If you've ever purchased industrial air filters, you've probably come across designations such as G4, M5, F7, F9, ISO Coarse, ISO ePM10, ISO ePM2.5, ISO ePM1 or H13.
For many maintenance managers, engineers and procurement departments these classifications can be confusing. Does a higher rating always mean a better filter? What happened to G4 and F7? Why do some specifications now refer to ISO 16890 instead?
Understanding these efficiency ratings makes it much easier to select the correct filter and avoid expensive mistakes.
Why Do Air Filters Have Efficiency Ratings?
Not every air filter is designed to perform the same task.
Some filters are intended to remove larger airborne particles such as dust, fibres and lint. Others are designed to capture much finer particles that affect indoor air quality or are critical to a manufacturing process.
Efficiency ratings provide a recognised method of describing how a filter performs under standardised test conditions.
It's important to remember that a higher efficiency filter is not automatically a better filter.
The correct filter is the one that has been selected for the application and forms part of the overall filtration system.
The EN779 Classification System
For many years, filters used in general ventilation systems were commonly classified using the EN779 standard.
Although EN779 has now been replaced by ISO 16890, it remains widely used throughout South Africa. Engineers, maintenance managers, consultants and procurement departments still regularly specify filters as G4, M5, F7 or F9.
If you're replacing an existing filter, you'll almost certainly continue to encounter these classifications.
EN779 broadly grouped filters into three categories.
G-Class Filters
G-Class filters were commonly used as coarse pre-filters.
Typical classifications included:
- G2
- G3
- G4
These filters are commonly used to remove larger airborne particles and to protect downstream filters and HVAC equipment.
M-Class Filters
M-Class filters provide finer filtration than G-Class filters and are commonly used as intermediate filters.
Typical classifications include:
- M5
- M6
They are frequently installed after a coarse pre-filter to improve air quality while reducing the loading on higher-efficiency filters.
F-Class Filters
F-Class filters provide finer filtration again.
Typical classifications include:
- F7
- F8
- F9
Depending on the application, these filters may be used as secondary filters, final filters within general ventilation systems or as pre-filters protecting HEPA filters.
Why Was EN779 Replaced?
EN779 served the industry well for many years, but it had limitations.
The standard largely classified filters using their efficiency at a single particle size under laboratory test conditions. As our understanding of air quality improved, it became clear that a new approach was needed to better represent the wide range of particle sizes found in real atmospheric air.
This led to the introduction of ISO 16890, which became the internationally recognised standard for classifying general ventilation air filters. Rather than focusing on a single test particle size, ISO 16890 evaluates filter performance across particle size fractions that are more representative of the air we actually breathe.
Understanding ISO 16890
ISO 16890 classifies filters into four main groups:
- ISO Coarse
- ISO ePM10
- ISO ePM2.5
- ISO ePM1
The "PM" stands for Particulate Matter, while the number refers to the aerodynamic diameter of particles, measured in micrometres (µm).
To put that into perspective, a human hair is typically between 50 and 100 micrometres in diameter.
Even PM10 particles are therefore much smaller than the width of a human hair.
| Classification | Typical Particle Fraction | Examples of Airborne Contaminants* |
|---|---|---|
| ISO Coarse | Larger airborne particles | Lint, textile fibres, insects, coarse dust and larger debris |
| ISO ePM10 | PM10 fraction | Pollen, mould spores, cement dust and many common industrial dusts |
| ISO ePM2.5 | PM2.5 fraction | Fine combustion particles, welding fumes, diesel soot and smoke |
| ISO ePM1 | PM1 fraction | Very fine combustion particles and other microscopic airborne contaminants |
*These examples are intended only as a general guide. Real airborne contamination usually consists of a mixture of many particle sizes.
Unlike EN779, ISO 16890 classifications are determined through standardised testing against these particulate matter fractions. A filter must achieve a minimum efficiency of 50% for the relevant particulate fraction before it can receive an ePM classification.
Can EN779 Be Converted to ISO 16890?
This is probably the question we are asked most often.
The honest answer is:
Not exactly.
There is no official one-to-one conversion table between EN779 and ISO 16890.
When ISO 16890 replaced EN779, filters had to be tested under the new standard to determine their ISO classification.
For that reason, two filters that were both previously described as F7 under EN779 may not necessarily achieve the same ISO 16890 classification.
If you see comparison tables published online, they should be regarded only as general guidance, not exact conversions.
When replacing an existing filter, it is usually best to work from the original specification or consult the filter manufacturer or supplier if there is any uncertainty.
Where Do HEPA Filters Fit In?
HEPA filters are different.
They are not classified under EN779 or ISO 16890.
Instead, HEPA filters are classified under standards such as EN1822 and ISO 29463, using classifications including:
- H13
- H14
HEPA filters are intended for applications requiring extremely high levels of particulate removal, including:
- Cleanrooms
- Pharmaceutical manufacturing
- Hospitals
- Laboratories
- Medical facilities
- Electronics manufacturing
Because of their very high efficiency, HEPA filters are normally used as the final stage of a multi-stage filtration system rather than as the first filter exposed to heavily contaminated air.
Does a Higher Efficiency Always Mean a Better Filter?
No.
One of the biggest misconceptions in industrial filtration is that selecting a higher efficiency filter will automatically improve system performance.
In reality, every filter creates resistance to airflow.
Higher efficiency filters generally have a higher resistance than coarse pre-filters and should only be used where the ventilation system has been designed to accommodate them.
Installing a higher efficiency filter than originally specified may affect airflow, increase pressure drop and reduce the performance of the ventilation system.
Likewise, selecting a lower efficiency filter where fine filtration is required may fail to provide the required air quality.
The correct filter is the one specified for the application—not simply the highest efficiency available.
Choosing the Correct Filter
If you're replacing an existing filter, the safest approach is normally to match the efficiency originally specified by the system designer or equipment manufacturer unless there is a specific engineering reason to make a change.
If you're designing a new system or considering changing filtration levels, the ventilation system should be evaluated as a whole rather than selecting filters based on efficiency alone.
Summary
- EN779 classifications such as G4, M5, F7 and F9 remain widely used throughout South Africa.
- ISO 16890 is the current international standard for classifying general ventilation filters.
- ISO 16890 evaluates filters against particulate matter fractions that better represent real atmospheric air than the older EN779 system.
- There is no official one-to-one conversion between EN779 and ISO 16890.
- HEPA filters are classified separately under standards such as EN1822 and ISO 29463.
- Higher efficiency does not automatically mean a better filter—the correct filter depends on the application and the design of the ventilation system.