Even in the post-pandemic era, cold and flu season on public transportation can still be an uneasy time. While the years spent thinking about the whereabouts of the SARS-CoV-2 virus might have left us with a greater awareness of airborne viral transmission, research is just beginning to provide a clearer picture of how particles move through the air in environments beyond hospitals and medical facilities.
Researchers from Drexel University recently published a paper in the journal sustainability that helps to fill in a few blanks in our understanding of airborne particle movement on buses, which could improve understanding of how germs can spread through the air. One of them, L. James Lo, PhD, an associate professor in Drexel’s College of Engineering, whose lab studies the effects of ventilation on indoor air quality, shared some of the insights he gleaned from the research with the Drexel News Blog — including some findings that may allay concerns about bus travel and a few strategies for evading whatever is in the air.
How does air exchange and ventilation affect air quality on a bus?
Air exchange rate typically means how much air is delivered and extracted by mechanical system (HVAC) in a space. Since a percentage of this air is outdoor air, and the entirety of this air is filtered — or it is supposed to be, if the system is functioning properly — on a bus, the particulate contaminants, smoke or viral droplets, for example will be diluted by outdoor air and partially removed by filtration. So, on a bus, a higher air exchange rate usually means better indoor air quality.
What are the primary sources of ventilation or air exchange on buses?
Buses usually have a roof-top based packaged HVAC system that might or might not have outdoor air exchange capability. If not, the system simply recirculates the air inside with filters that can remove the particles. This is why some mitigation efforts during the pandemic included installing germicidal components. Outdoor air may also be brought into the bus by hatches, opened windows and each time the bus door opens.
What factors affect how particles — from a sneeze or cigarette smoke, for example — move through a bus?
The most important factor is where the emitters of the particles are located. If they are near the vent, the small particles can be moving around the entire vehicle. If they are in a “dead zone,” which is an area that has little circulation — the back of the bus, for example — the pollutant might stick around the area instead of spreading. This is difficult to determine in a real environment however, because it is not yet possible to directly measure the amount of viral or bacterial particles in the air, thus it has only been projected by computer modeling to date.
One complication of increasing ventilation in a space, is that although it would dilute the entire space better, the increased flow could potentially blow viral or bacterial particles throughout the bus and toward other people.
This effect is not easily quantified and most risk assessment research does not account for spatial variability — it assumes the same risk for the entire space. The novelty of our recent paper is that we were able to represent some of variations within a bus environment to more accurately project particle movement in our modeling.
What air change rate should buses strive for to help mitigate the particle transport that could lead to spreading infectious germs?
It is impossible to nail down a number due to the complexity of variables based on guidelines from the Centers Disease Control and prevention, about four air exchanges over the course of 70 minutes would ensure 99% of the airborne pollutants are flushed out of a space, provided the emitting source is no longer present.
But this is also a lot of outdoor air coming into the bus that would need to be conditioned for thermal comfort, so it might not be feasible. Also, it is important to distinguish between outdoor air change rate and total air change rate, which includes recirculating air.
If a bus does not have good ventilation or a high air change rate overall, where would be the best place to sit or stand to be within the area that has relatively better air exchange dynamics?
The best place to be is by the door or near an air supply diffuser. Those areas will provide the best possible outcome, but that is relative to the entire bus. If the entire vehicle has poor ventilation, or little to no outdoor air exchange, these locations would still be risky. Potentially, with better filtration and in duct germicidal technology, the recirculated air could be much safer as well.
In your paper you note that little research has been conducted on particle flow on public transit — what factors can make it challenging to conduct this sort of research?
There has been little evidence that public transit is the key airborne transmission pathway, except for long-term exposure with little ventilation — a few hours on a tour bus with the same people and low ventilation, for example — as documented in some COVID literature.
One key reason is the “transit” nature of the public transit. Most people spend very little time onboard with very different people. So, it is more likely you would contract an airborne disease at work, at school or at home.
There is also no way to distinguish whether an infection actually started in a public transit vehicle. Both in vehicles and buildings, we would benefit from next-gen sensors, as well as more advanced real-time modelling so we can know more about the air around us.
Reporters interested in speaking with Lo, should contact Britt Faulstick, executive director, News & Media Relations, at bef29@drexel.edu, or 215.895.2617.
