Technical guide
Air filter classes: G4, F7, HEPA H13 — making sense of them
Air filters are classified under two families of standards. For general filtration, the former EN 779 standard (classes G, M and F: G4, M5, F7, F9 and so on) has been replaced by ISO 16890, which classifies filters by their efficiency against real particle sizes: ISO Coarse, ISO ePM10, ISO ePM2.5 and ISO ePM1. For absolute filtration, EN 1822 still governs HEPA filters (classes H13 and H14) and ULPA. The G4 and F7 designations remain in heavy use in the market: a G4 today corresponds to the ISO Coarse ≥ 60 % category, and an F7 to ISO ePM2.5 with an efficiency of roughly 65 to 80 % on fine particulate.
Filter class cross-reference table
| Common class | Original standard | Current equivalent | Role in the installation |
|---|---|---|---|
| G4 | EN 779 | ISO Coarse ≥ 60 % | Pre-filter: coarse dust, textile fibres, heavy pollen, sand, insects |
| M5 / M6 | EN 779 | ISO ePM10 | Medium filtration, protecting coils and heat exchangers |
| F7 | EN 779 | ISO ePM2.5 (≈ 65 – 80 %) | The reference fine filtration stage in professional ventilation |
| F9 | EN 779 | ISO ePM1 | Enhanced fine filtration, pre-filtration ahead of absolute filters |
| H13 / H14 | EN 1822 | HEPA absolute filter | Cleanrooms, operating theatres, laboratories, controlled-particulate zones |
Why stage the filtration?
A fine filter on its own loads too quickly and is expensive to replace. Good practice is to stage it: a G4 pre-filter stops the coarse particles and protects the filter behind it, an F7 deals with fine particulate, and an H13/H14 absolute filter comes in as the final stage only where it is genuinely required. This cascade arrangement extends the life of the most expensive filter considerably.
- G4 alone: equipment protection, undemanding environments, pre-filtration.
- G4 + F7: the reference arrangement in commercial buildings and professional ventilation, particularly in polluted urban areas.
- G4 + F9 + H13: controlled-particulate zones, healthcare, laboratories, certain food processing areas.
Which filter for which sector?
- Offices and retail: G4 as pre-filtration, F7 for fine filtration.
- General industry: G4 to M6 depending on the dust load, with enhanced pre-filtration if the outdoor air is dirty.
- Food processing: staging up to F9, or even H13 in clean zones held at positive pressure.
- Healthcare and laboratories: absolute H13 or H14 filtration depending on the cleanliness class being targeted.
- Commercial kitchens: metal baffle grease filters on the canopy, which are separate from the AHU filtration.
When to replace a filter
The right indicator is not the calendar but the pressure drop: every filter has an initial pressure drop and a final pressure drop recommended by the manufacturer. A differential pressure gauge fitted either side of the filtration stage gives you that reading continuously.
- Monitor pressure drop rather than elapsed service time alone.
- Replace a pre-filter more often than the fine filter it protects.
- Never clean and reuse a fine or absolute filter: the media is degraded.
- Record every replacement (class, date, pressure drop measured) — this is a requirement in regulated sectors.
Frequently asked questions
What is the difference between a G4 filter and an F7 filter?
A G4 filter is a pre-filter: it stops coarse particles (dust, textile fibres, heavy pollen, sand) and corresponds today to the ISO Coarse ≥ 60 % category. An F7 filter handles fine filtration, with an efficiency of roughly 65 to 80 % on fine particulate, that is to say the ISO ePM2.5 category. A G4 on its own is not enough to achieve good indoor air quality: it is used upstream of the F7 to protect it.
Is EN 779 still valid?
The G4, M5 and F7 designations from EN 779 remain very much present in the market and in specifications, but the classification standard in force for general filtration is ISO 16890, which classifies filters by their efficiency against PM10, PM2.5 and PM1 particulate. HEPA absolute filters remain classified to EN 1822.
When do you need a HEPA H13 filter?
An H13 or H14 absolute filter, classified to EN 1822, is required in controlled-particulate environments: operating theatres, laboratories, cleanrooms, certain sensitive food processing zones. It is always installed as the final stage, preceded by G4 pre-filtration and an F7 or F9 fine filter that protect it.
How often should the filters in an air handling unit be changed?
The frequency depends on the actual dust load, not the calendar. The replacement trigger is pressure drop: every filter has a final pressure drop recommended by its manufacturer, which you can read on a differential pressure gauge fitted either side of the filtration stage. The pre-filter is replaced more often than the fine filter it protects.
Further reading
Pre-filters, fine filters and absolute filters in stock.
Read more Air treatment serviceAir quality assessment and filtration staging.
Read more AHU or mechanical ventilation?Where the filtration stages sit inside a unit.
Read more Food processing ventilationFiltration and positive pressure in clean zones.
Read more