How Pore Size Is Designed in Pre-, Medium-, and High-Efficiency Filters

Pre-Filters: The Logic Behind a Coarse Structure—Capture the Large Particles and Protect the System

Pre-filters typically have pore sizes in the range of 5–100 μm, the largest of the three filter categories. Although this design may appear coarse, it serves a clear functional purpose.

Matching the pollutant: capturing large particles

Large airborne particles such as dust, hair, lint, pollen, and insect debris, generally 5 μm or larger, may not pose the same direct respiratory risk as fine particles, but they are numerous and occupy more space. If they enter medium- or high-efficiency filters directly, they quickly block the finer structure. It is much like removing stones before using a fine sieve for sand. A pre-filter's relatively large openings are designed specifically for these large particles, removing them mainly through sieving when the particles are larger than the available paths and reducing the load on downstream filters.

Matching material to pore size: an open structure maintains airflow

Pre-filters are commonly made of nonwoven fabric, nylon mesh, or metal mesh. These materials naturally form relatively open gaps of 5–100 μm and provide good permeability with low airflow resistance, typically no more than 50 Pa. If the openings were reduced to around 1 μm, more small particles could be captured, but resistance would rise sharply and equipment airflow would fall, reducing the efficiency of the system as a whole.

Medium-Efficiency Filters: A Precisely Positioned Transition Stage That Balances Efficiency

Medium-efficiency filters commonly have pore sizes of 1–5 μm. Positioned between pre-filters and high-efficiency filters, they serve as the central transition stage in the system. Their main purpose is to capture what the first stage misses.

Matching the pollutant: targeting medium-sized particles that pass through the pre-filter

Pre-filters are less efficient against particles below 5 μm, including fine fractions of PM10, mold spores, and some particles in cigarette smoke. If these particles enter the high-efficiency filter directly, they gradually block its ultrafine structure. The 1–5 μm range used in medium-efficiency filters covers this gap. Inertial impaction and interception allow particles to collide with fibers or contact them as the airflow passes, leaving the high-efficiency stage free to focus on smaller pollutants such as PM2.5 and virus-containing aerosols.

Coordinating pore size and material: high capture with acceptable airflow

Medium-efficiency filters are generally made from glass fiber or high-density nonwoven material. Their fibers are finer than those in pre-filters, and their pore distribution is more uniform. A medium-efficiency filter with a nominal 3 μm structure, for example, can capture more than 90% of particles in the 3–5 μm range while maintaining moderate resistance, typically 50–150 Pa. If the openings are too large, above 5 μm, the filter cannot perform its transition role. If they are too small, below 1 μm, resistance approaches that of a high-efficiency filter and the benefit of a separate intermediate stage is lost.

High-Efficiency Filters: Nanoscale Precision for the Smallest Threats

High-efficiency filters, including HEPA filters, commonly have effective pore structures in the range of 0.1–1 μm, while some ultrafine high-efficiency media reach an even finer scale. These are the most precise of the three filter categories, designed to target the smallest and most hazardous airborne pollutants.

Matching the pollutant: capturing invisible hazards

Fine airborne particles such as PM2.5, bacteria, and virus-containing aerosols in the approximate range of 0.1–2.5 μm pose a serious health risk because they can penetrate deeply into the respiratory tract. High-efficiency filters use very fine fibers and several capture mechanisms to remove them.

For particles around 0.3 μm, the reference size commonly used in efficiency testing, high-efficiency filters rely on Brownian diffusion, in which small particles move randomly and encounter fibers, as well as electrostatic attraction when electret media are used. Filtration efficiency can reach 99.97% or higher.

If a high-efficiency filter had a much more open structure, fine particles could pass through too easily and the filter would no longer provide high-efficiency purification.

Balancing Fine Structure and Airflow

A high-efficiency filter must be very fine without becoming impermeable. Excessive resistance, potentially above 300 Pa, can sharply increase energy consumption and reduce airflow. The design solution combines a fine fiber structure with a large pleated area. A high pleat density—for example, more than 100 pleats per meter—expands the available filtration area and reduces airflow velocity per unit area, helping balance filtration efficiency and resistance.

The System-Level Logic of Pore Design: Why One Filter Cannot Do Everything

It may seem simpler to use a high-efficiency filter alone, but staged design provides a better balance of performance and cost.

If a pre-filter used the fine structure of a high-efficiency filter, resistance would be too high for the equipment to maintain normal airflow. Large particles would also clog it rapidly, potentially requiring replacement within a week and creating very high operating costs.

If the high-efficiency stage used the open structure of a pre-filter, it would fail to capture fine particles and would no longer provide high-efficiency protection.

The pre-filter's coarse structure reduces the initial load, the medium-efficiency filter captures the next range of particles, and the high-efficiency filter performs the final fine filtration. Together, their graded structure forms a funnel-like filtration system that protects final air quality and extends the service life of the entire assembly. With effective upstream protection, high-efficiency filter replacement intervals can increase from about one month to six or even twelve months.

Conclusion: Pore-Size Design Is About Precise Matching

Pore-size design in pre-, medium-, and high-efficiency filters is not simply a progression from large to small. It is calculated around pollutant hazard levels and size distributions, equipment performance such as airflow resistance and energy consumption, and operating cost. This explains why a household air-conditioner filter looks relatively open—it is a pre-filter designed mainly to protect the equipment—while the inner filter of an air purifier feels much denser because it is a high-efficiency stage designed to protect health. Every filtration layer reflects a specific role in the overall air-purification strategy.