Air Cleaners:

How they work and what to look for when choosing one

We know that COVID-19 can stay in the air for a long time. It can float into your nose or mouth, even if you’re not in the same room or near someone who is sick. So it’s important to make sure that indoor air stays clean and refreshed. Portable air cleaners can effectively reduce the spread of the virus indoors, but there is an overwhelming amount of confusing information out there. We’ve partnered up with indoor air quality expert Dr. Shelly Miller, a Mechanical Engineering professor from the University of Colorado Boulder, to answer the most frequently asked questions about how to choose the right air cleaner for your space! We link to additional resources below from Shelly and others. 

Q: Are air purifiers the same thing as air cleaners?

Yes, they are synonymous, but technically these devices clean the air. You can’t actually “purify” air!

Q: Do air cleaners help stop the spread of airborne viruses? Which types do you recommend?

Stand alone or portable air cleaners can be purchased (or made) and operated in any space to reduce particulate matter concentrations within that space. They can be moved around to different locations too, so they are also called portable air cleaners.  These devices physically remove particles from an air stream, and this process is called filtration. They are effective against the virus because two factors play into the risk of catching COVID-19: 

1. The concentration of virus particles around you and 

2.  How long you spend in a space

If you cannot reduce how much time you spend indoors around people who are not in your bubble, you can minimize the indoor concentration of virus particles; this is where portable air cleaners can be useful. Portable air cleaners with high-efficiency particulate air (HEPA) filters physically trap virus particles within the filter material, thereby removing them from the air and reducing the concentration of the virus particles indoors. These filters do not have any negative side effects, and they can also reduce the concentration of other harmful particles like allergens and smoke pollution. 

Q: I’ve heard HEPA air cleaners are the best, but is there a cheaper alternative to purchasing a HEPA air cleaner?

Yes! HEPA filters are  >99% efficient, but an air cleaner with a >90% efficient filter will work as well.  And another option is to make your own portable air cleaner using a box fan and either 1 or 2 MERV 13 filters bought at the hardware store.  Learn how to do this with our Box-Fan Air Filter Hack. 

Here are some tips to make that DIY cleaner even better:

• Close the gaps between the filter and the fan with duct tape so that there’s no leakage

• Use a MERV 13 filter, as HEPA filters may put too much pressure on the fan and cause it to fail 

• Make sure your fan isn’t overheating by checking on it frequently and not running the filter-fan combo unattended. 

• Put the filter on the side pulling in the air; this allows for the most effective filtration but exposes virus particles on the outer surface where people may touch it. Although the virus dies over time, if the air cleaner is in an area where lots of people might come into contact with the filter’s surface, like a classroom, put the filter on the side that pushes the air out, so the virus particles are trapped inside and not exposed. 

• Use a lower setting of the fan, as putting the fan on high may put too much pressure on it. Lower settings will also make less noise. But they also clean less air. So you may need to make and use a few DIY air cleaners to effectively clean the air in your space. 

Q: How do I pick the right air cleaner? 

There are tons of products on the market, and they are all of different qualities, and not all the information provided about them is the same. It can be hard to see which features are actually important in a cleaner. Here are some things to keep in mind when picking an air cleaner: 

• Pick one that is recommended by certifying authorities (AHAM or CARB in the US)

• Pick one that is the right size for the space where you will use it, such as your bedroom or office.  

  • To do this, consult the Harvard-Univ. of Colorado guide on sizing air cleaners.

  • For a quicker estimate, use the instructions below to get a rough estimate of the size of your small or large space. Make sure the air cleaner is certified to clean an area at least as big as your space!

Q: How do I pick the right air cleaner? cont.

Whether a cleaner is right for your space can be best assessed by looking at the Clean Air Delivery Rate, which is a measure of both the filter’s efficiency at removing particles of a certain size and how much air passes through the filter. It is defined as the flow rate of air that has had all the particles of that size range removed. 

• The Clean Air Delivery Rate (CADR) should be indicated on the box or in the air cleaner’s manual, and it can vary with the setting used on the unit. 

• To see if the cleaner can effectively clean the air in your space, you need to determine how many air changes per hour (ACH) it can accomplish for what size of room. 5 ACH is recommended.

  • CADR / volume of room = air changes per hour 

  • The CADR is in feet/min and the volume is in cubic feet

• For a more advanced look at how the probability for infection is impacted by the air cleaner, use this Univ. of Colorado Aerosol Transmission Estimator.

• Pick one that uses just a filter. We do not recommend choosing a cleaner that uses ionizers or UV light because these technologies are add-ons that don’t improve the device’s air cleaning capability but can drive up the device’s cost. Also, ionizers have been shown to add ozone, a toxic indoor air pollutant, to the air. There is more information on these other cleaning mechanisms later in this guide. 

• Many air cleaners come with a carbon filter, which is a helpful addition as it  removes gaseous pollutants such as nitrogen oxides and volatile organic compounds from the air. 

• For a DIY air purifier, estimating the CADR is a little tricky. The box fan’s manual should indicate the flow rate (m3/h) for each setting of the fan. Using the filter will reduce this flow rate. A reasonable rule of thumb is that about 60% of the manual-indicated flow rate should be used as the CADR in the calculation above.

Q:  How often should I change the filter in my portable air cleaner?

The more particles the filter traps, the less air it can let through, which builds up the filter’s pressure, so less clean air is pushed out. Every air cleaner is different. Follow the instructions given with the air cleaner or on the manufacturer’s website. If you’re using the air cleaner in a place where you know there is a high concentration of virus particles or additional particulate air pollution like smoke, you may want to replace the filter more often.  

Wear protective equipment like masks and gloves when changing the filter outdoors. This is important because you don’t want people to come into contact with the particles caught in the filter without protection, and moving the filter may dislodge some particles. The risk of dislodging these particles is low, but we recommend taking precautions just to be safe. Place the used filters in a sealed plastic bag when throwing them out with the trash.

Q: Where should I place an air cleaner in a room?

Most commercial air cleaners draw air in from the side and push it out vertically. If this applies to your air cleaner, place it in the middle of the room and elevate it above the ground to ensure as much air at breathing height gets into the filter as possible. 

For DIY air cleaners that push air out of the side, elevating the unit is even more important, because blowing air directly over the floor can make virus particles that have already settled on the floor float around again. 

Q: Is germicidal ultraviolet light (UVC) effective at disinfecting the air from the COVID-19 virus?

Germicidal ultraviolet light (gUV, also UVC) is an effective technology to add to ventilation. It is useful for increasing the air change rates in crowded spaces and places that asymptomatic infected people are in. Although the exact efficacy of UVC at killing the COVID-19 virus in the air has yet to be verified by research, UVC has been shown to be effective against related airborne coronaviruses and against the virus on surfaces and in liquids.

As direct exposure to UVC light itself can pose a serious health risk, one way to safely add UVC to your space is with upper-room gUV lights. The CDC has authored a guideline for the use of upper-room gUV in healthcare. Germicidal UV can also be added to the ducts of the recirculating air in your HVAC system. There are several guidelines for making UVC work in a ducted ventilation system, so consult your HVAC expert to see if it would be right for your space. 

Dr. Shelly Miller has been studying gUV for over 20 years and here are informational slides from a presentation she gave in April 2020. For more information read this guide on how airborne transmission of COVID-19 indoors can be minimized authored by Dr. Lidia Morowska and other scientists. 

Q: Are portable air cleaners that don’t use filtration still useful?

Air cleaners that use ionization, plasmas, or oxidation, are not recommended because they can have unintended side effects on the air, such as producing ozone, a gas that is very harmful to your lungs. Please make sure any air cleaner you buy certifies that it does NOT produce ozone. Read this Twitter thread from Dr. Jose Jimenez for more details on what to avoid. Also there are not many independent scientific studies on the effectiveness of these technologies for indoor spaces. 

Q: Do disinfectant-spraying foggers help?

Spraying cleaning agents into the air is not a safe or effective way to remove the virus from the air. Read this Medium article for more details on the issues that disinfectant fogger machines can cause.

Disinfectants are great for sanitizing surfaces, especially surfaces with which many people come into contact. Even when disinfecting surfaces, make sure to do so when people are not actively using the space as the fumes can be harmful. The amount of time to wait before using the space again varies, but it is safe to assume at least 3 hours.