From F7–F9 to ISO ePM: Understanding the Evolution of European Air Filter Standards

For many years, G4, M5, M6, F7, F8 and F9 were widely used to specify air filters for general ventilation in Europe. These classifications came from EN 779, a standard that had been used for decades across HVAC and air-filtration applications. Today, however, specifications increasingly use terms such as ISO Coarse, ISO ePM10, ISO ePM2.5 and ISO ePM1. This change was not simply a replacement of old filter names with new ones. The way filter performance is tested and classified also changed significantly.

The Old European System: EN 779

The previous European standard for general ventilation filters was EN 779:2012. It divided filters into three groups: G1–G4, M5–M6 and F7–F9. Although these classes were part of the same standard, they were not evaluated using exactly the same method.

For the G1–G4 classes, filtration performance was primarily evaluated using a gravimetric method. A standardized test dust was introduced into the test system, and the amount of dust captured by the filter was determined by mass. This approach was mainly associated with filters intended to remove relatively coarse particles.

For M5–M6 and F7–F9, particle counting was used instead. The classification focused on filtration efficiency around a reference particle size of 0.4 μm. For the F classes, both average efficiency and minimum efficiency were considered. Under EN 779:2012, for example, an F9 filter required a mean efficiency of at least 95% at 0.4 μm, together with a specified minimum efficiency.

This made the F7–F9 system relatively straightforward for filter selection. However, it also had a limitation: describing filter performance mainly through a specific reference particle size did not fully represent how a filter behaved across the entire particle-size range encountered in real air.

Why Did Europe Move Away from EN 779?

Particles suspended in air have a wide range of sizes, and a filter’s efficiency can vary considerably with particle size. A filter that performs well at 0.4 μm may not have exactly the same relative performance at 0.2 μm, 1 μm or 2.5 μm. As a result, a single filter class could not fully describe the particle-removal characteristics of the filter.

At the same time, concepts such as PM10, PM2.5 and PM1 had become increasingly important in discussions of indoor air quality and particulate pollution. These terms describe particle-size fractions rather than traditional filter classes. A classification system that could directly communicate performance against these particle-size fractions therefore provided a more useful way to describe modern ventilation filters.

This was one of the important reasons behind the development and adoption of EN ISO 16890.

The New System: EN ISO 16890

EN ISO 16890 is the current European standard for air filters used in general ventilation. The international ISO standard was published in 2016, and Europe subsequently went through a transition period from EN 779 to EN ISO 16890. The transition ended on 30 June 2018, after which EN 779 was withdrawn as the applicable European classification standard for general ventilation filters.

The classification changed fundamentally. Instead of G, M and F classes, filters are now classified as ISO Coarse, ISO ePM10, ISO ePM2.5 and ISO ePM1.

The term ePM means particulate matter efficiency. Rather than assigning a filter to a class primarily based on its performance at one reference particle size, ISO 16890 measures the filter’s fractional efficiency across a range of particle sizes. The resulting efficiency curve is then used to determine the filter’s efficiency against the relevant PM fractions.

For example, a designation such as ISO ePM1 80% indicates that the filter has a calculated particulate-matter efficiency of 80% for the PM1 fraction according to the ISO 16890 test and calculation procedure. The classification therefore provides a direct connection between filter performance and particle-size fractions commonly used in air-quality discussions.

From a Single Reference Point to a Particle-Size Efficiency Curve

This is probably the most important technical difference between the two systems.

Under EN 779, the classification of M5–F9 filters was strongly associated with efficiency around 0.4 μm. ISO 16890 instead measures fractional efficiency over a range of particle sizes, producing a particle-size efficiency curve. This curve is then used to calculate the relevant ePM1, ePM2.5 and ePM10 values.

The difference matters because filtration efficiency is not constant across particle sizes. Depending on the filter media, construction and filtration mechanisms, efficiency may decrease toward a certain particle-size range and then increase again for larger particles. Looking at the efficiency curve therefore provides considerably more information about the actual filtration characteristics than a single efficiency value at one particle size.

Electrostatic Effects Are Also Considered

Modern synthetic filter media can use electrostatic effects to improve particle capture. A newly manufactured filter may therefore have a higher initial efficiency than the same filter after its electrostatic charge has been reduced.

ISO 16890 includes a conditioning procedure using isopropyl alcohol (IPA) to reduce the influence of electrostatic effects and determine minimum fractional efficiency. This is important because it makes the evaluation less dependent on the temporary electrostatic performance of certain synthetic filter media.

In other words, ISO 16890 does not simply ask how efficient a filter is when it is brand new. It also considers how the measured fractional efficiency changes after the prescribed conditioning process.

Are F7, F8 and F9 Equivalent to ISO ePM Classes?

This is where many filter specifications become confusing.

Industry documents often provide approximate relationships such as F7 ≈ ISO ePM1 50%, F8 ≈ ISO ePM1 65% and F9 ≈ ISO ePM1 80%. These figures can be useful when discussing the general performance range of legacy filters, but they should not be treated as official one-to-one conversions.

The reason is that EN 779 and ISO 16890 use different test and classification methods. EN 779 F9 was determined using its specified test procedure, including the 0.4 μm reference point, whereas ISO ePM1 is calculated from the filter’s fractional-efficiency curve using the ISO 16890 methodology.

Therefore, an EN 779 F9 filter does not automatically become an ISO ePM1 80% filter. If a current specification requires an ISO 16890 classification, the appropriate way to establish that classification is through the ISO 16890 test procedure rather than simply converting the old F-class designation.

What Happened to the Old F7–F9 Classifications?

The introduction of ISO 16890 did not make all existing F7–F9 filters unusable. The old classifications remain relevant when dealing with equipment designed under the previous standard, replacement filters for existing systems, historical test reports and older procurement specifications.

This is why F7, F8 and F9 can still be found in technical documents and product catalogs today. They are legacy classifications, not the current classification system for new general-ventilation filter specifications in Europe.

When an existing system specifies F9, the correct approach is therefore not necessarily to search for a product with a label that looks like a direct replacement. The required performance, airflow and pressure-drop characteristics should first be understood, and the replacement filter should then be selected according to the applicable current classification and application requirements.

EN ISO 16890 Does Not Replace EN 1822

Another important distinction is between general ventilation filters and high-efficiency filters.

EN ISO 16890 is intended for general ventilation air filters. It does not replace the European high-efficiency filtration standard EN 1822. High-efficiency filters such as E10, E11, E12, H13, H14, U15, U16 and U17 remain within a separate testing and classification framework.

EN 1822 uses the concept of MPPS — Most Penetrating Particle Size. Instead of classifying a high-efficiency filter according to PM1 or PM2.5, the test identifies the particle size at which the filter is most difficult to penetrate and evaluates performance at or around this point. For high-efficiency filters, overall efficiency and, where applicable, local leakage or integrity are also important.

The international ISO 29463 series provides a closely related international framework for high-efficiency filters and filter media. Therefore, ISO 16890 and EN 1822 should not be viewed as two stages of one continuous classification scale. They address different categories of filtration products.

The Evolution in One Table

	EN 779:2012	EN ISO 16890
Main application	General ventilation	General ventilation
Classification	G1–G4 / M5–M6 / F7–F9	ISO Coarse / ePM10 / ePM2.5 / ePM1
Main evaluation concept	Gravimetric and particle-counting methods	Fractional efficiency across particle sizes
Key reference	0.4 μm for M/F classes	Particle-size efficiency curve
PM1 / PM2.5 / PM10	Not the basis of classification	Core of the classification
Electrostatic conditioning	Different approach	Prescribed conditioning procedure
Pressure drop	Evaluated	Evaluated
European status	Withdrawn	Current

What Changed — and What Did Not?

The most important point is that Europe did not simply replace the label F9 with another label.

The transition from EN 779 to EN ISO 16890 changed the way filter performance is described. The old system was built around predefined filter classes and specified test conditions, while the new system uses fractional-efficiency data across particle sizes and expresses performance in terms of PM-related efficiency categories.

At the same time, the basic engineering questions remain the same: How efficient is the filter? At what airflow? Against which particle sizes? What is the pressure drop? How stable is the performance? And was the test performed on the media or on the complete filter?

A filter grade is therefore not the complete performance specification. The test method behind the grade is equally important.

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Understanding the standard is the first step to understanding the filter.