HEPA filters are primarily made of extremely fine borosilicate glass fibers, formed into a dense, non-woven mat that resembles paper. These glass microfibers, each typically less than a micrometer in diameter, are the core material responsible for trapping 99.97% of airborne particles down to 0.3 microns. But a complete HEPA filter includes more than just the filter media. Frames, separators, sealants, and gaskets all play a role, and each is made from different materials depending on the application.
The Glass Fiber Core
The filtering material itself is a mat of borosilicate glass fibers. Borosilicate glass is made primarily of silica (silicon dioxide) combined with boron oxide and smaller amounts of other minerals like aluminum oxide and sodium oxide. Two common types are used: sodium borosilicate (sometimes called C glass) and alumino borosilicate (E glass). Both produce fibers that are heat-resistant, chemically stable, and fine enough to create the dense web needed to capture microscopic particles.
This technology dates back to the 1950s, when the U.S. Naval Research Laboratory developed glass fiber filter paper as a replacement for asbestos-based filters. Asbestos is no longer permitted in HEPA filters. The glass fiber approach proved so effective that it remains the standard material for deep-pleated HEPA filters used in everything from hospital ventilation to nuclear facilities.
The fibers work not by acting like a simple sieve, but by creating a tangled maze where particles are captured through several physical mechanisms: they collide with fibers, get pulled toward fibers by air currents bending around them, or drift into fibers through random molecular motion. The extraordinary fineness of the glass fibers maximizes the surface area available for these interactions.
How the Filter Media Is Made
HEPA media is manufactured using a wet-laid process, essentially a modified papermaking technique. The glass fibers are first suspended in a diluted water solution. This slurry is then deposited onto a moving screen, sometimes called a forming fabric or wire, where the water drains away and vacuum suction pulls it through, leaving behind a thin, uniform sheet of tangled fibers.
The process involves several stages: fibers are dispersed in a pulper, blended to achieve the right mix of fiber diameters, and fed through a headbox onto the forming screen. Water is recirculated through the system so fibers remain in a highly diluted state during forming, which helps ensure an even distribution. The result is a flat, paper-like sheet that gets further processed with binders to hold the fibers together, then dried and cut to size.
Pleats, Separators, and Spacing
A flat sheet of filter media wouldn’t have enough surface area to handle real airflow without creating excessive resistance. That’s why HEPA filters are pleated, accordion-style, to pack a large amount of media into a compact space. A standard 24-by-24-inch HEPA filter panel can contain over 50 square feet of actual filter media once it’s folded.
To keep the pleats from collapsing against each other and blocking airflow, separators are placed between them. Traditional HEPA filters use thin corrugated aluminum foil strips as separators. These foil dividers create consistent air channels between the pleats, keeping resistance low and airflow smooth. Newer “mini-pleat” designs skip the aluminum and instead use small beads of hot-melt adhesive (a thermoplastic glue) applied in lines along the filter media to maintain pleat spacing. This approach saves weight and allows even more media to fit into the same frame size.
Frames, Sealants, and Hardware
The pleated filter pack needs a rigid frame to hold it in place and seal it into a duct or housing. Frame materials vary by application. Industrial and nuclear-grade HEPA filters typically use steel cases at least 1.8 mm thick, with stainless steel bolts, nuts, washers, and rivets (all 300-series stainless) holding everything together. Wood frames, including plywood meeting specific structural standards, are also acceptable in many settings. Particle board, however, is not permitted for framing because it lacks the necessary structural integrity.
Consumer HEPA filters in portable air purifiers often use lighter materials like molded plastic, cardboard, or aluminum frames, since they face far less mechanical stress than industrial units.
The sealant that bonds the filter pack to the frame is a critical component. It must be non-flammable or self-extinguishing, meaning it won’t continue burning once a flame source is removed. Some designs use a gelatinous seal that has no flash point below 450°F (232°C). Gaskets around the edges, typically made of closed-cell neoprene or similar rubber compounds, ensure an airtight fit when the filter is installed so that air can’t bypass the media.
PTFE as an Alternative to Glass
While glass fiber dominates the HEPA market, expanded polytetrafluoroethylene (ePTFE) membranes have emerged as a viable alternative. PTFE is the same material used in nonstick cookware coatings, and in its expanded form it becomes a thin membrane riddled with microscopic pores. These membranes can achieve the same filtration efficiency as glass fiber but with a lower initial pressure drop, meaning air passes through more easily for the same level of particle capture.
ePTFE membranes offer several advantages over glass. They’re highly resistant to chemicals, thermally stable, and strongly hydrophobic, so they repel moisture rather than absorbing it. The strong carbon-fluorine bonds in the polymer make it nearly inert, which is valuable in semiconductor manufacturing, pharmaceutical cleanrooms, and other environments where chemical exposure could degrade traditional glass media. The tradeoff is cost: ePTFE filters are significantly more expensive to produce than glass fiber versions, which is why glass remains the default for most applications.
Why Material Choice Matters
The materials in a HEPA filter directly affect its performance, lifespan, and safety profile. Glass fiber media handles high temperatures well and is inexpensive, but it’s relatively fragile. Rough handling can crack the media and create leaks that destroy the filter’s efficiency rating. PTFE membranes are more durable and easier to clean but cost more upfront. Frame materials determine whether a filter can survive high-humidity environments, elevated temperatures, or corrosive chemicals.
If you’re replacing a HEPA filter in a home air purifier or vacuum, the filter media is almost certainly glass fiber in a plastic or cardboard frame. For cleanroom, hospital, or industrial HVAC systems, the construction gets more robust: steel or aluminum frames, aluminum foil separators, fire-resistant sealants, and stainless steel fasteners. In both cases, the fundamental particle-trapping material is the same dense, paper-thin mat of microscopic fibers that has been the backbone of HEPA technology for over 70 years.