Stop focusing solely on ‘99 %efficiency’: to determine the true filtration efficiency of an air filter cartridge, first consider these three factors

The “efficiency” of an air filter cartridge is not a single number; it is a particle-size-dependent function locked to a set of test conditions. Without specifying the test standard, challenge particle size, and operating airflow, filtration efficiency is not comparable in engineering terms. A cartridge rated “99%” may fall below 60% when the particle-size basis changes; under a different airflow condition, the value may shift by several points. To determine the true filtration efficiency of an air filter cartridge, follow three steps: standard positioning, method matching, and operating-condition correction.

Step 1: Lock the standard system first, not the manufacturer’s nominal value

Different standards define “efficiency” over different domains, which directly determines what the data represent.

ISO 16890 is currently the international standard with the most engineering reference value for general ventilation filter cartridges. 

It no longer gives a single “total efficiency” but outputs three efficiency values by particle-size fraction: ePM1, ePM2.5, and ePM10, 

corresponding to the capture capability for 0.3–1 μm, 0.3–2.5 μm, and 0.3–10 μm particles, respectively. 

For example, a TNS cartridge certified to ISO ePM1 70% shows a clear particle-size dependence in actual capture rate: 73% for particles ≤1 μm, 79% for 2.5 μm particles, and 92% for 10 μm particles. 

The same cartridge, three particle-size bases, three efficiency values—this is direct evidence that “efficiency is not a single number.”

For sub-high-efficiency and higher grades, GB/T 6165-2021 and EN 1822 use the most penetrating particle size (MPPS, usually in the 0.1–0.3 μm range) as the efficiency criterion. 

Under this framework, the threshold for H13 is MPPS efficiency ≥99.95%, H14 requires ≥99.995%, and U17 reaches ≥99.9995%. If a cartridge claims “high efficiency” but cannot provide test data corresponding to MPPS, 

that claim does not hold in a professional context.

Step 2: The test method determines the physical meaning of efficiency

The core operation for determining efficiency is to simultaneously measure the particle concentration or mass upstream and downstream of the filter on a standard test rig, and then calculate the retention ratio. 

However, “measuring concentration” and “measuring mass” give fundamentally different meanings of efficiency.

The gravimetric method uses standard artificial dust and weighs the total mass of dust captured by the filter; it is suitable for coarse-efficiency grades. 

The gravimetric efficiency of G4-grade filter media for 5 μm dust is typically in the 80%–90% range. The gravimetric method reflects the mass retention capability for large particles and has almost no resolution for submicron particles.

The particle counting method uses a laser particle counter to measure the number concentration of particles upstream and downstream by particle-size bin, yielding particle-size-resolved efficiency. 

This is the core basis for selecting F-grade and H-grade cartridges: the counting efficiency of F5–F7 medium-efficiency cartridges for 1 μm particles is approximately 60%–95%; the counting efficiency of H10–H13 sub-high-efficiency cartridges for 0.3 μm particles covers 95%–99.9%. 

The counting method can reveal the U-shaped curve in which efficiency first decreases and then increases with particle size, whereas the gravimetric method completely flattens this curve.

An easily overlooked practical detail: counting-method efficiency is sensitive to test airflow. For the same cartridge, once the airflow deviates from the rated value, the position of the minimum point on the efficiency curve shifts; 

the greater the deviation, the larger the gap between the measured efficiency and the nominal value. Accepting certificate values without checking the test airflow conditions is the most common reason for misreading efficiency data.

Step 3: Read efficiency through a “grade–particle size–method” matrix, not as an isolated value

Integrating the above information, the determination of air filter cartridge efficiency should fall on a three-dimensional coordinate: filtration grade + target particle size + test method. The following matrix can serve as a quick interpretation tool:

Data source: filter media selection manuals and the EN 1822/GB/T 6165 classification systems.

In other words, the correct answer to “What is the efficiency of this cartridge?” is: “Under the ISO 16890 ePM1 basis, the counting efficiency for 0.3–1 μm particles is X at a test airflow of Y m³/h; under the EN 1822 MPPS basis, 

the efficiency for the most penetrating particle size is Z.” Without any one of these qualifying conditions, the efficiency value cannot serve as a basis for engineering comparison.

Figure suggestion: Insert an efficiency–particle size curve diagram, with particle size on the x-axis (log scale from 0.1 μm to 10 μm) and counting efficiency on the y-axis. 

Plot two cartridge curves—one F7 and one H13—to visually show “efficiency varies with particle size” and “the curve positions differ by grade.” 

The three integration intervals of ISO 16890 (ePM1/ePM2.5/ePM10) can also be marked on the figure so that the standard basis corresponds to the curves. 

This figure can convey the core argument in one screen: efficiency is a curve, not a point value.

The methodology for determining the filtration efficiency of an air filter cartridge ultimately comes down to one sentence: first ask “under what standard, for what particle size, by what method, and at what airflow,” 

and then read the percentage. Skipping the conditions and directly citing an efficiency number is equivalent to introducing an unquantified risk variable into engineering selection.