Fans vs. Plexiglass: Opposite Effects on Indoor Air Quality
While neither fans nor plexiglass directly purify the air in a space, both influence air circulation, impacting the concentration of pollutants and thereby affecting the potential for airborne disease transmission. Fans and plexiglass have opposite effects on indoor air quality. A better understanding of the risks and benefits of these two tools can assist in creating lower risk indoor spaces.
Direct Transmission
Short-range transmission poses a high risk due to the initial exhaled plume containing a concentrated amount of respiratory particles. You are sharing more air with the person in front of you.
Physical distancing offers a method to avoid inhaling the initial exhaled plume. An alternative approach involves installing a barrier, such as plexiglass, between individuals.
Imagine you were standing in front of a smoker on a windy day. The wind would rapidly disperse the initial plume, decreasing the concentration of inhaled smoke. Conversely, if someone is downwind and there is a light breeze, a higher concentration of smoke can be blown into their face.
A similar outcome can arise when using a fan. It can be used to dilute that initial exhaled plume, reducing the risk of short range transmission. However, if an individual is positioned downwind of the infectious person, the fan’s airflow might propel the concentrated plume of respiratory aerosols directly toward the susceptible person, heightening the risk of airborne transmission for them.
Air Distribution
How the air moves in a space matters. The goal in most systems is for the air to be well-mixed. This means that the clean air supplied to the space is evenly distributed throughout the space. Otherwise, there will be locations in the space with higher concentrations of pollutants which will increase the risk associated with those pollutants.
Fans serve as a means to circulate air within a space, effectively enhancing air distribution.
Contrarily, plexiglass has the potential to impede the even distribution of air. It could lead to locations within a space that have higher concentrations of pollutants and increases the risk associated with those pollutants.
More details about air distribution can be found here.
Summary of Effects
In brief, fans can generate direct air currents between individuals, while simultaneously enhancing air distribution to ensure uniform air supply within a space. Plexiglass can prevent air currents between two individuals at short range, but can hinder air distribution in a space creating areas with increased pollutant concentrations.
Bad Guidance
Because there are two competing effects here, direct air currents and air distribution, whether to use fans or plexiglass involves weighing the benefits and risks from the two strategies.
One of the assumptions from public health organizations early in the pandemic was that respiratory viruses like SARS-CoV-2 are not airborne and primarily transmit at close range through spray of larger droplets, although no direct evidence of that existed. Plexiglass would be an effective method to mitigate large droplet spray transmission.
Throughout the pandemic, numerous places have employed plexiglass in attempts to offer protection. The evidence shows that plexiglass can increase the risk of transmission of COVID-19.
While public health authorities might acknowledge aerosol transmission to varying degrees, there is still a large emphasis on mitigation of short range transmission to the expense of adequate attention to shared room transmission. There have been noted cases of fans contributing to transmission, but people never contact trace cases of transmission that never occurred because they were prevented through better airflow and air distribution. Consequently, guidance which stresses the dangers of airborne transmission associated with fans is based on selection bias as they can provide a net positive effect on the space, but only the detriments have been studied. For example, the PHO ventilation document emphasizes risks associated with direct currents from fans without adequately addressing their benefits.
In summary, based on ignorance of indoor air quality, people avoided using fans and instead used plexiglass. Despite good intentions, this failed to provide people with low risk spaces.
Good Guidance
Fans have a net benefit and plexiglass has a net detriment, but both can be used in effective or harmful manners. The CDC has guidance for both fans and plexiglass (see FAQs 11 & 12). ASHRAE has provided guidance on this as well.
Fans
For the use of fans, an ASHRAE Epidemic Task Force core recommendation is: “Where directional airflow is not specifically required, or not recommended as the result of a risk assessment, promote mixing of space air without causing strong air currents that increase direct transmission from person-to-person.” The ‘directional airflow’ aspect is based on the type of ventilation system described here in more detail. Most systems are well-mixed, so mixing the air with a fan is beneficial. Simply, the guidance is to use fans but avoid generating direct air currents between people. This can be done by not pointing fans at people, by having them point up and running them on lower speeds.
The CDC recommends using fans, but also running them on lower speed and ensuring they are not pointed towards people.
Plexiglass
ASHRAE provided guidance on barriers in the draft proposal for control of infectious aerosols: “Barriers may not be used unless actual in-space measurements show reduced concentrations in occupied spaces”. This was not included the published standard, but is still sound guidance.
CDC provides very similar guidance: “Any time barriers are deployed, airflow distribution testing with tracer “smoke” or handheld fog generators should be conducted. This testing can assist in evaluating airflow distribution within the occupied spaces. If stagnant air pockets are seen to occur, barrier redesign or reorientation can help to minimize the occurrence.”
For people who have many short range interactions, like receptionists or cashiers, there might be a potential for significant benefit from plexiglass barriers, but risks must be mitigated as they can still cause a net harm.
Summary
Fans are generally good to use. Try not to point them directly at people who are stationary and avoid generating air currents that can lead to direct transmission. Ceiling fans are ideal to use as they do not create direct currents between people.
Plexiglass is generally detrimental to use. If you want to use it to protect against short range transmission, perform a fog study and see if the fog concentration decreases at a similar rate as the rest of the room. If it decreases at a slower rate, then it should be avoided. Using a HEPA filter within the area covered by the plexiglass (where the eddies would occur) can mitigate issues with plexiglass.
