Post by Nadica (She/Her) on Aug 31, 2024 4:09:17 GMT
HEPA Filter FAQ - Published Feb 27, 2023
1. How do HEPA filters work?
A portable HEPA filter is a box with a filter and fan inside it. The fan pulls the air through the filter and then supplies the air back to the space. The filter removes particles from the air. These particles can include dust, allergens, smoke, mold, bacteria or respiratory particles that contain viruses.
It does not remove any gasses like carbon dioxide. A more detailed introduction to filtration can be found here.
2. How do you select a HEPA filter?
The most important property is the clean air delivery rate (CADR). AHAM recommends you follow the 2/3 rule. Take the area of the space in square feet, and multiply it by 2/3 to get the necessary CADR in cubic feet per minute (CFM). So if the space is 200 sq ft, CADR = 200*2/3 = 133 CFM. Getting one that is AHAM Verifide ensures 3rd party testing and reliable values.
In a typical commercial space with 9 foot ceilings, the 2/3 rule gives you 4.5 air changes per hour (ACH). This is a reasonable amount that would likely combine with ventilation for at least 6 ACH.
Be careful about some claims. For example, Austin HM400 advertises the fan is 400 CFM, but with the filter, the CADR (they call it True Air Flow Rate) is 250 CFM. By AHAM standards, that would be good for an area of 250*3/2 = 375 sq ft, but they use their own standard and claim it is sufficient for 1500 sq ft. That would give you 1.1 ACH in a typical commercial space with 9 foot ceilings, much lower than the 4.5 ACH used by AHAM.
The second important factor is noise. Here’s a rough guide as recommended by Clean Air Stars:
< 40 dBA — sleep
< 45 dbA — Classroom, Quiet Restaurant & Office
< 50 dBA —Loud office & Childcare
< 60 dBA — Loud restaurant or gym
> 60 dBA — Acceptable for loud environments
See the noise section below for how to select HEPA filters to ensure appropriate noise.
The third factor is cost. More expensive ones are often worse. Typical ones you can find at the hardware store are likely to be more cost effective than high end ones.
The final factor is additional features, which should be avoided. See FAQ #9.
FAQ #17 lists resources to select HEPA filters.
3. Do HEPA filters blow the virus around?
It’s already being blown around. People don’t usually feel or see that air is moving, but you can see airborne dust reflecting in sunlight to get a better idea how particles can move around a room. The same applies for smoking — if there’s a smoker in the room, the smoke will travel around you will eventually be inhaling the smoke. Typical indoor air speeds are 0.1–0.2 m/s. Consequently, any indoor air pollutant is already floating around the room.
The goal when designing ventilation systems is that the room is perfectly mixed — all pollutants are evenly distributed throughout the room. This also means that clean air is properly dispersed throughout the room and that when exhausting air, you will be extracting contaminated air instead of exhausting clean air and letting areas to build up with higher pollutant concentrations. Studies show that the well-mixed assumption is correct for small particle sizes.
This is why one of the core recommendations of ASHRAE is to mix the air in the room and the CDC recommends the use of fans (FAQ 11 on their webpage). Because HEPA filters use fans, they can increase airflow and mixing in a room which is a good thing.
The one caveat is that you do not want to create direct currents between people as anyone downwind will be placed at increased risk. The two options are to purchase a HEPA filter that blows up or place it where it is not blowing close to anyone (within 2 m or 6 ft).
4. Do HEPA filters make the virus travel further?
No. The virus will travel around the room with or without a HEPA fitler or fan. What the HEPA filter or fan will do is make it mix into the room at a quicker rate. This will help mitigate the risk of short-range transmission (see FAQ #13) and will help ensure clean air is better distributed throughout the room.
When placing a HEPA filter, ensure you are not creating direct currents between two people as the person downwind will be at increased risk.
5. Do HEPA filters interfere with ventilation?
Not if chosen properly. ASHRAE 241–2023 Control of Infectious Aerosols section 6 addresses the compatibility between in-room air cleaners and different ventilation systems.
As a brief summary, HEPA filters should be chosen with the correct discharge direction based on the ventilation system:
Well-mixed ventilation: any type
Natural ventilation: discharge up
Cross Flow: discharge horizontal
Downflow: N/A
Upflow: discharge up
Most ventilation systems are well-mixed ventilation and any type of in-room air cleaner can be used. A detailed description, including ventilation system types can be found here.
6. Does UV light help when combined with a HEPA filter?
Not really. To be rated a HEPA filter, it needs to remove more than 99.95% of particles. Even if the UV light successfully disinfects the remaining 0.05%, the effect on the clean air delivery rate would be negligible. Some UV devices are not properly designed and the UV light would not deliver a sufficient dose to inactivate pathogens. While there are portable air cleaners that exclusively use UV light to disinfect the air and are effective, the effect of combining HEPA filtration with UV light is unlikely to provide any meaningful benefit. An example highlighting this can be observed from data provided by Airpura.
According to their own data, the HEPA filter achieves a 100% removal rate, rendering the UV component ineffective in terms of overall air purification.
7. Do I need to worry about getting COVID from viruses coming off a HEPA filter?
No. This effect is known as resuspension. Virus particles are initially floating in the air and then deposit on the filter when pulled through. They can then become concentrated on the filter. If the filter is then disturbed, the virus particles can get resuspended and infect anyone nearby.
There is currently no evidence that this is a significant risk. Furthermore, virus particles decay over time, so if the filter has trapped virus particles, unless it was in the very recent past, it is unlikely to be able to cause infection. A much more significant risk would be to share a room with an infectious person without appropriate mitigation measures in place, like portable air cleaners.
When changing filters, it is good practice to wear a respirator and do it in a well ventilated environment as dust can become resuspended which can have harmful effects when inhaled.
8. Should I get a medical grade HEPA filter, True HEPA or H14?
Medical grade HEPA and True HEPA are just other names for HEPA filters rated H13. This is the standard rating for HEPA filters, so medical grade HEPA or True HEPA are just marketing names.
There are H14 HEPA filters which can filter 99.995% of particles instead of just 99.95%. This benefit is negligible in all normal situations, but might be relevant in certain laboratories. There is no need for general use of H14.
More information about HEPA filter ratings can be found here.
9. Should I get one with additional features like ionization, plasma, NCCO, PCO, hydroxylation?
No. Portable air cleaners often come with additional features. These involve some mechanism to supply reactive species to the space. The goal is they will interact with virus particles and cause them to inactivate or deposit on surfaces or filters.
Many of these technologies have poor evidence of effectiveness. Even claims of 99.999% reduction can still mean that HEPA filters are more than 10x more effective.
There is also evidence that many of these technologies can produce indoor air pollutants like formaldehyde and ozone. See this article for an in-depth review.
The CDC recommends caution with these technologies. The Ontario Society or Professional Engineers recommends these technologies are not used until there is a standard for safety and effectiveness.
10. What are the main problems with HEPA filters?
The three main problems with HEPA filters are noise, distribution and human operation.
See FAQ #11 to address noise.
There is a concern with short-circuiting — that clean air supplied directly returns to the intake of the HEPA filter and does not mix within the space. Proper air distribution can prevent short-circuiting. To achieve proper distribution, some of the following strategies can help:
multiple HEPA filters can be spread out throughout the space
they can be combined with fans that distribute the air better
they should be placed properly. See FAQ#11
Having portable air cleaners in a space requires people to turn them on and off. To ensure this is being done properly, the portable air cleaner needs to be selected to be noise appropriate for the space. This needs to be combined with proper training, signage and verification policies to ensure they are running properly.
11. How do you address noise?
The easiest way to address noise is to purchase multiple HEPA filters and run them on low. They still need to be sized properly for the space. More details can be found here.
Another alternative is to use a portable air cleaner with PC fans and either DIY it or purchase it from Clean Air Kits.
12. Where should it be placed?
It’s impossible to know the perfect place to put it without a computational fluid dynamics model or fog experiments, but they are still very effective, even if not in the ideal place. Here are some placement guidelines:
Best to have one that blows air up
If blowing sideways, best not to aim it at anyone.
If it’s in the direction of someone, keep at least 2m (6 ft) away
Centre of the room is best
Don’t blow into a wall or object
Raised is better than on the floor
Give at least 0.5 m (1.5’) clearance from walls or objects that could restrict flow
Keep away from other sources of clean air: open windows, supply air vents, other HEPA filters
If you have multiple, space them out.
Make sure there’s no obstruction to where the air is going (like under a table)
In classrooms, there’s usually plenty of room at the front.
Sometimes it’s better to move it a bit away from people if it allows you to run it on a higher speed because of noise
13. Are HEPA filters useless against short-range transmission?
No. Short range transmission is higher risk because the virus is more concentrated in the initial exhaled plume by the infectious person. To mitigate short-range transmission, this plume needs to be diluted quickly. In order to accomplish this, two things are required: clean air in the space and good airflow in the space. Good airflow will dilute the initial plume quickly and it should be replaced by clean air in the space.
A HEPA filter can assist on both of these measures. It will ensure the virus concentration in the space will be lower. Because it uses a fan, it will improve airflow throughout the space. Depending on the size of the space and the HEPA filter, this effect might be small, but it can still help.
Overall air quality improvements help mitigate short range transmission.
14. I heard it doesn’t work on viruses because it only works on particles down to 0.3 µm and viruses are smaller than that.
Capturing Particles below 0.3 µm
There are different physical mechanisms which allow filters to trap particles. The different mechanisms work best on different particle sizes.
The combination of these mechanisms is worst at 0.3 µm. Filtration is more effective on particles with both larger and smaller diameter. Because of this, HEPA filters are tested on particles 0.3 µm in diameter.
In summary, HEPA filters do a better job against particles below 0.3 µm. The misconception that they don’t work below 0.3 µm is because they perform worst on particles that are 0.3 µm, so that is the size they are tested on.
Size of Respiratory Aerosols
Even though the filters can trap particles smaller than viruses, the viruses are not floating around by themselves. This is known as the naked virus myth. The viruses are floating inside respiratory aerosols and the aerosols are much larger than viruses, so it is much easier to trap them through filtration. Here’s an example of the scale of the virus inside respiratory aerosols.
As far as the size of the respiratory aerosols, there is a range, but most are between 0.5 µm and 5 µm.
15. Is it dangerous to sit near a HEPA filter because it will draw the virus near you?
No. In general, the air in the room should be assumed to be well-mixed and the air that is being brought in to the HEPA filter is the same as the air you would be breathing if you were in the middle of the room.
However, short-circuiting is an established problem with air cleaning equipment that does not have proper distribution built in, like HEPA filters. Short-circuiting occurs when clean air supplied from the HEPA filter does not properly mix into the space, but instead circulates back to the return. This would mean that air near the HEPA filter is cleaner than the average air in the room and it would be lower risk to be adjacent to a HEPA filter.
16. “We have mechanical ventilation, so we don’t need a HEPA filter”
If a room has mechanical ventilation, it does not mean there is an appropriate level of clean air supplied. Mechanical ventilation can have various issues:
Current standards are not designed for airborne disease mitigation, only comfort. ASHRAE is currently developing a standard for airborne disease mitigation because its existing air quality standard does not address that. ASHRAE is used in building codes in North America.
Older buildings were not designed to comply with modern minimum requirements. Even with mechanical ventilation, they often have very poor ventilation levels.
Having mechanical ventilation does not mean it is operating properly.
Consequently, to provide appropriate risk, there are two things facilities should do. First, select an airborne mitigation standard and comply with it. Examples of standards are 6 air changes per hour (OSPE, Lancet, Harvard Healthy Buildings) or 10 litres/sec/person (WHO).
Second, there should be ongoing verification that the systems are operating as designed. The best way to do this is by monitoring ventilation with CO2.
If facilities are complying with an airborne disease mitigation standard and verifying proper operation, then it is reasonable for them to state that they have provided a reasonably low risk environment and do not want to add portable air cleaners to their strategy.
If there is no compliance to a standard for airborne disease mitigation, HEPA filters can assist in achieving those goals. If there is no verification, HEPA filters can provide an additional layer of protection if the systems are not running properly.
17. What good resources are there to pick HEPA filters?
Some good resources are:
Clean Air Stars
Clean Air Crew
Comparison of HEPA filters by Marwa Zaatari
18. Do Personal HEPA Filters Work?
In general, not really. There are two ways to use personal HEPA filters: a small HEPA filter in a large room that is close to you or having one blowing in your face.
If a small HEPA filter is used in a large room, the air close to it might be slightly cleaner than the rest of the room, but unlikely to be very meaningful.
If you are pointing one at your face, it depends on the filter and how close it is. This is data that was taken by Clean Air Stars using the QT3.
To get a meaningful reduction, the distance needs to be within 40 cm of your face. That is very close and not sustainable for a long period of time. They ran another test with a DIY PC fan array which showed much better results.
This one has over 80% particle reduction at 1.5 m away. A much more reasonable distance. It is likely that this option is better because the fan array gives it a much larger area. This is a legitimate option as opposed to most HEPA filters which supply air from a relatively smaller area with higher air speed.
This deals with a more general issue — when clean air is supplied from a point, how far will it travel before the clean air becomes well mixed with the surrounding air. One example which has good data is the gasper or personal air outlet found in airplanes. Does it work to use it like a personal HEPA filter and aim it at your face? This is what the ASHRAE epidemic task force has to say about it:
In summary, clean air mixes into the space pretty quickly. Using it might help a bit but not significantly.
19. Do HEPA Filters Interfere with Windows?
Alternatively, do windows negate the effectiveness of HEPA filters? Should you turn off HEPA filters if the windows are open?
No. There is no interference or need to stop using HEPA filters with open windows. Both windows and HEPA filters are sources of equivalent clean air in the room and using both together will increase the clean air delivery rate, creating a lower risk space.
Position of ASHRAE 241 — Control of Infectious Aerosols
In ASHRAE 241 — Control of Infectious Aerosols, section 6.5 discusses how in-room air cleaners work with different ventilation systems. One of these systems is natural ventilation (windows). Using the appropriate air cleaner will work with the natural ventilation system to create a lower risk space.
ASHRAE 241, section 6.1 provides the following formula for compliance with the standard:
Where:
Veaci is the required equivalent clean airflow
Vnv is the equivalent clean airflow rate from natural ventilation
Vacs is the equivalent clean airflow rate from air cleaner systems
Therefore, based on 241 section 6.1, both the air cleaner systems and natural ventilation can be added together to provide sufficient clean airflow to comply with the standard.
As discussed previously, it is best to position HEPA filters away from the windows since you want to be cleaning the dirty indoor air and not the clean air coming in through the windows.
Advertising Claims from HEPA Filter Manufacturers
Some HEPA filters come with a claim that they are more effective if windows are closed. This is true if the pollutant of concern is coming from outdoors, like wildfire smoke or pollen. HEPA filters would remove these pollutants, but they would continually be supplied to the space through the open windows. Therefore, in cases like wildfire smoke, windows should be kept closed while HEPA filters are run.
If the pollutant is concern is generated indoors, like infectious aerosols, both HEPA filters and windows are two separate methods to remove this pollutant and should be used together.
1. How do HEPA filters work?
A portable HEPA filter is a box with a filter and fan inside it. The fan pulls the air through the filter and then supplies the air back to the space. The filter removes particles from the air. These particles can include dust, allergens, smoke, mold, bacteria or respiratory particles that contain viruses.
It does not remove any gasses like carbon dioxide. A more detailed introduction to filtration can be found here.
2. How do you select a HEPA filter?
The most important property is the clean air delivery rate (CADR). AHAM recommends you follow the 2/3 rule. Take the area of the space in square feet, and multiply it by 2/3 to get the necessary CADR in cubic feet per minute (CFM). So if the space is 200 sq ft, CADR = 200*2/3 = 133 CFM. Getting one that is AHAM Verifide ensures 3rd party testing and reliable values.
In a typical commercial space with 9 foot ceilings, the 2/3 rule gives you 4.5 air changes per hour (ACH). This is a reasonable amount that would likely combine with ventilation for at least 6 ACH.
Be careful about some claims. For example, Austin HM400 advertises the fan is 400 CFM, but with the filter, the CADR (they call it True Air Flow Rate) is 250 CFM. By AHAM standards, that would be good for an area of 250*3/2 = 375 sq ft, but they use their own standard and claim it is sufficient for 1500 sq ft. That would give you 1.1 ACH in a typical commercial space with 9 foot ceilings, much lower than the 4.5 ACH used by AHAM.
The second important factor is noise. Here’s a rough guide as recommended by Clean Air Stars:
< 40 dBA — sleep
< 45 dbA — Classroom, Quiet Restaurant & Office
< 50 dBA —Loud office & Childcare
< 60 dBA — Loud restaurant or gym
> 60 dBA — Acceptable for loud environments
See the noise section below for how to select HEPA filters to ensure appropriate noise.
The third factor is cost. More expensive ones are often worse. Typical ones you can find at the hardware store are likely to be more cost effective than high end ones.
The final factor is additional features, which should be avoided. See FAQ #9.
FAQ #17 lists resources to select HEPA filters.
3. Do HEPA filters blow the virus around?
It’s already being blown around. People don’t usually feel or see that air is moving, but you can see airborne dust reflecting in sunlight to get a better idea how particles can move around a room. The same applies for smoking — if there’s a smoker in the room, the smoke will travel around you will eventually be inhaling the smoke. Typical indoor air speeds are 0.1–0.2 m/s. Consequently, any indoor air pollutant is already floating around the room.
The goal when designing ventilation systems is that the room is perfectly mixed — all pollutants are evenly distributed throughout the room. This also means that clean air is properly dispersed throughout the room and that when exhausting air, you will be extracting contaminated air instead of exhausting clean air and letting areas to build up with higher pollutant concentrations. Studies show that the well-mixed assumption is correct for small particle sizes.
This is why one of the core recommendations of ASHRAE is to mix the air in the room and the CDC recommends the use of fans (FAQ 11 on their webpage). Because HEPA filters use fans, they can increase airflow and mixing in a room which is a good thing.
The one caveat is that you do not want to create direct currents between people as anyone downwind will be placed at increased risk. The two options are to purchase a HEPA filter that blows up or place it where it is not blowing close to anyone (within 2 m or 6 ft).
4. Do HEPA filters make the virus travel further?
No. The virus will travel around the room with or without a HEPA fitler or fan. What the HEPA filter or fan will do is make it mix into the room at a quicker rate. This will help mitigate the risk of short-range transmission (see FAQ #13) and will help ensure clean air is better distributed throughout the room.
When placing a HEPA filter, ensure you are not creating direct currents between two people as the person downwind will be at increased risk.
5. Do HEPA filters interfere with ventilation?
Not if chosen properly. ASHRAE 241–2023 Control of Infectious Aerosols section 6 addresses the compatibility between in-room air cleaners and different ventilation systems.
As a brief summary, HEPA filters should be chosen with the correct discharge direction based on the ventilation system:
Well-mixed ventilation: any type
Natural ventilation: discharge up
Cross Flow: discharge horizontal
Downflow: N/A
Upflow: discharge up
Most ventilation systems are well-mixed ventilation and any type of in-room air cleaner can be used. A detailed description, including ventilation system types can be found here.
6. Does UV light help when combined with a HEPA filter?
Not really. To be rated a HEPA filter, it needs to remove more than 99.95% of particles. Even if the UV light successfully disinfects the remaining 0.05%, the effect on the clean air delivery rate would be negligible. Some UV devices are not properly designed and the UV light would not deliver a sufficient dose to inactivate pathogens. While there are portable air cleaners that exclusively use UV light to disinfect the air and are effective, the effect of combining HEPA filtration with UV light is unlikely to provide any meaningful benefit. An example highlighting this can be observed from data provided by Airpura.
According to their own data, the HEPA filter achieves a 100% removal rate, rendering the UV component ineffective in terms of overall air purification.
7. Do I need to worry about getting COVID from viruses coming off a HEPA filter?
No. This effect is known as resuspension. Virus particles are initially floating in the air and then deposit on the filter when pulled through. They can then become concentrated on the filter. If the filter is then disturbed, the virus particles can get resuspended and infect anyone nearby.
There is currently no evidence that this is a significant risk. Furthermore, virus particles decay over time, so if the filter has trapped virus particles, unless it was in the very recent past, it is unlikely to be able to cause infection. A much more significant risk would be to share a room with an infectious person without appropriate mitigation measures in place, like portable air cleaners.
When changing filters, it is good practice to wear a respirator and do it in a well ventilated environment as dust can become resuspended which can have harmful effects when inhaled.
8. Should I get a medical grade HEPA filter, True HEPA or H14?
Medical grade HEPA and True HEPA are just other names for HEPA filters rated H13. This is the standard rating for HEPA filters, so medical grade HEPA or True HEPA are just marketing names.
There are H14 HEPA filters which can filter 99.995% of particles instead of just 99.95%. This benefit is negligible in all normal situations, but might be relevant in certain laboratories. There is no need for general use of H14.
More information about HEPA filter ratings can be found here.
9. Should I get one with additional features like ionization, plasma, NCCO, PCO, hydroxylation?
No. Portable air cleaners often come with additional features. These involve some mechanism to supply reactive species to the space. The goal is they will interact with virus particles and cause them to inactivate or deposit on surfaces or filters.
Many of these technologies have poor evidence of effectiveness. Even claims of 99.999% reduction can still mean that HEPA filters are more than 10x more effective.
There is also evidence that many of these technologies can produce indoor air pollutants like formaldehyde and ozone. See this article for an in-depth review.
The CDC recommends caution with these technologies. The Ontario Society or Professional Engineers recommends these technologies are not used until there is a standard for safety and effectiveness.
10. What are the main problems with HEPA filters?
The three main problems with HEPA filters are noise, distribution and human operation.
See FAQ #11 to address noise.
There is a concern with short-circuiting — that clean air supplied directly returns to the intake of the HEPA filter and does not mix within the space. Proper air distribution can prevent short-circuiting. To achieve proper distribution, some of the following strategies can help:
multiple HEPA filters can be spread out throughout the space
they can be combined with fans that distribute the air better
they should be placed properly. See FAQ#11
Having portable air cleaners in a space requires people to turn them on and off. To ensure this is being done properly, the portable air cleaner needs to be selected to be noise appropriate for the space. This needs to be combined with proper training, signage and verification policies to ensure they are running properly.
11. How do you address noise?
The easiest way to address noise is to purchase multiple HEPA filters and run them on low. They still need to be sized properly for the space. More details can be found here.
Another alternative is to use a portable air cleaner with PC fans and either DIY it or purchase it from Clean Air Kits.
12. Where should it be placed?
It’s impossible to know the perfect place to put it without a computational fluid dynamics model or fog experiments, but they are still very effective, even if not in the ideal place. Here are some placement guidelines:
Best to have one that blows air up
If blowing sideways, best not to aim it at anyone.
If it’s in the direction of someone, keep at least 2m (6 ft) away
Centre of the room is best
Don’t blow into a wall or object
Raised is better than on the floor
Give at least 0.5 m (1.5’) clearance from walls or objects that could restrict flow
Keep away from other sources of clean air: open windows, supply air vents, other HEPA filters
If you have multiple, space them out.
Make sure there’s no obstruction to where the air is going (like under a table)
In classrooms, there’s usually plenty of room at the front.
Sometimes it’s better to move it a bit away from people if it allows you to run it on a higher speed because of noise
13. Are HEPA filters useless against short-range transmission?
No. Short range transmission is higher risk because the virus is more concentrated in the initial exhaled plume by the infectious person. To mitigate short-range transmission, this plume needs to be diluted quickly. In order to accomplish this, two things are required: clean air in the space and good airflow in the space. Good airflow will dilute the initial plume quickly and it should be replaced by clean air in the space.
A HEPA filter can assist on both of these measures. It will ensure the virus concentration in the space will be lower. Because it uses a fan, it will improve airflow throughout the space. Depending on the size of the space and the HEPA filter, this effect might be small, but it can still help.
Overall air quality improvements help mitigate short range transmission.
14. I heard it doesn’t work on viruses because it only works on particles down to 0.3 µm and viruses are smaller than that.
Capturing Particles below 0.3 µm
There are different physical mechanisms which allow filters to trap particles. The different mechanisms work best on different particle sizes.
The combination of these mechanisms is worst at 0.3 µm. Filtration is more effective on particles with both larger and smaller diameter. Because of this, HEPA filters are tested on particles 0.3 µm in diameter.
In summary, HEPA filters do a better job against particles below 0.3 µm. The misconception that they don’t work below 0.3 µm is because they perform worst on particles that are 0.3 µm, so that is the size they are tested on.
Size of Respiratory Aerosols
Even though the filters can trap particles smaller than viruses, the viruses are not floating around by themselves. This is known as the naked virus myth. The viruses are floating inside respiratory aerosols and the aerosols are much larger than viruses, so it is much easier to trap them through filtration. Here’s an example of the scale of the virus inside respiratory aerosols.
As far as the size of the respiratory aerosols, there is a range, but most are between 0.5 µm and 5 µm.
15. Is it dangerous to sit near a HEPA filter because it will draw the virus near you?
No. In general, the air in the room should be assumed to be well-mixed and the air that is being brought in to the HEPA filter is the same as the air you would be breathing if you were in the middle of the room.
However, short-circuiting is an established problem with air cleaning equipment that does not have proper distribution built in, like HEPA filters. Short-circuiting occurs when clean air supplied from the HEPA filter does not properly mix into the space, but instead circulates back to the return. This would mean that air near the HEPA filter is cleaner than the average air in the room and it would be lower risk to be adjacent to a HEPA filter.
16. “We have mechanical ventilation, so we don’t need a HEPA filter”
If a room has mechanical ventilation, it does not mean there is an appropriate level of clean air supplied. Mechanical ventilation can have various issues:
Current standards are not designed for airborne disease mitigation, only comfort. ASHRAE is currently developing a standard for airborne disease mitigation because its existing air quality standard does not address that. ASHRAE is used in building codes in North America.
Older buildings were not designed to comply with modern minimum requirements. Even with mechanical ventilation, they often have very poor ventilation levels.
Having mechanical ventilation does not mean it is operating properly.
Consequently, to provide appropriate risk, there are two things facilities should do. First, select an airborne mitigation standard and comply with it. Examples of standards are 6 air changes per hour (OSPE, Lancet, Harvard Healthy Buildings) or 10 litres/sec/person (WHO).
Second, there should be ongoing verification that the systems are operating as designed. The best way to do this is by monitoring ventilation with CO2.
If facilities are complying with an airborne disease mitigation standard and verifying proper operation, then it is reasonable for them to state that they have provided a reasonably low risk environment and do not want to add portable air cleaners to their strategy.
If there is no compliance to a standard for airborne disease mitigation, HEPA filters can assist in achieving those goals. If there is no verification, HEPA filters can provide an additional layer of protection if the systems are not running properly.
17. What good resources are there to pick HEPA filters?
Some good resources are:
Clean Air Stars
Clean Air Crew
Comparison of HEPA filters by Marwa Zaatari
18. Do Personal HEPA Filters Work?
In general, not really. There are two ways to use personal HEPA filters: a small HEPA filter in a large room that is close to you or having one blowing in your face.
If a small HEPA filter is used in a large room, the air close to it might be slightly cleaner than the rest of the room, but unlikely to be very meaningful.
If you are pointing one at your face, it depends on the filter and how close it is. This is data that was taken by Clean Air Stars using the QT3.
To get a meaningful reduction, the distance needs to be within 40 cm of your face. That is very close and not sustainable for a long period of time. They ran another test with a DIY PC fan array which showed much better results.
This one has over 80% particle reduction at 1.5 m away. A much more reasonable distance. It is likely that this option is better because the fan array gives it a much larger area. This is a legitimate option as opposed to most HEPA filters which supply air from a relatively smaller area with higher air speed.
This deals with a more general issue — when clean air is supplied from a point, how far will it travel before the clean air becomes well mixed with the surrounding air. One example which has good data is the gasper or personal air outlet found in airplanes. Does it work to use it like a personal HEPA filter and aim it at your face? This is what the ASHRAE epidemic task force has to say about it:
In summary, clean air mixes into the space pretty quickly. Using it might help a bit but not significantly.
19. Do HEPA Filters Interfere with Windows?
Alternatively, do windows negate the effectiveness of HEPA filters? Should you turn off HEPA filters if the windows are open?
No. There is no interference or need to stop using HEPA filters with open windows. Both windows and HEPA filters are sources of equivalent clean air in the room and using both together will increase the clean air delivery rate, creating a lower risk space.
Position of ASHRAE 241 — Control of Infectious Aerosols
In ASHRAE 241 — Control of Infectious Aerosols, section 6.5 discusses how in-room air cleaners work with different ventilation systems. One of these systems is natural ventilation (windows). Using the appropriate air cleaner will work with the natural ventilation system to create a lower risk space.
ASHRAE 241, section 6.1 provides the following formula for compliance with the standard:
Where:
Veaci is the required equivalent clean airflow
Vnv is the equivalent clean airflow rate from natural ventilation
Vacs is the equivalent clean airflow rate from air cleaner systems
Therefore, based on 241 section 6.1, both the air cleaner systems and natural ventilation can be added together to provide sufficient clean airflow to comply with the standard.
As discussed previously, it is best to position HEPA filters away from the windows since you want to be cleaning the dirty indoor air and not the clean air coming in through the windows.
Advertising Claims from HEPA Filter Manufacturers
Some HEPA filters come with a claim that they are more effective if windows are closed. This is true if the pollutant of concern is coming from outdoors, like wildfire smoke or pollen. HEPA filters would remove these pollutants, but they would continually be supplied to the space through the open windows. Therefore, in cases like wildfire smoke, windows should be kept closed while HEPA filters are run.
If the pollutant is concern is generated indoors, like infectious aerosols, both HEPA filters and windows are two separate methods to remove this pollutant and should be used together.