Particular Particles: Studying the movement of COVID-19 particles in the air

For occupational and environmental health science Professor Changjie Cai, the pandemic provided a myriad of research opportunities to study the best ways to protect people in different environments. As an aerosol scientist, most of Cai’s work involves studying suspended solid and/or liquid particles in the air. From a stationary airplane fuselage (also used for pilot training) on the Oklahoma State University campus to an enclosed toilet chamber in his lab at the Hudson College of Public Health, Cai and his team are collecting air samples to test for the presence of particles that might contain COVID-19 and analyzing how particles move through different spaces. Dr. Cai’s studies are funded by U.S. Centers for Disease Control and Prevention, Presbyterian Health Foundation, and the National Aeronautics and Space Administration.

One of Cai’s many projects includes analyzing the dispersion of small airborne particles in aircraft cabins. His goal is to find ways to reduce exposure to airborne pathogens during air travel. Cai is particularly interested in submicrometer particles, a.k.a. particles that are smaller than a micrometer. (For reference, a single strand of hair is between 100 and 150 micrometers wide.) “The reason I’m interested in submicrometer particles is that those particles can stay in the air for much longer than larger particles,” says Cai. For example, a previous study found that particles roughly 0.25-0.3 micrometers have the highest amount of COVID-19, and particles of this size can stay in the air for over three days without ventilation. For this project, Cai and his team aimed to develop an airflow controller for reducing particle concentrations. The first step was to analyze how different-sized particles moved throughout the aircraft cabin. In particular, they wanted to see how the aircraft ventilation system would impact particle distribution. Then the team looked at how different factors (e.g., the overhead seat fan, distance from the aerosol generation source, and seat direction) impacted the particle concentrations.

“We’ve found some interesting results,” says Cai. For example, they noticed that ventilation was effective at reducing particles less than 0.5 micrometers in size. However, it wasn’t very effective for large particles. They also found that turning on the overhead seat fan and directing it towards the face (and, more importantly, breath) can help to reduce over 90% of the aerosol exposure. “That’s something that we can recommend for people who are traveling,” says Cai. “Turn on the fan to reduce the risk of exposure to not just COVID-19 but any kind of airborne disease.” Cai and his team also found that source control is very effective. When a single fan was turned on over the source, it helped to significantly reduce the distribution of the particles. The next step is to develop a low-cost airflow controller that would be placed in front of each seat. Essentially, this controller would capture the aerosols expelled from a person’s mouth or nose and move the air through a ventilation system, ensuring that the particles never have a chance to circulate in the aircraft and infect others.

If you can’t find Cai in the test aircraft, you might look for him in the toilet lab on the fourth floor of the Hudson College of Public Health. This lab is aptly called the toilet lab because it contains a large chamber with a flushing toilet. Previous studies in the chamber found that flushing the toilet generates numerous aerosol particles and large droplets, which are mostly caused by the bubble bursting and splashing as the water is sucked into the toilet. It’s important to study these particles and droplets because they can contain bacteria and viruses that become aerosolized and land on surfaces each time a toilet is flushed. 

Unsurprisingly, different types of toilets can generate different amounts of particles—usually based on the water pressure. Cai and his team are looking at the different particle sizes created by flushing a commercial (flushometer-valve) toilet. This type of toilet is commonly found in commercial, industrial, and institutional restrooms (including hospitals). The aerosol measurement devices Cai and his team are using can measure a wide range of particle sizes, from 10 nanometers up to 20 micrometers, across 152 size bins. For reference, one nanometer is between two and 10 times bigger than the size of an atom.

Beyond simply measuring particle sizes, Cai and his team are also looking at ways to reduce the particles generated when the toilet is flushed. “Most institutional toilets don’t have a lid because lids can foster germ growth and are not easy to clean,” says Cai. However, this means that there is no barrier to keep particles from spewing into the air when someone flushes the toilet. To address this issue, Cai tested a temporary lid that acts as a blocker when a toilet is flushed, and he found a 50% reduction in aerosolized particles. He also observed that the lid was effective at reducing the number of droplets on nearby surfaces.

Next, Cai and his team used fluorescent dye to look at how the particles deposited on the bottom of the temporary lid. They discovered most of the particles on the back part of the lid. However, they noted a large number of small particles escaped from the sides since the temporary lid wasn’t completely sealed. Cai hopes to improve the lid by using a filter and a small vacuum to capture these runaway particles. He notes that this type of device would be most useful to janitorial staff who clean toilets in health care environments with sick patients. It would also help protect nurses or others who might be exposed to particles deposited on surfaces or left floating in the air. “Our department mainly focuses on the occupational environment,” says Cai. “So, we are especially interested in protecting workers.”

Cai acknowledges the master’s and doctoral students who have participated in these projects and helped analyze the data. Several of his students have used the research for their thesis or dissertation work. Occupational and environmental health students have also co-authored articles with Cai for peer-reviewed publications and presented research findings at national and international conferences. Since many of Cai’s projects are cross-disciplinary, his students have lots of opportunities to work with researchers from a variety of backgrounds and teams across the Health Sciences Center and OU Norman campuses.

When asked why he chose to study occupational health and environmental science, Cai notes his early interest in atmospheric science. “I was working with aerosols in the atmosphere and looking at how they impacted human health,” he says. At that time, Cai didn’t know about occupational and environmental health science. He laments that many people don’t know about this field. “Working environments impact almost every person at some point during their lives, so it’s important to make sure that working environments are safe and promote good health,” says Cai. He credits his doctoral advisor (Dr. Thomas Peter from the University of Iowa) for introducing him to aerosol exposure in occupational health science. Cai was immediately drawn to this field because he felt that he could directly help people reduce their risk of aerosol exposure.