What is a Hepa Filter & How Do They Work?

While the topic of air pollution is quite a hot one and has been for the past few decades, most conversations that take place focus on the environment and the air quality outside of our homes. However, indoor air quality can be just as bad as that outside, if not worse. The difference is that inside, we have the opportunity to change that via way of a good air filtration system.

If you’ve never owned an air purifier before, and never really read up about them, when you do go to look for one, it can be a little daunting with all the terminology and acronyms like HEPA, MERV, and CADR that are thrown around. In this guide, we hope to put some of those burning questions to bed and explain a little more about what some of these terms refer to.

HEPA-Type Filters Explained 

HEPA stands for High-Efficiency Particulate Air. It’s a kind of filter that’s found in most good air purifiers and vacuum cleaners and is very effective at trapping unwanted airborne particles. However, not all contaminants are captured. Particle size plays a big part here.

Filters that are deemed to meet the HEPA standard are those that are capable of removing 99.97% of all particles that are 0.3 microns or larger in size. Microns refer to the unit of measurement for particles and is the equivalent of one-millionth of a meter. Dust mites tend to be between 1-10 microns in size, although some can be much smaller.

A HEPA filter is made out of a series of pleated fibers. These fibers are often made out of glass fibers (otherwise known as fiberglass) or some kind of polypropylene or polyester non-woven fabric material and are held together in a frame. The most common type of frames used are those made from either plastic or cardboard. Quite often they will also feature a rubber or foam seal in which to prevent air from escaping.

The aim of a HEPA filtration system is to remove both small and large particles from the air. While larger particles such as pet hair and pet dander are a pain, they aren’t usually as harmful to you as smaller contaminants such as bacteria or viruses. And therefore, these are normally taken care of by way of a pre-filter.

Depending on the size of the contaminants, it depends on how it’s trapped. Larger particles are trapped by way of inertial impaction and sieving. These particles either get trapped on their own or they collide with other fibers which then get stuck trying to passing through the fibers. Medium-sized particles are seized by the fibers via interception. While smaller particles are diffused as they pass through the filter.

Air Filter Rating System Explained 

When looking to buy an air purifier, one term you’ll see around a lot is MERV. MERV stands for Minimum Efficiency Reporting Value and is essentially the quickest way to find out how effective an air purifier really is.  It’s a scale that was developed by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) back in 1987 and ranges from 1-20. The higher the rating, the less that can pass through it.

Simple dust filters tend to have a MERV rating of around 1-4. And while these are good for filtering out things like human hair and large dust particles, they have very little effect when it comes to removing smaller contaminants. Air filters with a MERV rating between 5-8 are generally your lower end disposal filters. These are slightly better and will filter things such as mold spores, and certain chemicals, they still aren’t very good at dealing with small particles. Air filters with a rating between 9-13 are better, especially when dealing with fumes, but still not really fit for use at home.

Really, when looking for an air purifier to use within the home, you should aim for one that has a MERV rating of 14+. Those that range between MERV 14-16 are capable of filtering up to 95% of particles 0.3 microns or larger. Which is not bad. They’re good for eliminating small and medium particles, as well as dust and mold spores. And, they’re good for anyone who suffers with low-moderate allergies.

However, if you’re someone who suffers from any high levels of allergies or any breathing conditions such as asthma or COPD, you should be looking to invest in an air purifier that has the best level of effectiveness. In which case you need to be looking at either a true HEPA filter or a ULPA filter.

A true HEPA filter has a MERV rating of 17+ and is the most common type used in air purifiers today. These filters are good for allergens and are very effective at removing mold spores, pollen, dust mites, and other microscopic contaminants. A ULPA (ultra-low particulate air) filter is the highest rated of them all and has a MERV rating of 18+. A filter of this caliber can remove even the smallest of particles with the greatest of efficiency. However, these filters don’t come cheap and are often only found in places such as clean rooms or biosafety cabinets.

air purifier filter being changed

Are HEPA Filters Washable? 

Yes….., some of them.

As you can probably imagine, HEPA air filters come in all kinds of shapes and sizes. In addition to that, they also vary in whether or not they are washable. Both the washable and washable types have their pros and cons, some of which we’ll go through here.

Washable HEPA filters

These consist of thick fibers that have been designed to get wet. The benefit of using these filters is you can wash them as often as you’d like and they will last quite a long time. However, just be aware that the more they are washed, the less effective they will become. So, while you want to make sure they stay clean enough to do the job, you don’t want to put more wear into them than needed. The other downside is the hassle of removing the filter, giving it a clean down and a rinse, and popping it back in your device.

Non-washable HEPA filters

While you will no doubt spend more money when using non-washable HEPA filters, they do have their advantages. The first is that you don’t need to keep scrubbing them, and they will still last a very long time. However, you can prolong their life a little longer by giving them a quick vacuum every now and then. And while it is still a bit of a pain having to remove the filter, at least there’s no scrubbing involved as there is with the washable kind.

Things to Consider Before Buying a HEPA Filter

You should now that you have a pretty good understanding of how HEPA filters work and why they’re so effective. So, in this section, we’ll go through some of the things to consider before making your next HEPA filter purchase:

Room Size

Before you part with any money, the most important factor to consider when buying an air purifier is whether or not it will be effective in the space you want to place it. Once you’ve decided on where it’s going, the next step is to get the room’s measurements. Any air purifier that’s worth investing in, will tell you the maximum room size it’s capable of filtering. Never buy an air purifier that’s too powerful for the room it’s going as is just a waste of energy. On the flip side, don’t get one that can’t handle the size of the room it’s going in.


Another factor to bear in mind is the air purifier’s Clean Air Delivery Rate (CADR). This metric was developed by the Association of Home Appliance Manufacturers (AHAM) and essentially refers to the volume of filtered air a device can deliver. It is only ever measured in the highest fan speed but is a good indication as to how much filtered air you’ll get. The CADR scale ranges from 0-450 and is measured in cubic feet per minute (CFM). However, any air purifier with a CADR rating of less than around 300, really isn’t worth its money. Just remember that the higher the CADR rating, the more powerful the fans are.

Activated Carbon Filter

While HEPA filters are exceptionally good at removing contaminants from the air, they’re not very good at tackling odors. Therefore, a lot of good quality HEPA filters are used in conjunction with activated carbon filters. These filters were designed specifically to remove gas and odors from the air. And while they don’t make any real difference in terms of dealing with larger airborne particles, they are fantastic when it comes to filtering out cigarette smoke, mold spores, or even those lingering pet smells.

UV-C Light

There is a wide range of air purifiers on the market today, all varying in performance, budget, and features and functions. And while many of these are just added extras to enhance the user experience, one feature that is thought to make a difference is the addition of a UV-C light. When used correctly, this form of ultraviolet light can be very effective in killing germs, mold, bacteria, and more. The downside is that UV-C light is the most dangerous form for humans and other living organisms.

It works by destroying the nucleic acids in cells, and as a result, disrupts the DNA. This causes the cell to become incapacitated or die. Air purifiers that use UV-C lights first draw the air in through a filter of some kind. Then, as air passes through the purifier’s chamber it’s subjected to the UV light. With a design such as this, there’s no releasing of the UV light into the room, so exposure is prohibited.

We hope you found the above useful. There isn’t really much to HEPA filters when you break it down. But as simple as they are, they are highly effective air cleaning machines that require very little maintenance other than a replacement filter every now and then. You can pick a decent one up for less than $200, and once you’ve got one, you’ll never look back as you sit back, relax, and breathe in air cleaner than you ever have!