HEPA Filter Testing: DOP/PAO, Leak Tests and EN 1822 Explained
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04-11-2026
A HEPA filter is only as good as its last test. A filter that looks perfect can still carry a pinhole leak, a damaged seal or a loose gasket, and any of these can let unfiltered air into a pharmaceutical, biotech or semiconductor cleanroom. That is why every serious facility treats HEPA filter testing as a core part of validation, not an optional extra.
This guide explains the standards, the tests, the DOP/PAO scan method and the acceptance limits in plain steps. It is written by the engineering team at Rayshen Environmental, a HEPA filter manufacturer in India that builds and tests filters for pharma HVAC and cleanroom projects.
Why HEPA Filter Testing Matters in a Cleanroom
A terminal HEPA filter is the last barrier between your supply air and your product. If it leaks, particle count rises, pressure cascades can be disturbed, and batches, wafers or research samples are put at risk. Regulators expect documented proof that filters work both at the factory and after installation.
Testing does three jobs:
Confirms efficiency: the filter media captures particles at the required level.
Confirms integrity: no leaks in the media, frame, sealant or gasket.
Confirms performance: airflow and pressure drop match the design, so the room keeps its air changes and classification.
This is why HEPA filter testing is part of GMP compliance and cleanroom qualification under ISO 14644.
EN 1822 and ISO 29463: The Standards Behind HEPA Filter Efficiency
Two standard families define how HEPA and ULPA filters are classified and tested. EN 1822 is the European standard that has long been the reference in India for HEPA filter manufacturing and testing. ISO 29463 is the international standard built on the same principles, and the two classification systems map closely to each other.
HEPA and ULPA Filter Classes at a Glance
EN 1822 classifies filters by their efficiency at the most penetrating particle size. The minimum values for common classes are:
| EN 1822 Class | Type | Overall (Integral) Efficiency | Local (Leak) Efficiency | Approx. ISO 29463 Equivalent |
| E11 | EPA | 95% | – | ISO 15 E |
| E12 | EPA | 99.5% | – | ISO 25 E |
| H13 | HEPA | 99.95% | 99.75% | ISO 35 H |
| H14 | HEPA | 99.995% | 99.975% | ISO 45 H |
| U15 | ULPA | 99.9995% | 99.9975% | ISO 55 U |
| U16 | ULPA | 99.99995% | 99.99975% | ISO 65 U |
Always confirm limits against the current edition of the standard for your project specification.
MPPS vs "99.97% at 0.3 Micron
You will often see "99.97% at 0.3 micron" quoted for HEPA filters. That is the older US definition, based on a fixed 0.3 micron test particle. EN 1822 and ISO 29463 are stricter in principle, because they test at the Most Penetrating Particle Size (MPPS).
MPPS is the particle size at which the filter media is least efficient. For glass fibre filter media it is typically around 0.1 to 0.3 micron, and it shifts with the media and the air velocity. Testing at MPPS gives the worst-case efficiency, which is more honest than a single fixed size. Particles larger or smaller than MPPS are captured even more effectively.
Types of HEPA Filter Tests
A HEPA filter is tested at more than one stage of its life. Here are the four tests you should know.
1. Factory Efficiency Test (MPPS)
The manufacturer determines the MPPS of the filter media and measures the filter's efficiency at that size using particle counters, following EN 1822. This test establishes the filter class (H13, H14, U15 and so on). For higher classes, each filter is individually tested.
2. Factory Leak (Scan) Test
Each filter is scanned across its face and frame with a probe while challenged with test aerosol. Any local penetration above the class limit means rejection. A good manufacturer ships every filter with its own test report so you can trace performance back to a serial number.
3. Pressure Drop and Airflow Test
The filter is run at its rated airflow and the resistance across the filter media is recorded in pascals. A mini pleat HEPA filter usually offers a lower initial pressure drop than older separator designs for the same face area, which saves fan energy and helps hold airflow steady as the filter loads. Pressure drop is also your best on-site indicator of when a filter is loading up and nearing replacement.
4. In-Situ Integrity Test with DOP/PAO
After installation, the filter is tested in place. This checks the whole assembly: media, frame, sealant, gasket and housing seal. It is the test regulators and auditors ask about most, and it is usually repeated at set intervals through the life of the cleanroom. ISO 14644-3 describes the installed filter leak test method.
DOP vs PAO: Which Test Aerosol Should You Use?
The leak test needs a measurable aerosol upstream of the filter. DOP (dioctyl phthalate) was the traditional choice. PAO (polyalphaolefin) is now the common replacement, because DOP has health and handling concerns and is restricted in many facilities.
| Factor | DOP | PAO |
| Full name | Dioctyl phthalate | Polyalphaolefin |
| Common use today | Older specs, legacy procedures | Standard in most new cleanroom projects |
| Health and safety | Handled with care; restricted in many facilities | Generally preferred for lower hazard |
| Generation | Thermal or cold (Laskin nozzle) | Cold generation (Laskin nozzle) |
| Detection | Aerosol photometer | Aerosol photometer |
| Acceptance principle | Same leak limit | Same leak limit |
Both work with an aerosol photometer. Use the aerosol your project specification names, and do not mix aerosols between tests without documenting the change.
Step-by-Step: How to Perform a HEPA Filter Leak Test
This is the standard scan-test sequence. Always follow your validated SOP and the current ISO 14644-3 method.
Check prerequisites. Confirm the HVAC system is balanced and running, the room is at its design airflow, and the filters are fully installed.
Calibrate the equipment. Use a calibrated aerosol photometer and aerosol generator, with valid certificates.
Measure airflow. Record the airflow through each filter to confirm it is within the design range.
Set up the aerosol challenge. Introduce the aerosol upstream of the filter so the concentration is stable and uniform. A typical upstream challenge is in the range of 10 to 20 µg/L, but follow your specification.
Take the upstream reference. Measure the upstream concentration and set the photometer to 100% reference.
Scan the downstream face. Move the probe in overlapping strokes across the whole filter face at a slow, steady speed, keeping it a few centimetres from the filter surface. Typical guidance is a scan speed of no more than about 5 cm/s.
Scan the frame and seals. Pass the probe along the frame, gasket and the filter-to-grid joint.
Record any leaks. Mark the location and the peak reading of any hit.
Repair and retest. Seal the defect only if permitted by the specification, then retest the repaired area.
Document everything. Record filter IDs, airflow, aerosol, challenge concentration, results and operator details for the validation report.
Acceptance Criteria and Why HEPA Filters Fail Validation
For a standard scan test, the commonly applied limit is that downstream penetration must not exceed 0.01% of the upstream concentration at any point. Some specifications set different limits for specific zones, so always use the number in your approved protocol.
Common Causes of Failure
Gasket or sealant leaks: poor seating, uneven compression or gaps at the grid joint.
Damaged media: pinholes from handling, impact or poor transport packaging.
Frame defects: gaps or cracks between the frame and the filter pack.
Housing leaks: the problem is in the terminal housing, not the filter.
Uneven aerosol or airflow: poor upstream mixing causes false results.
Dirty or loaded filters: filters run past their service life lose reliability.
Most failures are caused by handling or installation, not by the filter media itself. Careful unpacking and installation by trained crews prevent most re-tests.
How Often Should HEPA Filters Be Tested?
Testing frequency depends on your cleanroom grade, regulatory framework and risk assessment. As a common practice:
After installation or replacement: always.
Grade A and B / ISO 5 areas in pharma: typically every six months.
Grade C and D / ISO 7 and 8 areas: typically every twelve months.
After any major maintenance, relocation or repair: retest.
Replacement is driven by pressure drop, test results and airflow loss. Many facilities replace a filter when the pressure drop reaches roughly twice the initial value, or sooner if it fails an integrity test. Follow the manufacturer's guidance and your own SOP.
Choosing the Right HEPA Filter for Your Cleanroom
Testing is easier when you start with the right filter. Here is what to weigh for a HEPA filter for cleanroom use.
Mini Pleat HEPA Filter vs Separator Type
A mini pleat HEPA filter uses closely folded glass fibre media held apart by thin beads of thermoplastic or similar spacers. It has a compact depth, a lower pressure drop and a lighter weight, which suits ceiling-mounted terminals and fan filter units. Separator type filters use corrugated aluminium separators between deep pleats, which gives higher dust holding capacity and tolerance of higher-temperature or humid conditions. Choose based on space, airflow, temperature and cleaning needs.
Explore our mini pleat HEPA filters and the full HEPA filter range.
When to Use a ULPA Filter
A ULPA filter (U15 and above) gives much higher efficiency at MPPS and is chosen for semiconductor fabs, advanced microelectronics and some critical aseptic processes. The trade-off is a higher pressure drop and higher cost, so use ULPA only where the process truly needs it. For most pharma and biotech rooms, H13 or H14 is the right grade.
What Affects HEPA Filter Price?
The HEPA filter price depends on several factors, so be careful comparing quotes line by line:
Filter class (H13, H14, U15)
Size, depth and face area
Filter media type and pleat design (mini pleat or separator)
Frame material (aluminium, stainless steel, galvanised) and gasket type
Individual test report and scan-test documentation
Airflow rating and pressure drop at rated flow
Order quantity and customisation
A very low price often signals thin documentation, untested filters or inferior media. Ask for the test report before you compare cost.
Why Buyers Choose Rayshen Environmental
Rayshen Environmental Private Limited manufactures HEPA filters, fan filter units, air showers, pass boxes, laminar airflow units, biosafety cabinets, cleanroom panels and modular cleanroom systems from our facility in Greater Noida. We supply pharmaceutical manufacturers, biotech firms, HVAC consultants, cleanroom contractors, EPC companies, solar and medical device producers and semiconductor facilities across India.
Direct manufacturer: transparent pricing and direct access to engineers.
Factory-tested filters: test documentation supplied so your validation team has what it needs.
Custom sizes and specifications: built around your housing, grid and airflow.
Support through validation: guidance for installation, leak testing and audits.
Need a HEPA filter specification reviewed or a quote? Contact Rayshen Environmental and our team will help you choose the right class, size and test documentation.
Frequently Asked Questions
Q1. What is the difference between DOP and PAO testing?
Both are test aerosols used to challenge a HEPA filter upstream during an integrity test. DOP is the traditional material, while PAO is the common modern alternative with fewer handling concerns. The test method and the leak limit are essentially the same.
Q2. What does MPPS mean in HEPA filter testing?
MPPS is the Most Penetrating Particle Size, the size at which a filter is least efficient. EN 1822 and ISO 29463 measure efficiency at MPPS to give a worst-case result.
Q3. Is a H14 filter the same as 99.97% at 0.3 micron?
No. The 99.97% at 0.3 micron figure is the older US definition of HEPA. A H14 filter must reach at least 99.995% at its MPPS under EN 1822, which is a stricter and more complete test.
Q4. What is an acceptable leak in a HEPA filter scan test?
The commonly used limit is a downstream penetration of 0.01% of the upstream concentration. Your approved protocol or regulatory requirement sets the final number.
Q5. Can I repair a leaking HEPA filter?
Small leaks at the frame or seal can often be repaired with approved sealant if your specification allows it, followed by a retest. Leaks in the filter media usually mean the filter must be replaced.
Conclusion
HEPA filter testing is how you turn a good filter into a validated one. Understand EN 1822 and ISO 29463 classes, test at MPPS, run a proper PAO or DOP scan test after installation, monitor pressure drop and airflow, and keep clear records. Do that and your cleanroom stays compliant and your product stays protected.
Looking for a reliable HEPA filter manufacturer in India? Talk to Rayshen Environmental for tested filters and full documentation support.
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