Primary vs Medium vs HEPA Air Filters: Key Differences Explained
TL;DR — Key Differences at a Glance
Primary, medium, and Hepa Air Filters serve fundamentally different roles in an HVAC system, and no single filter type can replace another. Primary filters (G1–G4 / MERV 1–8) capture coarse particles >10 μm at low cost (≤$8/unit) and serve as pre-filters to protect downstream stages. Medium filters (M5–M6 / F7–F9 / MERV 9–15) capture 2–10 μm particles and are the main workhorses for commercial IAQ. Hepa Filters (H13–H14 / MERV 16–17+) capture ≥99.95% of particles at 0.3 μm MPPS and are reserved for critical applications.
The cost difference is extreme: a G4 primary filter costs $3–$8 and creates an initial ΔP of 40–55 Pa. An F9 medium filter costs $25–$45 with an initial ΔP of 100–130 Pa. A HEPA H13 filter costs $60–$150 with an initial ΔP of 120–200 Pa. This 15–20x price spread exists because the capture mechanisms differ: primary filters use impaction, medium filters use diffusion and interception, and HEPA filters rely on all three plus electrostatic forces.
The most cost-effective system design is a staged approach: G4 pre-filter (protects investment) + F7–F9 main filter (delivers IAQ) + optional HEPA final filter (for critical zones). Each stage extends the life of the next, reducing total cost of ownership by 40–60% compared to running fine filters alone.
I have been building air filters at KWS since 2011, and the single most common misunderstanding I encounter from buyers is the assumption that "better" means "use the highest grade everywhere." A facility manager in Malaysia once ordered HEPA filters for his entire office building because he wanted "the cleanest possible air." He spent $38,000 on filters, installed them without pre-filtration, and had to replace every single HEPA unit within 4 months because they loaded with coarse dust that a $5 G4 pre-filter would have stopped.
Primary, medium, and HEPA air filters are not different grades of the same product — they are different products designed for different jobs. Primary filters are the coarse guard. Medium filters are the precision workhorse. HEPA filters are the surgical tool. Using any of them incorrectly wastes money and degrades performance.
This article explains exactly how they differ in construction, capture mechanism, performance, cost, and application — so you can make informed decisions about which filter type belongs where in your system.
Primary air filters (also called pre-filters or coarse filters) are the first line of defense in any HVAC system. They are classified under EN 779 as G1–G4 or under ISO 16890 as ISO Coarse 40%–90%. Under ASHRAE 52.2, they correspond to MERV 1–8.
What they capture: Coarse particles larger than approximately 10 μm — dust, lint, pollen, textile fibers, insect fragments, and large spores. These are the particles you can see floating in a shaft of sunlight.
How they work: Primary filters rely primarily on inertial impaction. Air flows through relatively large openings in the filter media (woven or non-woven synthetic fibers), and particles that are too large to follow the air stream collide with the fibers and stick.
Typical construction: Pleated synthetic sheet media in a cardboard or metal frame. Panel filters, roll filters, and some bag-type designs. Media thickness is typically 4–15 mm.
Initial ΔP: 25–55 Pa at rated airflow (depending on filter class and pleat count).
Standard sizes: 595×595×46 mm (most common), 595×595×96 mm (higher capacity), and many custom dimensions.
Cost: $2–$8 per unit (G3–G4, quantity pricing). The lowest-cost filter class by a wide margin.
Medium air filters (also called fine filters or pocket/bag filters) are the primary air cleaning stage in most commercial HVAC systems. They are classified as M5–M6 (medium) and F7–F9 (fine) under EN 779, or ePM10 ≥50% to ePM1 ≥85% under ISO 16890. Corresponding MERV values are 9–15.
What they capture: Fine particles in the 0.4–10 μm range — mould spores, cement dust, fine pollen, bacteria (individual cells are 0.5–5 μm), and most smoke particles. F7–F9 filters capture the majority of inhalable particulate matter (PM2.5 and PM10).
How they work: Medium filters use a combination of interception (particles following air stream touch fiber surfaces) and diffusion (sub-micron particles move randomly due to Brownian motion and collide with fibers). The electrostatic charge built into the synthetic media attracts oppositely charged particles, improving capture efficiency without increasing ΔP.
Typical construction: Multi-pocket bag designs (the most common, offering 8–12 times the media area of a flat panel), mini-pleat rigid boxes, and V-bank configurations. Media thickness typically 15–30 mm for bag filters.
Initial ΔP: 70–130 Pa depending on class. F7 bag filters typically start at 80–90 Pa; F9 at 110–130 Pa.
Cost: $12–$45 per unit (M5–F9). F9 bag filters cost roughly 5–6 times more than G4 panel filters.
HEPA (High Efficiency Particulate Air) filters are the highest grade of dry particulate air filter used in commercial and industrial applications. They are classified under EN 1822 as H13 (99.95% efficiency at MPPS), H14 (99.995%), U15 (99.9995%), and U16 (99.99995%). Under ASHRAE 52.2, H13 corresponds approximately to MERV 16–17+.
What they capture: Everything that a primary and medium filter can capture, plus ultra-fine particles down to 0.1 μm. This includes viruses (0.02–0.3 μm), fine smoke particles, diesel exhaust particulates, and nanoparticle (<0.1 μm) contamination. A HEPA H13 filter captures 99.95% of all particles at its Most Penetrating Particle Size (MPPS), which is approximately 0.15–0.3 μm for most HEPA media.
How they work: HEPA filters use all four mechanisms simultaneously — inertial impaction, interception, diffusion, and electrostatic attraction. The media consists of ultra-fine glass microfiber (borosilicate glass) or e-PTFE (expanded polytetrafluoroethylene) fibers arranged in a dense, randomly oriented mat. The fiber diameter is 0.5–2 μm, and the inter-fiber spacing is measured in microns, creating a tortuous path that captures particles through multiple mechanisms.
Typical construction: Deep-pleat (60–150 mm pleat depth) mini-pleat designs with aluminum separators or hot-melt glue bead separators between pleats to maintain airflow channels. Frames are typically galvanised steel, stainless steel, or anodised aluminium. Gel-seal frames are preferred for critical applications to prevent bypass leakage.
Initial ΔP: 120–250 Pa for H13–H14 at rated airflow.
Cost: $60–$300+ per unit, with the widest price range driven by frame material, seal type, and certification documentation. A HEPA H14 with gel-seal frame costs roughly 8–20 times more than a G4 primary filter.
| Parameter | Primary (G4 / MERV 8) | Medium (F7–F9 / MERV 13–15) | HEPA (H13–H14 / MERV 16+) |
|---|---|---|---|
| Min particle capture | >10 μm | >0.4 μm | >0.1 μm (≥99.95% at 0.3 μm) |
| Primary mechanism | Inertial impaction | Interception + diffusion | All four mechanisms |
| Media material | Synthetic fibers | Synthetic microfibers | Borosilicate glass / e-PTFE |
| Media area ratio | 1x (baseline) | 8–12x (bag design) | 15–25x (mini-pleat) |
| Initial ΔP | 40–55 Pa | 80–130 Pa | 120–250 Pa |
| Final ΔP | 100–110 Pa | 200–250 Pa | 400–600 Pa |
| Unit cost | $3–$8 | $15–$45 | $60–$300+ |
| Service life (office) | 3–6 months | 6–12 months | 12–36 months |
| Energy cost/yr per filter | ≤$8 | $15–$35 | $30–$80 |
| Application | Pre-filter, low-risk | Main filter, commercial IAQ | Critical zones |
| Standard | EN 779 G1–G4 / ISO Coarse | EN 779 M5–F9 / ISO ePM | EN 1822 H13–H14 |
- You need to protect downstream fine filters from coarse dust
- The application is general ventilation with no specific IAQ requirements (warehouses, parking garages, storage)
- You are in a high-particulate environment and need a low-cost disposable pre-filter replaced frequently
- Your budget prioritises the lowest possible upfront filter cost
- You are filtering outdoor intake air in a region with moderate to high coarse particulate levels
- You need to meet commercial IAQ standards (ASHRAE 62.1, LEED, WELL)
- The space is occupied by people — offices, schools, retail, hotels, restaurants
- PM2.5 and PM10 control is required for occupant health and productivity
- You are designing a staged filtration system and need the main filtration stage
- The application requires F7–F9 protection as pre-filtration before HEPA
- Regulatory standards require it — GMP Grade A/B, hospital ORs, ISO Class 5–8
- You are protecting a product or process from airborne contamination
- The particle size of concern is below 0.5 μm (viruses, fine smoke, nanoparticles)
- Airborne infection control is the priority — isolation rooms, TB wards
| Application | Primary | Medium | HEPA | Rationale |
|---|---|---|---|---|
| Office building, general | G4 (pre) | F7 (main) | — | F7 sufficient; HEPA overkill |
| Hospital operating theatre | G4 (pre) | F9 (pre) | H14 (final) | Three-stage to protect HEPA |
| Pharma cleanroom Grade A | G4 (pre) | F9 (pre) | H14 (final) | EU GMP Annex 1 requirement |
| School classroom | G4 (pre) | F7–F9 (main) | — | F9 controls PM2.5 adequately |
| Semiconductor fab | G4 (pre) | F9 (pre) | H13–H14 (final) | ISO Class 3–5 coverage |
| Food RTE packaging | G4 (pre) | F9 (main) | H13 (final) | Listeria control standard |
| Data center cooling | G4 (pre) | F7 (main) | — | HEPA adds unnecessary ΔP |
| Warehouse / storage | G4 only | — | — | Cost is the priority |
The most cost-effective approach is staging filters in ascending efficiency, and the reason is purely economic: coarse particles are expensive to capture with fine filters.
- F7 bag filter: $28/unit
- Service life without pre-filter: 4–5 months (loads with coarse dust)
- Annual filter cost: $67–$84 per filter position
- Annual energy cost: ~$22
- Total annual: ~$89–$106
Two-stage G4 + F7:
- G4 panel: $5/unit + F7 bag: $28/unit = $33 total upfront
- G4 replaced every 4 months ($15/yr), F7 replaced every 10 months ($34/yr)
- Annual filter cost: ~$49 per position
- Annual energy cost: ~$25 (slightly higher ΔP from two stages)
- Total annual: ~$74
Annual saving with staged approach: $15–$32 per filter position (17–30%)
For a building with 200 filter positions, that is $3,000–$6,400 saved per year simply by adding a $5 pre-filter. Because the cost of a G4 pre-filter is 5–7% of a HEPA filter, adding a pre-filter extends HEPA life by 3–5 times. The simple causal relationship is: more coarse particles captured upstream = fewer coarse particles loading the expensive downstream filter = longer service life = lower total cost of ownership.
| Filter Type | EN 779 Class | ISO 16890 Class | ASHRAE MERV | Min Efficiency at: |
|---|---|---|---|---|
| Primary | G1 | ISO Coarse 40% | MERV 1–4 | Not tested below 3 μm |
| Primary | G2 | ISO Coarse 50% | MERV 5 | 20–34% at 3–10 μm |
| Primary | G3 | ISO Coarse 65% | MERV 6–7 | 35–49% at 3–10 μm |
| Primary | G4 | ISO Coarse 75% | MERV 8 | ≥70% at 3–10 μm |
| Medium | M5 | ePM10 50% | MERV 9–10 | ≥50% at 1–3 μm |
| Medium | M6 | ePM10 65% | MERV 11–12 | ≥65% at 1–3 μm |
| Medium | F7 | ePM1 50% | MERV 13 | ≥50% at 0.3–1 μm |
| Medium | F8 | ePM1 65% | MERV 14 | ≥65% at 0.3–1 μm |
| Medium | F9 | ePM1 85% | MERV 15 | ≥85% at 0.3–1 μm |
| HEPA | H13 (EN 1822) | Not classified | MERV 16+ | ≥99.95% at 0.3 μm MPPS |
| HEPA | H14 (EN 1822) | Not classified | MERV 17+ | ≥99.995% at 0.3 μm MPPS |
Note: ISO 16890 does not cover HEPA-grade filtration. HEPA classification is governed by EN 1822:2019 or IEST-RP-CC001.6. When procuring internationally, always specify both the local standard and the equivalent international standard to avoid misclassification.
In 2023, a 500-bed private hospital in Ho Chi Minh City was planning its HVAC upgrade and the procurement team initially specified HEPA H13 filters for every air handling unit. The budget quoted for the complete HEPA installation was $1.2 million — with recurring replacement costs of $800,000/year.
I visited the site and proposed a staged approach:
- General wards and public areas: G4 pre-filters + F7 bag filters (annual cost: $48,000)
- ICU and operating theatres (16 rooms): G4 + F9 + H14 HEPA gel-seal (annual cost: $65,000)
- Isolation rooms (8 rooms): G4 + F9 + H13 HEPA + UV-C (annual cost: $38,000)
Total annual filter cost: $151,000 vs $800,000 — a saving of $649,000 per year. And because the HEPA filters were protected by pre-filters, service life extended from 12–18 months to 36–48 months in critical zones.
The principle is simple: use the cheapest filter that can do the job in each zone. Reserve expensive HEPA filtration for the zones that genuinely require it.
At our Xiamen factory, we produce primary, medium, and HEPA filters on dedicated production lines because the manufacturing processes are fundamentally different.
- Primary filter line: Automated pleating of synthetic sheet media, hot-melt adhesive bonding, cardboard or metal frame assembly. Production speed: ~600–800 units/shift. QC focus: pleat height consistency and frame seal integrity.
- Medium filter line: Multi-pocket bag sewing or V-bank assembly. Ultrasonic welding or adhesive bonding. Production speed: ~200–300 units/shift. QC focus: pocket depth uniformity and initial ΔP verification on every batch.
- HEPA filter line (ISO Class 7 cleanroom): Mini-pleat forming with aluminum separators. Media handled in cleanroom to prevent pre-contamination. Each filter individually tested (DOP/PAO aerosol challenge) and scanned for pinhole leaks. Production speed: 30–50 units/shift. QC focus: 100% scan testing per EN 1822.
Every filter carries a QC certificate with measured initial ΔP at rated airflow. HEPA filters include the individual efficiency test result at MPPS.
Technically yes, but practically no. A HEPA filter exposed to coarse dust without pre-filtration will load to final ΔP in 2–4 months instead of 24–36 months. Given that a HEPA filter costs 15–20 times more than a G4 pre-filter, the economics are clear: always stage a pre-filter before any HEPA installation.
MERV stands for Minimum Efficiency Reporting Value, defined by ASHRAE Standard 52.2. It ranges from 1 (lowest) to 20 (HEPA-grade). EN 779 classes G1–G4 and M5–F9 are broadly equivalent, with G4 = MERV 8 and F9 = MERV 15 as the most common reference points. ISO 16890 is gradually replacing both as the global standard.
For PM2.5 (particles ≤2.5 μm), you need at minimum an F7 (MERV 13) filter capturing ≥50% at 0.3–1 μm. For effective PM2.5 control (>80% capture), use F9 (MERV 15). Primary filters (G1–G4) capture virtually no PM2.5 because their minimum particle capture threshold is >10 μm.
You need HEPA when: (1) a regulatory standard explicitly requires it (pharma GMP, hospital ORs, ISO cleanrooms), (2) your process cannot tolerate >1 particle per cubic metre above 0.5 μm, or (3) airborne infection control is required. For general commercial IAQ, F7–F9 is sufficient and significantly more economical.
MPPS is the particle size at which a filter is least efficient. For HEPA glass microfiber media, MPPS is approximately 0.15–0.3 μm. This is why HEPA filters are rated at 0.3 μm. Particles above and below MPPS are captured more efficiently because different mechanisms dominate at different sizes.
Because HEPA media manufacturing is fundamentally more complex. Primary filter media uses coarse synthetic fibers in a random web — a simple, high-speed process. HEPA media requires precision borosilicate glass microfiber formation with controlled fiber diameter (0.5–2 μm) and multiple quality tests per roll. HEPA media costs roughly 30–50 times more per square metre than primary filter media.
Not typically. HEPA filters require deeper housings (pleat depth 60–150 mm vs 20–46 mm), different sealing mechanisms (gel-seal vs gasket), and higher structural support. Installing a HEPA filter in a primary housing will almost certainly cause bypass leakage, rendering the HEPA ineffective. Always confirm housing compatibility before upgrading.
Primary, medium, and HEPA air filters are not interchangeable. Each serves a distinct purpose in a properly designed staged filtration system. Primary filters protect your investment in downstream stages. Medium filters deliver the IAQ that occupants need. HEPA filters provide the surgical cleanliness that critical applications demand.
The most expensive mistake is not buying the wrong filter type — it is buying the highest grade filter for every position. That Malaysian facility manager learned this the hard way. The Ho Chi Minh City hospital avoided it by applying the staged approach.
At KWS, we manufacture all three filter types and we will tell you honestly which one belongs where. If you need G4 pre-filters for your warehouse intake, we supply them. If you need H14 HEPA for your pharmaceutical cleanroom, we test and certify every single one.
Contact KWS at kws.airfliter@gmail.com or visit www.kwsairfilter.com.