Air Filter Media: Big Purification Power in a Small Package

Core Materials Used in Air-Filter Media

Filtration efficiency is closely related to the media material. Common materials include the following.

Synthetic Fibers, Including Polyester and Polypropylene

These media are made by hot-melt bonding or needle punching. Interlaced fibers create a porous structure with low cost and high dust-holding capacity, making them suitable for primary- and medium-efficiency filtration. Polyester-fiber media, for example, resists moisture and can be used in humid environments.

Glass Fiber

The smaller fiber diameter creates a denser filtration network. Glass fiber is commonly used in medium- and high-efficiency filter media and can capture smaller particles in the 1-5 micrometer range. However, it has relatively poor flex resistance and must not be excessively bent during installation.

Activated-Carbon Fiber or Activated-Carbon-Particle Composites

Adding activated carbon to fiber media provides functions beyond particle filtration. It also adsorbs odors and volatile organic compounds from the air. This type of media is often used in cleanrooms that require odor removal, including pharmaceutical workshops and food plants.

Classification by Filtration Efficiency

Under international standards such as ISO 16890 and ASHRAE 52.2, filter media can be divided into efficiency groups for different purification requirements.

Category | Primary-efficiency media | Medium-efficiency media | High-efficiency, or sub-HEPA, media

Materials | Polyester fiber, nylon mesh, or nonwoven | Dense synthetic fiber or glass fiber | Ultrafine glass fiber or polypropylene composite fiber

Particle size filtered | At least 5 micrometers, including large dust, hair, and insects | At least 1 micrometer, including pollen, smoke, and bacterial cells | At least 0.5 micrometer, including virus-bearing droplets and submicron particles

Efficiency grade | G1-G4, with G3 and G4 as common primary grades | F5-F9, with F7 and F8 as common medium grades | At least 80% efficiency for ePM10 or ePM2.5 in sub-HEPA applications

Applications | Front end of fresh-air systems and primary stage of air-handling units | Intermediate stage of central clean-air systems and pre-filtration for fan filter units | Pre-filtration for clean benches and laminar-flow hoods

Functional characteristics | High dust capacity and low resistance protect downstream high-efficiency equipment | Balances filtration efficiency and energy use while intercepting medium-sized particles | Approaches HEPA performance and extends HEPA filter life

How Air-Filter Media Purifies Air

Although filter media looks simple, its purification process combines several physical and physicochemical mechanisms.

Inertial Impaction

Larger airborne particles, including dust and pollen, cannot follow a curved airflow path because of their inertia. They strike the filter fibers directly and are retained. Primary-efficiency media relies mainly on this mechanism.

Diffusion and Interception

Fine particles, including virus-bearing particles and bacteria, move randomly through Brownian motion and are captured when they contact fibers. This effect is more significant in medium- and high-efficiency media because of its denser fiber network.

Electrostatic Attraction

Some media receives an electrostatic treatment such as electret processing. The charged fibers attract oppositely charged particles and can even capture nanoscale particles smaller than 0.1 micrometer.

Gravitational Settling

Particles settle onto fiber surfaces under the force of gravity. This mechanism is especially effective for large particles.

Critical Cleanroom Applications

Filter media does not operate alone. It works together with other cleanroom equipment, providing the following core functions.

Protecting HEPA Filters and Extending System Life

Primary- and medium-efficiency media first captures 80%-90% of large particles. This reduces the load on downstream HEPA or ULPA filters and can extend their replacement interval from three months to one or two years, lowering operating and maintenance costs.

Precisely Controlling the Cleanliness Class

In ISO Class 5, or Class 100, and cleaner rooms, sub-HEPA media used as a layer upstream of the HEPA filter can help keep the concentration of particles measuring at least 0.5 micrometer below 3,520 particles per cubic meter. Together with fan filter units, it supports continuous airflow purification.

Meeting Special Environmental Requirements

Heat-resistant filter media: Glass-fiber or ceramic-fiber media is used in high-temperature applications such as oven exhaust systems and can withstand temperatures above 200 degrees C.

Water- and oil-resistant filter media: A surface coating enables use in humid or oily workshops, such as frying areas in food-processing plants. It prevents the media from developing mold when damp or becoming blocked by oil.

Maintenance and Selection

Determining the Replacement Interval

Use a differential-pressure gauge to monitor the pressure difference across the media. Replace it when resistance reaches twice the initial value. For example, primary media with an initial resistance of 50 Pa should be replaced once resistance exceeds 100 Pa. Replacement is also required when the media is visibly black or heavily loaded with dust. Timely replacement prevents excessive resistance from reducing airflow and avoids microbial growth in accumulated dust.

Selection Principles

Choose filter-media efficiency according to the cleanroom class. An ISO Class 8, or Class 100,000, workshop can use G4 primary plus F7 medium-efficiency filtration. An ISO Class 5, or Class 100, cleanroom requires a combination of G4 primary, F8 medium-efficiency, and sub-HEPA media to minimize the load on the final HEPA filter.

From the primary filter pad at a fresh-air intake to the sub-HEPA layer ahead of a fan filter unit, air-filter media quietly protects the sterile environment of a cleanroom through its low cost and strong value. It serves as the first line of defense in an air-purification system. Although it does not directly determine the highest cleanliness class, its successive capture stages reduce the burden on high-efficiency equipment and allow the entire cleanroom system to operate reliably. This is how a small component delivers major purification performance.