Air Distribution — Fans, Personal HEPA Filters, Plexiglass & Short Range Transmission
A lack of understanding of air distribution is a major source of misinformation and bad decisions about ventilation. In order to create low risk environments and ensure our interventions are effective, addressing air distribution is an essential part of the plan which is often neglected.
What is air distribution?
You can supply clean air to a space, but that doesn’t necessarily mean that people will be breathing it in. It’s possible the air will never make it into their breathing zone. Air distribution deals with how well that clean air is supplied throughout the space. It can also refer to issues of dead-spots — places within the zone where the air is not well mixed and the concentration of pollutants is higher.
Causes of poor air distribution
While there are different types of air distribution systems, the vast majority are mixing ventilation systems and consequently, I will be dealing with mixing ventilation systems in this post. The goal is for all the clean air and pollutants to be evenly distributed throughout the space. If this doesn’t happen, the concentration of pollutants will be higher in some parts of the space and they will pose an increased risk to anyone located there. When it comes to supplying air to the room, there is an issue called short-circuiting where air being supplied goes straight to the return without mixing into the space.
The risk of short circuiting is based on how the system is designed and operated. ASHRAE 62.1, the standard for ventilation and air quality, quantifies how much short-circuiting can occur based on the different types of system designs and operations. Here’s table 6-4 for zone distribution effectiveness.
The most common problem is #3, supplying hot air at the ceiling. Warm air will be buoyant and remain in the upper part of the room and won’t mix into the lower part of the room (breathing zone). It reduces air distribution by 20%.
The most egregious distribution problem is #9 — supply and return on the same side of the room. It’s common and a complete engineering design failure. Here’s what it would look like.
The exhaust and supply are right beside each other. Imagine being on the other side of the room. How helpful would this be? How much would the unit just clean the air beside it and not have much of an effect on the whole space? Based on #9 in ASHRAE table 6.1, the assumption is that it’s only 50% effective on the space as a whole, so if it is supplying 400 cubic feet per minute (CFM) of air to the space, it is only as effective as a unit that would be supplying 200 CFM with proper air distribution ductwork and diffusers. Ignoring air distribution is a way to cut corners when installing new ventilation systems. It can save money on the installation, but significantly hamper the effectiveness of the ventilation.
Calculating air distribution
This section requires math. It helps conceptualize the issues at hand. Understanding the math is not essential to understand these concepts. You can skip to the next section.
ASHRAE 62.1 Appendix C provides an equation to calculate the air distribution effectiveness throughout the space. It can be calculated as:
Distribution Effectiveness = (Ce-Cs)/(C-Cs)
Where:
Ce is the concentration of a pollutant in the exhaust air
Cs is the concentration of a pollutant in the supply air
C is the concentration of a pollutant in the space
Here are some examples.
If all the air from the supply goes straight into the exhaust and there is no mixing into the space, then Ce = Cs and the distribution effectiveness = 0. This will never exist as there will always be some mixing.
If the space is perfectly mixed, then the concentration of the pollutants in the exhaust will be the same as the concentration of pollutants in the space or Ce = C. The equation will then be (C-Cs)/(C-Cs) = 1. Therefore, perfect air distribution in mixing ventilation systems has the same pollutant concentration in the exhaust as the space and should have uniform pollutant concentration throughout the space.
Here’s an example with numbers. You are supplying 100% outdoor air to the space with a carbon dioxide (CO2) concentration of 420 parts per million (ppm). You measure the CO2 concentration in the space at 1200 ppm and the CO2 in the exhaust at 900 ppm. What’s the air distribution effectiveness?
Distribution effectiveness = (900–420)/(1200–420)=0.62
Therefore, the ventilation supplied to the space is only 62% effective. This is a huge waste. 38% of the air which you are already using up energy to get into the space is not making it to people’s noses, but instead going straight into the return and bypassing everyone. What would happen here if you designed the space better to ensure proper distribution?
I won’t derive the CO2 equations again, but you can find the equations here.
For a classroom at 1200 ppm,
Outdoor airflow rate per person = (0.0045 lps x 1000000)/(1200–420)=5.8 lps /person, where lps = litres per second.
For a classroom at 900 ppm,
Outdoor airflow rate per person = (0.0045 lps x 1000000)/(900–420)=9.4 lps /person
What’s the actual outdoor airflow rate per person supplied to the space?
5.8 x 0.62 + 9.4 x 0.38 = 7.2 lps/person.
If there was perfect distribution, the CO2 level would be:
Indoor CO2 = 420 ppm + (0.0045 lps × 1 000 000) / 7.2 lps/person = 1045 ppm
Therefore, the occupants will be exposed to CO2 levels around 200 ppm higher than what would be expected in a properly designed space with no added energy cost.
Diagnosing air distribution problems
A proper diagnosis of air distribution problems requires measuring the pollutants in the space. To know if it’s air distribution and not just poor ventilation, you need to compare the pollutant in the space and the air that is being exhausted from the space. Therefore, verifying the ventilation equipment is operating properly does not necessarily identify air distribution issues. There are three main options to identify air distribution issues.
CO2 monitoring
CO2 monitoring is one of the best tools to ensure ventilation is working properly. Measuring the CO2 in the space will tell you how well outdoor air is being supplied to the space. If you measure the CO2 in the return duct, or if the CO2 concentrations are much higher than expected and the equipment is working properly, it can indicate an air distribution issue. A full introduction to CO2 monitoring can be found here.
Pollutant decay
First, a pollutant is placed into the space and the concentration increases. Then you remove the pollutant from the space and measure how quickly the pollutant is removed. This can be done with dry ice and CO2 sensors. It is explained here on page 16.
Fog study
The previous two methods can identify how well the air is distributed from the ventilation system to a location within the breathing zone. Unless you measure at many points throughout the space, which is generally impractical, it doesn’t give you a complete picture. A fog study can be used to see how well the fog is cleared from the whole space. This is the best way to identify dead-spots, where the ventilation is not properly removing pollutants.
Improving air distribution
The simplest way to improve air distribution is a fan. It will help mix the air throughout the space and ensure the pollutant concentration and clean air supply is uniform. Pedestal fans can be used for this, but you should avoid creating direct currents between people. Having it move air vertically can help mix the dirtier air in the breathing zone with the cleaner air in the upper room. A ceiling fan is ideal for this purpose and should be used. This can also be used when a fog study identifies dead-spots in the room. The CDC recommends using fans here FAQ 9. The ASHRAE Epidemic Task Force recommends mixing the air in the space as their third core recommendation.
Addressing the issues previously shown in ASHRAE 62.1 Table 6–4 can also improve air distribution. Systems should never be designed with the supply and the return on the same side of the room. Ideally, have a proper air distribution system designed and if not, at least place them on opposite sides of the room (see table 6–4 #8). It takes the ventilation from being 50% effective and makes it 80% effective.
For systems where the air is being supplied from the ceiling, it’s common for equipment to just supply really hot air to the space. Trying to reduce the temperature as much as comfortably allowable will help the air mix into the lower room instead of remaining buoyant in the upper room. Ventilation equipment which tightly controls the air temperature can help with this.
Short range transmission and air currents
There is a higher risk of transmission of airborne diseases when a susceptible person is in short-range from an infector. This is because the initial exhaled plume has a higher concentration of virus particles.
The issue can also be rephrased this way — risk is increased because a person is located within the space in a room that has a higher virus concentration. Seen in this light, short range transmission is just an air distribution issue. The way to mitigate short range transmission is to try to quickly make the virus concentration at short range similar to the virus concentration in the room. For example, a ceiling fan above the people at short range will quickly push the aerosols out of the breathing zone and mix it into the rest of the room.
A concern can exist when creating air currents from one person to another. This higher concentration of virus particles can be pushed towards someone who is not at short range. Pedestal fans can help with air distribution, but can also create air currents when not setup properly. The best way to mitigate this is to avoid creating direct currents between people. Aiming fans up or not directly at people will help reduce the risk. If it is aimed towards someone stationary, like in an office or classroom, it should be kept two meter away. This concern should not exist for ceiling fans or properly designed air distribution systems.
A misconception is that you are creating an increased risk for everyone in the room by doing this. It’s based on ignorance and thinking that these viruses won’t travel throughout the room without a fan. For example, the Health Agency of Canada claims fans “make the virus travel further”. In general, air eventually mixes throughout the room. Using a fan, even in this case, will not change the steady state concentration of viruses in the room. It will just ensure that they are more uniform, thereby reducing risk associated with places that are higher concentration and ensure that the clean air supplied is properly mixed into the space.
In general, using a fan or having better air distribution will not increase risk, but will lower risk by ensuring the clean air is better mixed throughout the space and ensuring there are no spaces with a higher virus concentration.
HEPA filters & blowing the virus around
HEPA filters are just a fan with a filter. The fan will draw air through the filter and the filter will remove particulate matter from the air, including virus laden respiratory aerosols. The fan then operates just as any other fan would. Consequently, a HEPA filter can also be seen as a method to improve air distribution.
A common repeated misconception is that they blow the virus around, but this is the same as previously discussed. The virus is already travelling around the room. The fan with the HEPA filter will just assist in ensuring that the clean air supplied is better mixed throughout the space and that there are fewer locations with higher concentrations of virus. This is in addition to the cleaner air that it supplies to the space. Overall, HEPA filters reduce risk even though their use has been prevented by public health organizations due to their ignorance about air distribution.
HEPA filters can be seen as working with the ventilation system to mix the air in the space. However, alone by themselves, they are unlikely to mix the air will into the space. Going back to table 6–4, HEPA filters are the case where the supply and return are on the same side of the room (they are actually right beside each other). This means according to 62.1, you can assume they are about 50% effective, so a HEPA filter supplying 200 cubic feet per minute (CFM) of clean air is similar to a well-distributed ventilation system supplying 100 CFM of clean air throughout the space.
Sometimes HEPA filters can help clean the air throughout the room, but sometimes they would just make the air in the corner of the room a little cleaner while being ineffective at greater distances. The solution is to ensure the air is properly distributed throughout the room with the use of fans or a functioning ventilation system.
Personal HEPA Filters and airplane air outlets
The goal with personal HEPA filters is to take advantage of poor air distribution and create a zone of lower virus concentration. If the air coming out of the filter does not mix that well into the space, then the air in the vicinity will be cleaner. Clean Air Stars has tested this out with various models, for example, this is what they found with the QT3.
Within 1 m, there is a reduction in particle concentration. It’s more effective within 40 cm, but that would be uncomfortable to do this for an extended period of time. In general, unless you are holding it close to your face and blowing at you, we don’t have evidence that personal HEPA filters are effective.
If you are using a large HEPA filter, it will likely create an area closer to it with lower particle concentrations as they do not have great air distribution by themselves. Some people are concerned that HEPA filters will draw the virus towards you, but because of short circuiting, the opposite would likely be true. Air closer to the HEPA filter will have a higher concentration of clean air and be lower risk than the rest of the space. The air that would be brought towards the HEPA filter would be mixed air from the space, so the worst case scenario, being near the HEPA filter would be the same risk as being away from the HEPA filter.
Personal air outlets (gaspers) on airplanes are also an example of clean air being supplied. In general, the air mixes quickly into the space and they do not provide much protection. Guidance from the ASHRAE epidemic task force is that their use is not necessarily recommended, but not discouraged.
Plexiglass barriers
Any barriers in a space can interfere with air distribution. They create eddies which can trap particles and prevent them from being cleared out of the space.
Desk shields were a tool that were used in schools at the beginning of the pandemic based on ignorance of air distribution and the flawed assumption that large droplet spray poses a higher risk than airborne aerosols. Consequently, the desk shields increased the risk of transmission.
Barriers on the one hand can be used to mitigate short range transmission by preventing inhalation of the initial concentrated plume. However, they also increase risk from shared room transmission by inhibiting air distribution. In my opinion, for situations where there are many short range interactions, like receptionists or grocery checkout personnel, plexiglass barriers can be used provided there are portable air cleaners used close to the employee or if a fog study is done to confirm that general air distribution isn’t hampered. For situations like office space and classrooms, plexiglass barriers increase risk of transmission.
Conclusion
Much of the focus in guidance is simply dealing with clean air delivery rates, but is not as focused on how the air moves throughout the space. Failure in understanding air distribution has lead to bad guidance and poor decisions throughout the COVID-19 pandemic, including reluctance to use fans, prohibitions on HEPA filters, use of plexiglass barriers, poor HVAC installations ignoring air distribution and leaving air distribution improvements out of guidance.
Providing clean air to a space requires either ventilation, filtration or UV light. While air distribution alone does not provide a low risk space, it’s an essential component to ensure the air cleaning measures are effective.
