Buying an Air Purifier to Remove Germs

germ air purifier

Protecting yourself from germs may very well be a priority for you as you consider which air purifier to purchase. So what exactly do you need to look out for in an air purifier to protect your family from germs?

First a few definitions…


Any microorganism – microscopic organism – can be called a germ. Although we usually only refer to disease causing microorganisms when talking about germs.

In other words, a germ is usually

  • Microscopic
  • A living organism
  • Disease causing

The Mayo Clinic gives five categories of germs:

  • Bacteria
  • Viruses
  • Fungi
  • Protozoans
  • Helminths

Of these five categories, only three are frequently airborne – bacteria, viruses, and fungi. Two are not – protozoans and helminths.

Let’s take a look at these last two first:

Protozoans are sufficiently small to easily become airborne –anywhere from 1 to 150 μm – but do not usually spread through the air. Instead, you’re more likely to find protozoans in food, soil, water, or insects.  Malaria is perhaps the most easily recognized disease caused by protozoans (the protozoan is Plasmodium – which measures only 1 to 2 microns). This disease, is of course transmitted by mosquitos.

Helminths, on the other hand, are too large to become airborne. Tapeworms and roundworms are good examples of helminths.

That leaves us with bacteria, viruses, and fungi. Let’s take a closer look at these three next:


All fungi reproduce through spores. And spores easily become airborne. Mold is probably the first fungus that comes to mind when you think of airborne fungal spores. And indeed mold spores are small enough to easily become airborne (2 to 30 microns). Mold is also common enough to be an issue for a lot of people. For this reason, we actually have a full article dedicated to mold which you can find here.

Here’s the thing: mold is not just one fungus. “Mold” is simply the common name given to a wide variety of different fungi that spread via spores common in indoor air. Specific fungi, collectively referred to as “mold” include:

  • Cladosporium (genus)
  • Penicillium (genus)
  • Aspergillus (genus)

Note that even the names given above are still non-specific to a certain extent. These are only genuses containing many different (specific) species. An example of a specific species of fungus called a mold:

  • Stachybotrys chartarum (specific species - black mold)

Are there other fungi that can become airborne that are NOT molds? Sure, but the molds listed above are by far the most common fungi found in indoor air in the United States.

An example of a fungus that’s not called a mold but can still spread via air is Blastomyces dermatitidis. This fungus causes a disease called “blastomycosis”. People get the disease from breathing in mold spores. However, these mold spores are typically found only outside in “moist soil and in decomposing matter such as wood and leaves.”


Most bacteria range in size from 0.2 up to 20 μm in diameter. There certainly are bacteria that are larger and/or longer than 20 μm but most bacteria are in the 0.2 to 20 μm range.

Bacteria near the smaller end of the spectrum – closer to 0.2 to 6 micron – are of greatest concern here. Why? Because of their small size they become airborne more easily and if small enough can stay airborne more easily as well.

Some common diseases caused by airborne bacteria include:

Caused bySize
Pertussis (commonly called Whooping cough) Bordetella pertussisapproximately 0.8 μm by 0.4 μm
DiphtheriaCorynebacterium diphtheriae0.5 μm diameter and 2 to 6 μm long
Bacterial meningitisone of several bacteria; an example is Streptococcus pneumoniae0.5 and 1.25 μm
Tuberculosismycobacterium tuberculosisrods 0.2 to 0.5 μm wide and 2 to 4 μm long
Bacterial pneumoniaone of several bacteria; the most common is streptococcus (pneumococcus)0.2 to 2 μm

The bacteria which cause these diseases spread through the air by several means including:

  • Direct methods and
  • Indirect methods

Spread By Direct Methods

1. airborne droplets can be aimed directly towards someone

e.g. coughing/sneezing directly towards someone NOTE even talking/laughing etc. can do it but coughing/sneezing most egregious

2. saliva/bodily fluids can be directly transferred to someone

e.g. kissing

Spread By Indirect Methods

As it relates to general air quality (and air purifiers), direct transfer of bacteria is NOT of primary concern. Instead, our focus needs to be on indirect transfer of bacteria.

In other words, an air purifier can’t help to stop the spread of bacteria via direct methods – it can’t stand in between you and someone else if they sneeze or cough on you. It can’t get in the middle of you and someone else if they directly transfer saliva or bodily fluids to you.

However, an air purifier can help to remove the bacteria from the air you breathe if the bacteria got there (as a particle floating in the air) indirectly. How does the bacteria get there indirectly? By first entering the equation directly (by the two methods above) and then interacting with the air, surfaces, and/or other particles before interacting with you.

With that in mind, let’s take a look at what exactly happens and how it happens when the two direct methods listed above become indirect methods of bacteria transfer:

1. airborne droplets, instead of directly landing on someone, enter the air and indirectly come into contact with someone nearby

eg. coughing/sneezing in the general vicinity of someone

The bacteria itself is certainly small enough to stay airborne. At around 3 μm we know that particles stay airborne for about an hour. At 0.5 microns particles can stay airborne for a few days. Thus, all of the bacteria we listed above could theoretically stay airborne for at least a few hours if not a few days.

However, bacteria don’t leave the body as isolated particles. Instead, bacteria are contained within droplets. Thus, we need to consider the size of the droplet (at least initially), not the size of the bacteria, when evaluating its behavior in the air (namely how long it will stay airborne) and the type of filter required to remove it from the air.

Scientific studies show that droplets produced by humans may contain:

  • various cell types (e.g. epithelial cells and cells of the immune system)
  • physiological electrolytes contained in mucous and saliva (e.g. Na+, K+, Cl-)
  • and of course the infectious agent (in this case bacteria)

They also show that much of these droplets evaporates after they are coughed or sneezed out of the body - thus the droplets become smaller and smaller from the moment they leave the body.

These same studies also show that a single sneeze can produce as many as 40,000 droplets between 0.5 μm and 12 μm in diameter. A single cough can produce as many as 3000 droplets equal to or less than 5 μm in diameter. That’s a lot of very small particles entering the air at once!

The scientific research that’s been conducted on this topic also cites several ways in which these droplets can move around an indoor environment including

  • People moving – e.g. walking
  • Opening/closing a door between different rooms
  • Differences in air temperature between different rooms

At 0.5 μm to 12 μm sneeze droplets can very much stay airborne for long periods of time. At equal to or less than 5 μm, cough droplets can also stay airborne. And, as we just showed, they can move around quite easily around the home. Doing something as simple as walking or opening or closing a door can move these droplets around the home.

All of this means that

  • airborne droplets can very effectively stay in the air and
  • travel through different parts of the home in the air

The question is - what happens when these droplets eventually come into contact with a surface? Well, the same thing that happens when our second direct method of disease transfer – saliva/bodily fluids – come into contact with a surface – they dry up. More on that next.

2. saliva/bodily fluids - or airborne droplets – instead of directly coming into contact with someone, land on a surface and dry up. The substance that remains becomes airborne and infects someone nearby 

e.g. saliva/bodily fluids on surfaces or airborne droplets from coughing/sneezing land on a surface and dry up

All these liquids dry up over time in most indoor environments. Why? Because your home is much drier than the inside of your body.

To put it scientifically: “Desiccation (drying) is experienced by all airborne microbes”.

Once the droplet/fluid dries up the bacteria can latch onto anything, including a myriad of microscopic particles. The problem here is that nobody really knows onto exactly which particles the bacteria latch onto. We find current research on this topic to be lacking as even the most complete studies on the topic have to concede: “The correlation between surfaces and aerosols, and the surface-aerosol interactions in indoor environments, require further investigation. In particular, the volume of particles deposited on and then detached from a particular surface is poorly understood.”

Our advice: assume anything is possible. The bacteria itself may become airborne or it may latch onto any type of particle to become airborne. It, of course, becomes airborne via some type of disturbance – e.g. walking over carpet, jumping on a couch, etc.

3. airborne droplets, instead of directly landing on someone, interact with other particles while still airborne

Again, little is known about this type of interaction. Again, we err on the side of caution: Assume anything is possible and therefore limit particles in general.

In any case, whether it’s bacteria inside an airborne droplet, bacteria released from a dried up droplet, bacteria that’s latched onto other particles or bacteria inside airborne droplets that have interacted with other airborne particles – the “particle” of concern – the particle you want to remove from the air – is going to have a particular size. That size will be no smaller than the smallest bacteria – 0.2 microns. On the other end of the spectrum, bacteria may latch onto a particle as large as 100+ microns – though such a particle would not stay airborne very long.

HEPA filtration is the answer

The good news here is that no matter how the bacteria is presented and no matter what its size an air purifier equipped with a HEPA filter will be able to remove it from the air. Whether it’s a single isolated bacterium as small as 0.2 microns or one that has latched onto larger particles as large as 100+ microns, a HEPA filter will remove all particle types and all particle sizes.

HEPA filter efficiency is usually given in terms of 0.3 microns: a HEPA filter is usually said to be 99.97% effective removing particles 0.3 microns and larger. Clearly, 0.3 microns is more than the smallest bacteria at 0.2 microns. Does this mean that HEPA filters are ineffective removing 0.2 micron bacteria from the air? Absolutely not.

The 0.3 micron threshold is given because it represents the size range of particles that a HEPA filter finds most difficult to filter. Particles larger than 0.3 microns are easier to filter. Particles smaller than 0.3 microns are easier to filter. Particles right around 0.3 microns are the most difficult to filter. The filter’s efficiency is given in terms of 0.3 microns to demonstrate how it’s still highly efficient removing the most difficult to remove particles (particles around 0.3 microns) from the air.

So rest assured that a HEPA filter will still be able to remove the smallest bacteria – around 0.2 microns in diameter – from the air.

The best HEPA equipped air purifier for airborne bacteria

A HEPA filter removes bacteria from the air but it requires some “means” to push bacteria laden air through it. That “means” is provided by an air purifier.

An air purifier is specifically designed to be able to push large amounts of air through a HEPA filter. Your home’s HVAC system cannot efficiently push air through a HEPA filter. Installing a HEPA filter in an HVAC system will damage the system.

In all fairness, an HVAC system usually services a whole house. A single residential air purifier can only process air for a single room. But that’s exactly the point of an air purifier. It’s intended to be used as a means for an extreme level of filtration in smaller spaces.

We recommend 250 CFM air purifiers as the best all-around performers on the market. These units process 250 Cubic Feet of air per Minute. This is sufficient processing power for a room up to about 300 sq. ft. If the room you need to service is larger than 300 sq. ft. we recommend running multiple 250 CFM units. If it’s smaller, we still recommend a 250 CFM unit.

The best 250 CFM unit – the best air purifier for bacteria - currently on the market is the Winix 5500-2.

Special recommendation for those especially concerned about airborne bacteria

Disease causing airborne bacteria represent a serious health concern for many people. If you’re especially concerned about airborne bacteria in your home we advise you to halve the square footage recommendations we listed above.

In other words, if you’re especially concerned about removing airborne bacteria from your home, we would recommend one 250 CFM unit per 150 sq. ft., instead of our normal recommendation of one unit per 300 sq. ft.

This effectively doubles the rate at which the unit can process a given quantity of air in a particular size space. For example, using our original recommendation a single 250 CFM unit can process all the air in a 300 sq. ft. space with 10 ft. ceilings (300x10=3,000 cu. ft.) once every 12 minutes (3,000/250=12). By using our special recommendation above instead you would need to install two 250 CFM units in such a space. Thus, you would have a total output of 500 CFM and be able to process all the air in the room in only 6 minutes (3,000/500=6).

This effectively doubles the chance the air purifier has to remove any particular airborne bacteria particle from the air.

Common bacterial disease causing bacteria are not the only bacteria

Note that thus far we’ve only considered bacteria causing the diseases listed earlier. We’ve also limited the scope of our discussion to disease transfer originating in liquid form and caused by an infected human being – airborne droplets (coughing/sneezing), saliva, bodily fluids.

But what if we removed these limitations?

We’d find many other sources and means of bacteria transfer within the home:

Source 1: Humans

You don’t need to be sick and coughing or sneezing to transfer bacteria. In fact, the majority of bacteria found in your home come from your skin and intestinal tract. Transfer can occur either directly – your skin on a surface – or indirectly – using the bathroom where fecal matter can become airborne. Most of this bacteria is just not the nasty disease causing kind we discussed earlier.

Source 2: Water

Bacteria love water. And when you take a shower, a bath, use the toilet, or get some tap water there’s always a risk of bacteria being in the water. Again, most of the bacteria in the water is harmless and your local municipality (assuming you live in the US) is likely chlorinating the water that comes into your home which drastically reduces the likelihood of problematic disease causing bacteria being in the water

Source 3: The Outdoors

Your home is surrounded by the outdoors. And bacteria are everywhere outdoors. Bacteria can enter your home through doors and windows but they can also enter via more subtle means – on you, your clothes, your shoes – anything you, your family, or your pets carry into your house.

No matter the source, we’ve seen that bacteria in your home can easily become airborne. Either the bacteria itself is small enough to become airborne or it latches onto particles that can become airborne. In either case, the best way to protect yourself from airborne bacteria is an air purifier equipped with a HEPA filter. Not all airborne bacteria is harmful but why take the chance?

Our bodies contain all the bacteria we need. Breathing in airborne bacteria at best is unnecessary and at worst can make us very sick.


Most viruses are between 0.02 and 0.4 microns in size – much smaller than bacteria which can be as small as 0.2 microns but as large as 20 microns.

Bacteria of concern are on the smaller side – between 0.2 and 6 microns. That’s because smaller bacteria are more likely to become airborne and stay airborne.

We don’t need to make this distinction with viruses. At 0.02 to 0.4 microns any virus can easily become airborne and stay airborne. Some, like the ebola virus, are thankfully not transferred via the air but theoretically, only taking into account its size, any virus can become airborne.

As we did for bacteria, perhaps it would be most helpful to look at the more common viruses and assess their composition and primary means of transfer to determine how we can combat their spreading with an air purifier.

Note that our concern is common airborne viruses. Viruses like the ebola virus get a lot of news coverage not because they’re common but because of the severity of symptoms when individuals are infected with the disease. The ebola virus also does not spread via the air. Instead, it spreads by direct contact with blood or other body fluids. 

The most common viral infections, their respective viruses, and their sizes are listed in the table below:

Common coldcan be caused by a variety of different viruses including adenovirus (0.09 - 0.1 microns), coronavirus (0.078 microns), and rhinovirus (0.03 microns)
Flu can be caused by Influenza A (0.08-0.12 microns) among others
Bronchitiscan be caused by certain chemicals, bacteria, and several different viruses (85% to 95% of acute bronchitis cases in healthy adults caused by viruses) - adenovirus and influenza A are some examples
Stomachcan be caused by a variety of different viruses including norovirus (0.023-0.04 microns)

When it comes to the spread of these diseases (viral infections) we see similar trends as we do with the spread of bacterial infections. We can once again break down transfer of the disease into two primary categories:

  • direct transfer - the virus is directly transferred (e.g. you cough/sneeze directly onto somebody else)
  • indirect transfer - the virus travels a particular path before it is transferred (e.g. you cough/sneeze airborne droplets that remain airborne in a room and another person inhales some air containing those droplets a few hours later)

Indirect transfer is the concern

As was true for bacteria, our concern – when it comes to air quality – is primarily with indirect airborne transfer of viruses.

There is nothing an air purifier can do in a direct transfer scenario. For example, there is nothing an air purifier can do when an infected individual coughs on you and you immediately inhale the infected air.

An air purifier can only help when it comes to indirect airborne transfer of viruses.

In order to assess how an air purifier can help, note that a virus can be indirectly transferred by

1. airborne droplets

e.g. coughing/sneezing nearby someone else. The droplets remain airborne and infect someone else)

2. dried up droplets and also saliva/bodily fluids

e.g. an infected individual touches their eyes or mouth – they then touch a surface. The end result? Bodily secretions land on a surface, dry up, and leave a virus behind that later becomes airborne either by itself or after latching onto other particle

Let’s take a closer look at each of these transfer methods, starting with

Airborne droplets

When an infected individual coughs, sneezes or even talks liquid droplets containing the virus enter the air. The droplets can

be directly transferred to another individual

e.g. you cough/sneeze directly onto somebody else – again, this is not our concern when it comes to air purifiers

stay airborne and then indirectly transfer when another person breathes in that same air
land on a surface

be disturbed and then be indirectly transferred to someone else

Here we’re concerned with scenario 2.

We know from earlier research (on bacteria) that one sneeze can produce up to 40,000 droplets between 0.5 μm and 12 μm (microns) in diameter. One cough can produce up to 3,000 droplets equal to or less than 5 μm in diameter. Not to mention the droplets produced when infected individuals talk, laugh, or just breathe.

At these sizes - 0.5 μm to 12 μm - the droplets can easily stay airborne. They are also easy to filter with an air purifier. A HEPA equipped air purifier can easily handle particles in this size range.

Other means by which viruses can become airborne - dried up droplets and also saliva/bodily fluids

Some of the sneeze/cough/etc. droplets will remain airborne while others will land on a surface. If the droplet lands and surrounding liquid dries up, the virus remains, ready to become airborne should it be disturbed.

Bodily fluids including saliva can also contain viruses. These fluids can also land on a surface and dry up. The virus can become airborne in the very same way in which a dried up droplet virus can become airborne. Again, if it’s on a surface it needs to be disturbed to become airborne.

A disturbance can be anything from a puff of air from an AC vent blowing over the surface to jumping on, running over, or even walking over a surface.

At approximately 0.03 to 0.1 microns most of the viruses that cause common viral infections are small enough to not only become airborne (after being disturbed) but also stay airborne extremely easily. However, as was true for bacteria, we need to consider the complex environment surrounding the viruses. The viruses are likely to interact with a myriad of different particles. In the home these “other particles” are usually dust particles.

We know that the virus is likely to interact with (latch onto) particles larger than itself. Why larger particles? Well, for one, because it’s highly unlikely for there to be any particles smaller than a virus in the home.

Studies have shown the Influenza A virus to associate with particles ranging from 0.4 microns all the way up to 10 microns. Interestingly, in the same study, respiratory syndrome virus (PRRSV) was also associated with particles in the same size range except for particles between 0.7 and 2.1 microns. As a final note from the study, scientists found that while the viruses studies can be found in a wide range of particle sizes,“virus viability is size dependent”.

HEPA takes care of all

The bottom line is that a HEPA equipped air filter will take care of all particle sizes involved. Whether it’s airborne droplets (0.5 to 12 microns), stand-alone common viruses (0.03 to 0.1 microns), or particles associated with viruses (0.4 to 10 microns) – a HEPA filter is up to the task.

In terms of specific models, output, etc. the same recommendations that applied to bacteria apply for viruses.

That is to say we recommend the Winix 5500-2 as the best air purifier for viruses currently on the market.

Killing microbes

Certain air purifier manufacturers market their HEPA filters as being antibacterial or antimicrobial. Blue Air, for example, manufacturers HEPA filters made of polypropylene – a material that is naturally antibacterial. Alen manufacturers filters made with HEPA-Silver threads. Alen claims that these threads are antimicrobial.

Other manufacturers have introduced new “technology” that is marketed to destroy bacteria and viruses. GermGuardian makes air purifiers equipped with “UV-C light technology” that “works with Titanium Dioxide to reduce airborne bacteria, viruses, germs and mold spores.”

Our response?

You don’t need to kill germs to filter germs. A regular HEPA filter will do just fine filtering both bacteria and viruses. If HEPA filters alone are good enough for hospitals, it follows that they should be good enough for your home.

An antibacterial filter is certainly not a bad thing. But it isn’t very necessary either. A critical component for bacterial growth is moisture. Now think about what goes on inside an air purifier – a lot of air movement. This is not an environment that allows for the accumulation of moisture. In fact, the inside of an air purifier is probably the last place in your home you’re likely to find moisture.

You may also be concerned about mold (fungal) growth on your air purifier’s filters. Again, moisture is a critical component for fungal growth and the inside of an air purifier is a very dry place. You do not need to be concerned about mold growing inside your air purifier.

“Technology” like UVC-light can be effective in killing germs but

Finally, remember that an air purifier’s HEPA filter should be replaced frequently. Yes, if viruses and bacteria are properly filtered they will “build up” on the filter BUT not for long. With each filter replacement the “build up” process resets.

The Bottom Line

HEPA filtration is good enough. You do not need an antibacterial/antimicrobial filter or UVC light to kill germs. You just need to filter them and a HEPA filter does this very well. Again, if it’s good enough for hospitals it’s good enough for your home.

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Comments (2)

Chad Mooresays...

Another great article. Keep it up!

Josh LEEsays...

Thank you so much for your thorough review. I am a dentist and this is the exact answer I was looking for.