Why is the novel coronavirus spreading so quickly and how can we slow its transmission?

Purdue College of Engineering
Purdue Engineering Review
7 min readJul 12, 2020


Droplet dispersion in a coffee shop

Qingyan (“Yan”) Chen, the James G. Dwyer Professor of Mechanical Engineering at Purdue University, has been studying the spread of infectious diseases in enclosed spaces for 16 years. Given his experience and expertise, he has become a go-to source for commentary and advice on how to curb the spread of the novel coronavirus, particularly on airplanes, on cruise ships and in other enclosed spaces.

Quoted by such top-tier media as Fast Company, Forbes, CNN, USA Today and The Washington Post, he shares his insights below. In addition to discussing virus transmission on planes and ships, he offers his thoughts on what all of us need to do to keep COVID-19 at bay.

Currently in the limelight as a COVID-19 authority, you have been researching the spread of diseases in enclosed environments for more than a decade. How did you develop your expertise?

It has indeed been a great honor to be interviewed, consulted, and quoted by TV stations, newspapers and other media outlets on work related to how the novel coronavirus spreads in enclosed spaces, such as commercial airplanes, cruise ships and hospital ships, and home and working environments.

I started to look at airborne infectious disease transmission through the FAA Center of Excellence for Airliner Cabin Environment Research, which the FAA sponsored under congressional mandate. I served as a co-principal director for the center from 2004 to 2010. My students and I worked with a group of professionals from other universities representing different disciplines, such as mechanical engineers, public health scientists, chemists, materials engineers, environmental engineers, and electrical engineers. Because I had used my knowledge on fluid mechanics to study airflow in enclosed spaces, I collaborated with those colleagues in the center to develop my expertise in infectious disease transmission.

What other research projects have you worked on that contribute to your knowledge of how to slow the spread of the novel coronavirus?

My students and I looked into SARS transmission in airliner cabins, where there was clear evidence that a patient could spread the disease. We started the investigation in the FAA center, and then Boeing continued to support our research.

In addition, in the past few years, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) has sponsored our research to improve indoor air quality in buildings, such as offices, workshops, classrooms and restaurants. Working with researchers from Boeing and other industry partners, my students and I have developed advanced ventilation systems, such as displacement ventilation and personalized ventilation systems, for both airplanes and buildings to slow the spread of airborne infectious disease viruses.

What misconception about the spread of the novel coronavirus has surprised you most?

I am surprised by the skepticism about novel coronavirus transmission being airborne, via small droplets, although research has shown the risk. Early studies found that all the masks used by COVID-19 patients in hospitals in Wuhan, China, contained the coronavirus. This proved that droplets from coughing, speaking and even breathing contain the virus.

If large droplets contain the virus, the small ones should, too. The only difference is that large droplets may contain more virus than small ones. Small droplets, or aerosols, can be airborne and could make someone sick if the person is exposed in an indoor space for a long time. Now other studies have also shown that droplets can be transported to a distance beyond six feet and that small droplets can be airborne for a few seconds to a few hours, depending on the droplet size.

So, may six feet not suffice for social distancing, even if people wear masks? What precautions should be taken in an environment, such as a restaurant, where masks for customers may not be practical?

Our SARS study found that during a four-hour flight, about 20 percent of passengers would be infected if one SARS patient is on the plane. By assuming that the patient still produced the same amount of virus and did not wear a mask but everyone else on the plane wore an N95 mask, our research showed that none of the passengers would become sick. Such a scenario is unlikely real, but it indicates the importance of wearing a mask.

With everybody wearing a mask, social distancing could be reduced, although we would need more research to provide a concrete answer as to how much. In a restaurant, where the customers cannot wear masks while eating or drinking, it is important to change ventilation systems or to install Plexiglas screens in the middle of a table.

Is it safe for people to travel by plane now? If so, under what circumstances? Also, what more do cruise ship operators need to do to protect passengers?

For commercial airplanes, it’s safe if you have sufficient PPE (personal protective equipment). For an airplane traveler, virus transmission occurs not only in an airplane cabin but also in the entire door-to-door process, including taking public transport from home to airport, checking in at an airport counter, being screened for safety, waiting in a lounge, boarding an airplane, deplaning, picking up checked luggage, and taking public transport to your destination. Social distancing is important, but it is impossible in some cases, such as in a bus or an airplane cabin. Thus, it is critical to wear a mask from door to door, to wash your hands thoroughly and frequently, to clean surfaces you touch with disinfectant wipes, and to close the toilet lid when you flush. I cannot emphasize enough the importance of wearing a mask.

Unlike airplanes, which use high-efficiency particulate air (HEPA) filters to clean supply air, cruise ships use conventional filters that cannot remove small droplets/particles. Because a passenger will stay on a cruise ship for days rather than hours, low-dose exposure to small droplets in the air that may contain the novel coronavirus is my major concern. The cruise ship industry should consider installing HEPA filters.

As we reopen businesses and ease social distancing guidelines, what changes and safeguards are needed, for example, in hotels, offices and retail stores?

Six-foot social distancing will help since research indicates large droplets will not travel farther than that. However, small droplets, although they may contain less of the novel coronavirus, could be airborne. Because we stay in hotels and offices for a long time, I doubt that it will be safe in those spaces without any protection. If everyone wears a mask, it will greatly reduce infection risk. Although a customer may spend only a few minutes in a retail store, it is very difficult to maintain social distancing. Thus, I recommend wearing a mask. For all the indoor spaces, air-conditioning systems should run with 100 percent outside air to prevent possible cross-contamination between spaces.

What do those reopening businesses most need to do to avoid a serious COVID-19 resurgence?

Business owners should carefully examine whether their business is vulnerable to the four possible infection routes: 1) direct contact, or indirect contact (for example, via a cup); 2) large droplets; 3) small airborne droplets, and 4) fecal-oral transmission. The four routes of transmission, and respective solutions, depend very much on business type.

What precautions do you recommend for people visiting a grocery store?

I recommend wearing masks and keeping social distancing. Whenever I visit a grocery store, I wear a mask and gloves; I disinfect all the surfaces I touch in my car by using a 70 percent ethanol solution; and I wash my hands thoroughly after coming back home. I also disinfect all my grocery packages by wiping them with a 70 percent ethanol solution.

Qingyan (“Yan”) Chen, PhD

James G. Dwyer Professor of Mechanical Engineering

College of Engineering, Purdue University

Editor-in-Chief, Building and Environment

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