This post is our sincere effort to answer a common question people have – “What is HEPA filter?”. Prior knowledge about a product that you are interested in buying is crucial. Even your search for the Best air purifier would be directionless otherwise. What you end up buying may be some low quality air purifier without the “real deal”. I am referring to the True HEPA filters which are the most commonly used quality filters. Its always better to invest sometime before simply sitting down to browse the product online. The quality of the filters that an air purifier holds inside is utmost important. So, you must invest time in this post where I’d tell you how HEPA filters work and various grades of HEPA filters.
Not just the HEPA filter meaning, I’d also discuss other improvements that have been brought about in recent years. Take for example the ULPA filters.
HEPA STANDS FOR?
Firstly you should know that HEPA is an abbreviation. Do you know what is the full form of HEPA? The funny thing is that there are many! The most technically correct full form of HEPA is “High Efficiency Particulate Arrestance“. It simply means a filter which is highly efficient in arresting or capturing particulate matter. Many a times you’d see “High Efficiency Particulate Air” on some websites which is totally nonsensical and has no logic. But, the US Department of Energy likes it so. Actually try adding the word ‘filter’ infront of that full form and it starts to make sense. HEPA stands for High Efficiency Particulate Air <Filter>.
Particulate matter can be anything which is suspended in the air and poses a risk to our health upon breathing. Usually knowing about PM2.5 and PM10, which you hear about most often in News or otherwise is sufficient. PM in PM2.5 is “Particulate Matter”. PM2.5 simply means such dust particles or pollutants which are smaller than 2.5 microns in diameter. Microns means micrometers or 1 millionth part of a meter.
WHAT IS TRUE HEPA FILTER?
Now that you atleast know its full form, time now to know what is a True HEPA filter. See, there are many qualities of an air purifier filter. These can be the crappy ones like “HEPA-type”, “HEPA-like” (other such similar misleading terms) and then there’s True HEPA. A True HEPA filter can filter out particles of sizes upto 0.3 microns with atleast 99.97% efficiency. Have I have talked about what is the efficiency of HEPA filter? Not yet! I’ll discuss it ahead about what percentage of particles does a HEPA filter remove from the air.
HEPA FILTER MATERIAL
If you happen to visualize a HEPA filter, it would look like a synthetic material or even paper to you. On a closer look maybe under a microscope, the internal structure of a HEPA filter can become visible. There would just be a haphazard mesh of fibers. The filter is pleated and this may be due to a technical requirement or to increase the surface area. HEPA filters are not something newly introduced to the World. Infact, the first HEPA filters were made way back during the World wars. There were many accidental discoveries during that time in the quest to make new weapons, chemicals or materials for war.
The synthetic material made HEPA filters are the most common ones. The others have fiberglass as their building blocks. I would not ask you to go deeper into the HEPA filter material. Its not easy to find out anywhere in the user manual, brochures or the product descriptions. You just need to ascertain the HEPA filter grade. Besides, another thing to look for is whether there is any air leakages near the edges or the frame of the filter.
COATINGS ON HEPA FILTERS
Some HEPA filters come with coatings of special compounds or extracts to enhance their utility. Catechin filters are good examples commonly found in ACs. Catechin is extracted from green tea and inhibits the virulence of viruses. It surrounds the virus and makes it incapable to attach to any host cell. We’ve seen catechin layer HEPA filters, filters with Gingko and Sumac coating and many others. These coatings are usually meant to destroy virus and bacteria from the filter surface and prevent their growth. The Airmega filter is the best HEPA filter I’ve seen in quality. It has a green anti-flu layer of extracts from Japanese plants.
HEPA FILTER EFFICIENCY
Do not just think an air purifier has excellent filters if you see the word “HEPA” in the product description. To ensure that your air purifier has a True HEPA filter, there are other things to scrutinize. There are different GRADES of HEPA filters too.
For home air purifiers, E12 and H13 grades are most prevalent. Some air purifiers do use H14 grade True HEPA filters too, but rarely so. E12 or H12 filter is not a HEPA but an EPA filter. Scientific and medical applications may even use U15 filters. What do these grades of HEPA filters like E12, H13, H14 and U15 denote? True HEPA filters can a range of efficiencies in trapping dust particles ranging from 85% to 99.9999%.
GRADES OF HEPA FILTERS
The table below would clear up this subject better. H13 grade HEPA filters are the ones you’d find without breaking a sweat. H14 grade HEPA filters are the ultimate good ones for home and dental clinics and hospitals. The grades like U15, U16 denote ULPA filters with very high efficiencies.
HEPA FILTER H13
All good brands offer HEPA filters of H13 grade in their air purifiers. Once you are sure of this, you can rest assured that as far as the filter quality is concerned, there’s nothing to worry. A HEPA filter H13 grade can remove particulate matter with a 99.97% efficiency. This % figure is what you see all over the place on online portals. Once the HEPA filter quality is sorted out, look at the other stages of filtration. The carbon filter should be checked for its quality and its good to have actual carbon pellets rather than just a layer of carbon over the fibers.
HEPA FILTER H14
If your air purifier has HEPA filters of H14 grade, you should feel lucky to find it. If other things are suitable as well, you should grab it with both hands. Having H14 grade filters in a home air purifier is the best it can get. Although, percentage wise there’s a minute difference between the efficiencies of H13 vs H14 filters. But, this is how it is and better is called better no matter by how much margin. I’ve only seen Medify and Sharp air purifiers using H14 HEPA filters.
HEPA FILTER PORE SIZE
One would think that to be able to stop such small particles, the HEPA filter pore size would be still smaller. Well this is not true. HEPA filter is not a sieve per se so there’s no fixed pore size as such. As I mentioned above, its all haphazardly woven fibers that constitute the filter. Particles ranging a few microns in size are pulled in by the air purifier fan towards the filter. These particles get stuck to the fibers no matter they are larger or smaller. I’ll now explain a couple of phenomena like straining, interception and diffusion.
- INTERCEPTION – Particles which are nearly the size of the HEPA pore size stop by interception. Herein, particles of dust or bacteria sized between 0.3 micron and 1 micron are arrested by the filter. Just like police intercepts the criminals, similarly the particles find the fibers which trap them after collision.
- STRAINING – Larger particles over 1 micron size like PM2.5 and PM10 particles are easily caught. Such particles stand no chance to get through the gap between any two fibers. Hence, the outermost layer of the HEPA filter would have these particles. The finer particles would be lodged somewhere in the inner layers of the filter.
MOST PENETRATING PARTICLES
HEPA filters are not very good at stopping sub-0.3 micron particles. The reason is obvious. Such small particles are not easy to capture by the fibers. You know gaseous molecules move by diffusion and their size is the size of their molecules, way smaller than 0.3 microns infact in angstroms (Å). What is diffusion, I’ll explains ahead. For now, just note that particles smaller than 0.3 microns are the most penetrating particles. These travel deep inside the filters before being captured if at all. Else they may end up in our lungs and from their to our blood steam via the capillaries.
- DIFFUSION – For Ultra-small particles well below 0.3 micron (< 0.3 µm size), their movement in air does not take place in linear direction. Infact, they are so small to take a straight path in their motion. These ultra-small particles move in (roughly speaking) Brownian motion or simply you can say haphazard way. The gaps in the fibers may not necessarily be so small as to capture such particles. But, even if such particles escape from one gap, more gaps and fibers are waiting for them. Eventually, they meet and collide with a fiber and stick then and there. These are the most dangerous of pollutants which can damage the brain through our lungs via the blood.