When selecting air filters, you will often see terms such as G4, F7, F9, H13, H14, EN 779, EN 1822, and MPPS on product specifications. These labels are all related to filtration performance, but they differ in terms of the types of filters they apply to, testing methods, and evaluation criteria.

If you only compare the “efficiency percentage” without confirming the testing standard, test particle size, airflow rate, and pressure drop, you may end up selecting a product that does not meet the requirements of the actual equipment or cleanroom environment.

This article explains the relationship between EN 779, EN 1822, and MPPS from a practical customer-oriented filter selection perspective. It also provides an overview of HEPA filter testing procedures, how to interpret test reports, and key considerations when purchasing air filters.

Quick Overview in One Minute

• EN 779: Previously used for coarse and medium-efficiency filters in general ventilation and HVAC applications. Common legacy classifications included G4, M5, M6, F7, F8, and F9.
• EN 1822: Used for high-efficiency EPA, HEPA, and ULPA air filters. Common classifications include E10–E12, H13–H14, and U15–U17.
• MPPS: Not a filter class or an independent standard. It stands for Most Penetrating Particle Size and is an important concept for evaluating high-efficiency filter performance under EN 1822.
• ISO 16890: Has replaced EN 779 and is now the primary classification standard for general ventilation air filters.

In simple terms:

General HVAC filters are classified according to the legacy EN 779 system or the current ISO 16890 standard; HEPA and ULPA filters are evaluated according to EN 1822; and MPPS is an important concept for identifying the particle size at which a high-efficiency filter is most vulnerable to penetration.

1. What Is EN 779?

EN 779 was a European test standard for particulate air filters used in general ventilation. It was mainly applied to:

• Air Handling Units (AHUs)
• Make-Up Air Units (MAUs)
• Industrial ventilation systems
• HVAC systems in commercial and public buildings
• Pre-filtration in cleanroom systems
• Pre-filtration for process equipment

Common Legacy Classifications Under EN 779

EN 779 evaluated dust-holding capacity, particle-size efficiency, and dust-loading performance. However, it was intended for general ventilation filters and cannot be used to classify HEPA filters such as H13 or H14.

EN 779 Has Been Replaced by ISO 16890

EN 779:2012 was withdrawn in 2018, and general ventilation filters have progressively transitioned to the ISO 16890 classification system. BSI EN 779 Standard Information

ISO 16890 classifies filter performance according to particulate matter size fractions:

G4, F7, and F9 designations are still widely used in the market because legacy specifications, equipment drawings, and customer purchasing practices continue to use these classifications.

It is important to note that EN 779 and ISO 16890 use different testing and classification methods. They should not be directly converted using a simple equivalency table. When converting between legacy and current specifications, actual test reports and equipment requirements should be considered.

2. What Is EN 1822?

EN 1822 is a European standard for high-efficiency EPA, HEPA, and ULPA air filters. It covers product classification, filter media testing, test aerosols, particle counting, overall filter efficiency, and local leak testing.

Typical applications include:

• Hospital operating rooms and specialized patient rooms
• Pharmaceutical and biotechnology manufacturing facilities
• Aseptic workstations and biological safety cabinets
• Laboratories and clean work areas
• Electronics and semiconductor cleanrooms
• Fan Filter Units (FFUs)
• High-cleanliness manufacturing processes and equipment

The current EN 1822-1:2019 evaluates high-efficiency filters using particle-counting methods and classifies them based on overall and local efficiency. BSI EN 1822-1 Standard Information

EN 1822 High-Efficiency Filter Classifications

Is There Really Only a 0.045% Difference Between H13 and H14?

Based on the efficiency figures alone, H13 has an efficiency of 99.95%, while H14 has an efficiency of 99.995%—a difference of only 0.045 percentage points.

However, the difference becomes much more significant when comparing penetration:

• H13 maximum overall penetration: 0.05%
• H14 maximum overall penetration: 0.005%

In other words, the allowable particle penetration of H14 is only one-tenth that of H13.

Therefore, when evaluating HEPA filter performance, customers should consider not only filtration efficiency but also penetration rate and local leakage requirements.

3. What Is MPPS?

MPPS stands for Most Penetrating Particle Size. It refers to the particle size that is most difficult for a filter to capture under specific filter media, face velocity, and test conditions.

MPPS is not a standard or filter classification. Instead, it represents the particle size range at which particles are most likely to penetrate the filter media.

In other words, MPPS can be viewed as the filter’s weakest point in terms of particle filtration under the specified test conditions.

Why Is There a Most Penetrating Particle Size?

Air filters do not simply capture particles based on the size of the openings in the filter media. Multiple filtration mechanisms work together:

1. Inertial impaction: Larger particles have greater inertia and cannot completely follow changes in airflow direction, causing them to collide with filter fibers.
2. Interception: Particles following the airflow come sufficiently close to a fiber and are intercepted by it.
3. Diffusion: Very small particles are affected by Brownian motion, resulting in irregular movement and an increased probability of colliding with fibers.
4. Electrostatic attraction: Some filter media use electrostatic charges to attract particles, although performance may be affected by time, temperature, humidity, and contaminants.

Larger particles are more readily captured through inertial impaction and interception, while very small particles are more effectively captured through diffusion. Between these two ranges, there can be a particle-size range that is more likely to pass through the filter media. This is the MPPS.

Is MPPS Equal to 0.3 μm?

Not necessarily.

MPPS can be affected by:

• Filter media type
• Fiber diameter and structure
• Media thickness
• Face velocity
• Electrostatic characteristics
• Temperature and relative humidity
• Test aerosol and measuring equipment

MPPS is often found around 0.1–0.3 μm, although it may be below 0.1 μm for some filter media. Therefore, the MPPS of every HEPA filter should not automatically be interpreted as 0.3 μm.

ISO 29463-5 also uses MPPS as an important basis for evaluating the efficiency of high-efficiency finished filters and provides testing guidance for filters with an MPPS below 0.1 μm and charged synthetic-fiber filter media. ISO 29463-5:2022

4. How Do HEPA Filters Pass EN 1822 Testing?

EN 1822 does not simply measure a single filtration-efficiency figure. Instead, it establishes a comprehensive testing framework covering filter media, MPPS, finished-filter efficiency, and local leakage.

Structure of the EN 1822 Series

The overall testing process can be divided into three main stages.

5. Stage 1: Filter Media Efficiency and MPPS Testing

The first step is to test the flat-sheet filter media before it is assembled into a finished filter.

The test equipment generates an artificial aerosol with a controlled concentration and particle-size distribution. The aerosol is passed through the filter media, while particle-counting equipment measures the particle concentrations upstream and downstream.

The formulas are:

Penetration = Downstream Particle Concentration ÷ Upstream Particle Concentration × 100%

Filtration Efficiency = 100% − Penetration

Penetration is measured across different particle sizes to identify the particle size with the highest penetration and therefore the lowest filtration efficiency. This particle size is the MPPS of the filter media.

Finished-filter efficiency and leakage testing are subsequently evaluated using MPPS or a particle-size range around the MPPS as an important reference.

6. Stage 2: Local Leak Scanning of Finished Filters

After the filter media has been assembled into a HEPA or ULPA filter, the finished product must be checked for local defects that may have occurred during manufacturing and assembly.

During testing, a uniform and stable test aerosol is introduced upstream of the filter. A scanning probe then scans the downstream surface of the filter along the specified path.

Typical scanning areas include:

• The entire downstream surface of the filter media
• Filter pleats and pleat peaks
• Filter-media joints
• The interface between the filter media and frame
• Areas around the sealant
• Frame corners and other structural weak points

Common causes of local leakage include:

The value of local leak scanning lies in identifying weak points that may not be reflected by the average overall efficiency. ISO 29463-4 also specifies the scanning method as an important test method for leakage testing of finished high-efficiency filters. ISO 29463-4

7. Stage 3: Overall Efficiency Testing of Finished Filters

Local scanning is used to identify the weakest points, while overall efficiency testing evaluates the average filtration performance of the entire finished filter.

A typical test procedure includes:

1. Install the finished filter in the test system.
2. Set the specified test airflow or rated airflow.
3. Introduce a stable test aerosol upstream.
4. Confirm a uniform distribution of the upstream aerosol.
5. Measure upstream and downstream particle concentrations.
6. Calculate overall penetration and filtration efficiency.
7. Determine the filter classification according to the EN 1822 criteria.

For example, if 1,000,000 particles are measured upstream and 50 particles are measured downstream:

Penetration = 50 ÷ 1,000,000 × 100% = 0.005%

Filtration Efficiency = 100% − 0.005% = 99.995%

This result meets the overall efficiency threshold for H14. However, whether the filter ultimately qualifies as H14 must also be determined by confirming local efficiency and other required test conditions.

8. What Test Aerosols Are Used in EN 1822?

High-efficiency filter testing requires artificial aerosols with controlled particle-size distributions and concentrations. Common test media include:

• DEHS
• PAO-type test oils
• PSL solid particles
• Other liquid or solid aerosols that comply with the applicable standard

The purpose of the test aerosol is not to simulate ordinary dust, but to generate stable, measurable challenge particles covering the MPPS range.

It is important to note that using PAO or DOP does not automatically mean that an EN 1822 test has been completed.

When determining whether a test meets the required standard, customers should also verify:

• The applicable test standard and version
• Type of test aerosol
• Test particle size or MPPS range
• Test airflow and face velocity
• Whether a particle counter or photometer was used
• Whether overall efficiency or local leakage was measured
• Instrument calibration status
• Acceptance criteria

9. What Is the Difference Between Factory Testing and On-Site PAO Testing?

These two types of testing have different purposes and test subjects and cannot replace one another.

Passing factory testing means that the filter itself meets the requirements under the specified test conditions. However, transportation, handling, and on-site installation may still cause frame deformation, filter-media damage, gasket displacement, or uneven clamping.

Therefore, in critical environments such as healthcare, pharmaceutical, biotechnology, and cleanroom facilities, leakage testing, airflow, differential pressure, and cleanliness should still be verified according to applicable site requirements after HEPA filter installation or replacement.

10. How Should Customers Interpret HEPA Filter Test Reports?

A useful test report or factory test document should ideally include at least the following information:

• Filter model, dimensions, and serial number
• Declared classification, such as H13 or H14
• Applicable test standard and standard version
• Rated airflow and actual test airflow
• Test face velocity
• Initial pressure drop
• Test aerosol type
• MPPS or test particle-size range
• Upstream and downstream particle concentrations
• Overall efficiency and penetration
• Local efficiency or maximum local penetration
• Leakage scan results
• Test date and instrument information
• Pass/fail determination

Simply stating “H13, 99.95%” is not sufficient information.

A clearer specification would state:

H13 high-efficiency filter, tested in accordance with the specified EN 1822 requirements, with an overall efficiency of ≥99.95% at MPPS, together with the rated airflow, initial pressure drop, and test results.

This provides a more reliable basis for comparing products from different suppliers, filter media, and manufacturers.

11. Comparison of EN 779, EN 1822, and MPPS

12. Practical Checklist for Customers Purchasing Air Filters

When selecting an air filter, customers should not simply pursue the highest efficiency class. The selection should be based on the equipment and actual operating environment.

General HVAC Filters

Consider the following:

• Whether the original equipment uses EN 779 or ISO 16890 specifications
• Whether the target is PM10, PM2.5, or PM1 control
• Actual airflow and filter dimensions
• Initial pressure drop and available fan capacity
• Dust concentration in the operating environment
• Dust-holding capacity and replacement interval
• Upstream and downstream filter configuration

HEPA and ULPA Filters

Consider the following:

• Whether H13, H14, or a ULPA classification is required
• Applicable testing standard
• Overall and local efficiency requirements
• Rated airflow and initial pressure drop
• Filter media, frame, and sealant materials
• Whether high-temperature or chemical resistance is required
• Whether factory test documentation is provided
• Whether on-site leakage testing is required after installation
• Whether the existing fan can handle the filter pressure drop

A higher classification is not necessarily more suitable.

If the filter pressure drop exceeds the system design capacity, it may result in insufficient airflow, reduced air changes per hour, increased fan energy consumption, or even an imbalance in cleanroom pressure.

The correct selection principle is:

While meeting the required cleanliness and process requirements, the filter should also be evaluated in terms of filtration efficiency, airflow, pressure drop, installation airtightness, energy consumption, and maintenance cycle.

Frequently Asked Questions (FAQ)

Which Is More Stringent, EN 779 or EN 1822?

The two standards have different scopes and cannot be directly compared. EN 779 was intended for general ventilation filters, while EN 1822 is used for EPA, HEPA, and ULPA high-efficiency filters and places greater emphasis on MPPS efficiency and local leakage control.

Can F9 Replace an H13 HEPA Filter?

No, they cannot be directly substituted.

F9 belongs to the legacy EN 779 classification system for general ventilation filters, whereas H13 is a HEPA classification under EN 1822. The two differ in efficiency thresholds, test particle sizes, and testing methods.

Is H14 Always Better Than H13?

H14 has a lower allowable penetration rate, but pressure drop, system airflow, and cost must also be considered.

The appropriate classification should be selected according to the cleanroom classification, process risk, equipment capability, and validation requirements rather than simply choosing the highest available efficiency class.

Is MPPS Fixed at 0.3 μm?

No.

MPPS varies depending on the filter media, face velocity, fiber structure, and test conditions. Although 0.3 μm is commonly referenced in discussions of high-efficiency filtration, it should not automatically be regarded as the MPPS for every EN 1822 filter.

Is On-Site PAO Testing Still Necessary After Factory Testing?

For critical clean environments, it is generally still necessary.

Factory testing verifies the performance of the filter itself, while on-site testing verifies whether the filter, frame, gasket, and installation interfaces remain leak-free after transportation and installation.

Filter Classification Is Only the Starting Point—System Compatibility Is the Key

EN 779, EN 1822, and MPPS represent different technical concepts:

• EN 779 was the legacy classification system for general ventilation filters and has now been replaced by ISO 16890.
• EN 1822 is used for the classification and performance testing of EPA, HEPA, and ULPA high-efficiency filters.
• MPPS represents the particle size most likely to penetrate the filter and is an important indicator of the weakest point in high-efficiency filtration.

Ultimately, what matters is not simply whether a filter is labeled H13 or H14, but whether the product achieves the required performance at the correct airflow and under the specified test conditions, while integrating effectively with the existing HVAC or cleanroom system.