Air filters are devices that trap airborne particles. These are particles that can exacerbate allergy and asthma symptoms and even present more serious health hazards with extended exposure. By removing them, you improve indoor air quality and promote a healthier environment. HEPA filters are a particular type of air filter used in many air purification systems.

What Is a HEPA Filter?

HEPA is an acronym that stands for high-efficiency particulate air. In the U.S., the Department of Energy (DOE) established and maintains the standard for HEPA filters. HEPA filters are a style of pleated filter. These are filters made from cloth, paper or a similar material. The manufacturer folds that material into a series of pleats in order to increase surface area. The industry commonly uses pleated filters for central HVAC systems and for portable and fixed air purifiers. A HEPA filter will trap at least 99.97% of particles that pass through it. The DOE measures air filters based on minimum performance. You can think of this effectiveness as 99.97% or better.

The History of HEPA Filters

The U.S. Army Chemical Corps and National Defense Research Committee developed the original HEPA filter as a joint project. This project was part of the larger Manhattan Project in the 1940s. The mission of the Manhattan Project was the development of nuclear weapons. The lab in Oak Ridge, Tennessee needed an air filter that would protect the more than 90,000 workers there. From that need, the HEPA filter was born.

The industry didn’t coin the term high-efficiency particulate air until the 1950s. This is when the U.S. declassified the technology, and it became commercialized. HEPA was a generic trademark. Shortly after, HEPA filters were in use in many hospitals and in air purifiers for homes. By the 1960s, HEPA was the filtration standard for the microelectronics and nuclear power industries. More than 80 years after its invention, HEPA remains the standard for mechanical air filtration.

How Does an HVAC Filter Work?

The core material used for HEPA filters is a mat of fibers. The manufacturer weaves those fibers in a random arrangement. The pleated form factor expands the surface area, but it’s the web-like arrangement of fibers that catches the particles. It does this through three main mechanisms: interception, impaction and diffusion.

Interception is when particles in the air bump into a fiber and stick to it. This can occur on the surface of the HEPA filter but also within the web of fibers. Impaction is when a particle penetrates a fiber and becomes embedded. Particles that are 0.1 microns and smaller are not prone to interception and impaction if they have a direct path. This is where diffusion comes in. The web of fibers delays those molecules. The longer they’re delayed, the greater the chance that interception or impaction occurs.

The 0.3-Micron Benchmark

HEPA filters can trap at least 99.97% of airborne particles that are equal to 0.3 microns in size but also smaller and larger. A HEPA filter is actually more effective than 99.97% at trapping smaller particles, such as viruses, which average 0.1 microns. This may seem counterintuitive at first, but it originates with the Manhattan Project. Irving Langmuir was an American chemist, physicist and engineer who identified the 0.3-micron-diameter particle as the most difficult to capture. This became the basis for how we measure air filters and later for the minimum efficiency reporting value (MERV) scale.

MERV Rating

The MERV scale indicates the minimum performance of an air filter to trap particles between 0.3 and 10.0 microns in size. The scale ranges from 1 to 16. In the MERV 1-7 range, a filter can trap particles between 3.0 and 10.0 microns. Starting at MERV 9, air filters can trap particles as small as 1.0 microns. Starting a MERV 11, air filters trap particles as small as 0.3 microns. At MERV 16, an air filter provides 95% or better efficiency trapping particles 0.3 microns and larger.

As mentioned, HEPA filters provide 99.97% effectiveness trapping particles in the 0.3-micron range. This is beyond the MERV scale. Some brands will indicate this as MERV 17-20, but these aren’t official ratings. The DOE and American National Standards Institute (ANSI) don’t recognize those additions. Neither does the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), which developed and maintains the MERV scale.

What’s the Difference Between True HEPA and Other Labels?

It’s important to understand that there’s no official HEPA certification in the U.S. There are three main organizations that test HEPA filters, including the aforementioned ASHRAE. The others are the American Society for Testing and Materials (ASTM) and the Institute of Environmental Sciences and Technology (IEST). Many of the leading brands will indicate that their products meet ASHRAE, ASTM and IEST standards in addition to DOE standards.

You may encounter labels like True HEPA, Absolute HEPA and HEPA Type. These are all marketing labels. That said, the industry uses True HEPA to indicate filters that conform to DOE standards. When shopping filters from reputable brands, you can generally rely on it. Absolute HEPA is generally unreliable as a designation. Permanent HEPA means the product is washable, which most experts advise against. Labels like HEPA Type and HEPA Like are largely meaningless.

Applications for HEPA Filters

HEPA filters are widely used in both portable and whole-house air purification systems. The exception would be air purifiers that don’t use mechanical filtration, such as ionizers. HEPA is generally not recommend for HVAC. Most HVAC systems support up to MERV 13. Beyond that, the air resistance is too great and leads to inefficient operation and increased equipment wear. Many vacuums use HEPA filters or HEPA bags. This ensures that the vacuum traps the dust and other particles rather than recirculates it.

What Can’t a HEPA Filter Trap?

HEPA filters are highly effective at trapping particulate matter. That includes dust, pollen, pet dander, textile fibers and even smoke. Two concerning pollutants that a HEPA filter doesn’t trap well are volatile organic compounds (VOCs) and ozone (O3). VOCs are a carcinogen that off-gases from building products and exist in many cleaning and beauty products. O3 is a naturally occurring gas but one that reacts with VOCs to make it even more dangerous. It does this by making VOCs smaller and better able to penetrate our lungs. Both VOCs and O3 tend to bounce off HEPA fibers rather than stick to them. Many air purifiers will include an activated charcoal stage to absorb VOCs and other gases and airborne particles.

Would You Like to Upgrade Your Home With HEPA Filtration?

Level 9 Heating & Cooling installs HEPA air purifiers throughout Des Peres, St. Louis, Washington and the surrounding areas. Our team also specializes in hydroxyl-based air purification and whole-house dehumidifiers and humidifiers. Our NATE-certified HVAC technicians install and service all manner of heating and cooling technologies, including geothermal heat pumps.

We’re available 24/7 for emergency repairs and offer a maintenance plan that can save you money and time. We have plumbers that specialize in camera inspections, piping and repiping water, sewer and gas lines, drain cleaning and leak detection. Our plumbers also install tank and tankless water heaters, garbage disposals, sump pumps, boilers and water filtration and softening systems.

Call today or contact us online with questions or to schedule an appointment.