerosolized) while others fall and come to rest on the surfaces of furniture, fixtures, or the floor?
- How do the thermal characteristics of our own bodies, along with heating and air conditioning (HVAC), influence particle movement?
- How do mitigation measures such as masks, social distancing, handwashing affect the likelihood of infection, and how effective are they?
In the remainder of this article, I will review an important early result from our work. In our simulations, airborne virus particles do not generally fill a space evenly. Depending on the 3D shape of the building, particles can accumulate to form dense clouds in some areas, while their concentration is sparser in others. Consider these images:
These image show two views of the common meeting space of a conference center in a major European city, set up with refreshment tables and full of people milling around and networking. One individual, highlighted with magenta color, in the center of the left image, has COVID-19 and is shedding virus particles, visible as the yellow plume of particles forming above her head. After some time, a fraction of the particles she breathes form a cloud, visible in the lower part of the right image. People who spend time in the vicinity of that cloud are much more likely to become infected than those standing in areas where the accumulation of airborne particles is not as concentrated.
We ran several scenarios within the 3D model of the conference center and measured the accumulated virus particle concentration over time. The results are illustrated in the heat map below.
Some of the results were as expected: airborne particle concentration was greatest in high-traffic, high-occupancy areas such as the entrance area, the refreshment tables and the toilets. But there were surprises as well. An area of high particle concentration formed in the enclosed area visible at the top left of the image above. This was not a high-traffic area so the high level of accumulation could not be attributed to a higher-than-average number of particles being released in that area. So why the accumulation?
It seems likely that this kind of accumulation occurs at the end of an inadequately ventilated corridor, or any partially enclosed space. If the normal airflow is blocked by a wall or other obstruction, a zone is created where particles are “trapped” and their density increases. This effect is illustrated in a simple fluid dynamics simulation, shown below, where a region of high-density forms against the wall of a space enclosed on 3 of 4 sides
While preliminary and in need of validation, these results nonetheless suggest that the specific design of a building’s interior (along with other factors such as the HVAC system) can create invisible “hot zones” where concentrations of airborne agents, such as pathogens, CO, CO 2 , and radioactive decay products, for example, can be considerably higher than average levels measured elsewhere in the building.
Anyone concerned with keeping the occupants of buildings safe needs to be aware of how the particular structural design of a given space may result in hidden “hot zones”. In any existing structures, be it a school, a store, a factory, office, warehouse, or any interior space, traffic flow should be modified to keep people away from locations where the risk of airborne threats is unusually high. For future buildings, certification standards such as the WELL certification International Well Building Institute (IWBI), should require that pathogenic particle accumulation characteristics be simulated, understood and mitigated for by design revisions, HVAC changes, or access restrictions. This will not only impact the infection rate during outbreaks of lethal pandemics, but most likely have a measurable impact on the general health of the building’s occupants lowering low-level illnesses and the productivity losses due to them.
Today, in order to meet safety code, buildings must protect their occupants from fires, earthquakes and other risks, and to meet sustainability standards, they must be efficient and clean. The events of 2020 have shown us that buildings have always exposed their occupants to another class of risk—infection from airborne agents. Design-specific, agent-based simulations incorporating computational fluid dynamics provide an effective way of assessing airborne infection risk in both existing and planned structures. They promise to provide architects, engineers, and certification authorities with a powerful new tool for creating spaces that are both safe, and conducive to the wellness of everyone who occupies them.
Please subscribe to this blog to receive my next article in the series, which describes how heat created by human bodies, HVAC, and more affect particle movement.