We have all heard the term HEPA filter before. Most likely when buying a vacuum cleaner or air purifier. But what is a HEPA filter exactly? And why do there seem to be huge price discrepancies between some of them? For instance, you can look on Amazon and find a HEPA filter for $29.99. And yet there are others you can buy for $459.99. So what gives? What is the right HEPA filter for HVAC applications?
We have had a huge influx of calls from clients inquiring about different types of HEPA filters and whether or not they could get some to put in their offices, businesses, hospitals, etc. So we decided to outline what the differences are in HEPA filters and which ones are right for your application...and which ones are not.
But first, let’s identify what exactly a HEPA filter is. HEPA is an acronym for High-Efficiency Particulate Air. The Institute of Environmental Sciences Technology (IEST) defines a HEPA filter as:
In other words, a HEPA filter is one that can capture at least 99.97% of particles that are in the 0.3-micron range. Now that is pretty small. To put it into perspective the average cross-section of human hair is anywhere from 50 - 70 microns. And the human eye cannot see anything smaller than 40 microns in size.
So why is the 0.3-micron range the magic number?
So as you now know, for a filter to be considered a HEPA filter, it must capture a minimum of 99.97% of particulates that are in the 0.3-micron range. But why that specific micron range? Well, it’s because this size range is the most penetrating particle size (MPPS) that we know of and according to scientists, “can evade filters more than particles that are larger, or even smaller in size.” So HEPA standards are based on how efficient they can capture particles in this 0.3-micron range.
So what is a HEPA filter media made out of?
HEPA filter media is a paper composed of micro glass fibers, synthetic fibers, expanded film such as polytetrafluoroethylene (PTFE), or other types of fibers that can be pleated back and forth to form a compact filter element. Close pleating is necessary to fit all the required media into the desired space because the paper has a high resistance to airflow and the media velocity is usually in the range of 200 to 500 CFM.
So because there is so much media compacted in a limited space, the initial pressure drop of a clean HEPA filter can range between a 0.5ΔP to 1.1ΔP. With the final ΔP being as high as a 2.3ΔP. So HEPA filters are not designed with energy efficiency in mind, rather, particle penetration protection.
There are 4 different ways particles can travel through a HEPA filter.
When particles are traveling through the filter, there are several ways the particle is trapped and taken out of circulation.
This is when large, high-density particles are captured. As air flows through the filter, it must bend or change direction many times to flow around the filter fibers. Because of their inertia, large particles resist change in direction and attempt to continue in their original directions. Because of this, they collide with and adhere to the filter fiber.
This happens when the particle follows the airstream but still comes in contact with the filter fiber as it passes around it. If the forces of attraction between the fiber and the particle are greater than the force of the airflow to dislodge it, the particle will stick to the filter fiber.
This is where small particles at low air velocity are being captured. As the contaminated air passes through the filter media, minute particles will tend to move from areas of higher concentration and will take an erratic path described as Brownian Motion. This erratic path increases the probability that particles will come in contact with fibers and will stay attached to them. Diffusion works best with fine filter fibers and very low air velocities.
This occurs when the smallest dimension of a particle is greater than the distance between adjoining filter media fibers.
A HEPA filter is designated as such due to its ability to capture particulates in the 0.3-micron range (99.97%). The phrase “True HEPA” was originally coined to separate the difference in particle capture efficiency. European standards define a HEPA filter as a filter that can capture 85% of particles in the 0.3-micron range whereas the United States requires 99.97% to be HEPA certified.
The American standard is more commonly dubbed as the “True HEPA” due to its higher particulate capture percentage in the 0.3-micron range. That being said, “True HEPA,” “Ultra HEPA,” and other HEPA name variations are not recognized by the accrediting bodies in the U.S or Europe.
So we have to keep in mind, for a filter to be considered a (true) HEPA filter, it must be rated at a 99.97% rating for capturing particles in the 0.3-micron range.
So what about HEPA’s that capture less than 99.97%?
There are websites that have HEPA filters for sale right now for $29.99. They claim it can catch 99.9% of particles. This is good, right? Not necessarily. While this $29.99 HEPA filter can capture 99.9% of particles you have to ask yourself of what size particles is it 99.9% efficient in?
The 99.9% efficiency this filter is referring to is large particle capture, but not so much at capturing ultra-fine particles such as bacteria and viruses in the 0.3-micron range. And as we’ve learned, particles in the 0.3-micron range are the most troublesome to catch and take out of circulation.
So when searching for HEPA filters make sure its rating is at least 99.97% for the 0.3-micron range.
Which HEPA is the best for me?
So, as you know by now, HEPA filters are classified by their minimum collection efficiency. And to be considered a HEPA it must capture 99.97% of particles in the 0.3-micron range. But some particles are much smaller than that. And there are HEPA’s for that too.
Commercially available HEPA filters can control particulates in the 0.01-micron range at efficiencies up to 99.99+% and in the 0.1-micron range at 99.9999+% (considered to be a ULPA or Ultra Low Penetration Air filter). These efficiencies apply to the 0.3-micron range as well. And it may not seem like much of a difference between 99.97% and 99.99%, or a 99.9999% HEPA but the resistance to airflow between these seemingly small numbers is compelling. So making sure you have the right HEPA for the right application is very important.
Which area is best for HEPA filters?
HEPA filters are best applied in situations where high collection efficiency of submicron particles (PM) is required. HEPA filters are typically utilized for applications involving chemical, biological, and radioactive particles.
HEPA filters are installed as the final component (or final filter) in the air system. Filters before the HEPA are designed to capture larger particles and have a higher change-out frequency.
Common applications for HEPA filters are for the following areas-
Hospital Operating Rooms
The filters can be utilized in any of the above applications where particles are generated and can be collected and ducted to a central location.
Keep in mind that HEPA filters are designed to capture ultrafine particles and not designed for airflow efficiency.
So it would not be ideal to put a 3” HEPA in place of your 4” pleat in your AHU because you want to capture smaller ultrafine particles. You must have the right application to warrant a HEPA filter. Doing so will overwork your air system drastically reducing the airflow because it’s trying to maintain its airflow through a dense filter.
In summary, the use of a “true” HEPA filter is limited to applications where ultrafine filtration is needed and where the air handling systems have both the filter holding capacity and the face capacity to push air through your HEPA filter.