HEPA Filter DOP/PAO Testing and Leak Testing Explained
DOP/PAO testing is a leak test that determines whether a HEPA filter leaks through its media, its frame or its mounting gasket, by introducing an aerosol upstream and scanning the downstream face. A controlled aerosol concentration is fed to the upstream side, the downstream surface is scanned with a probe, and any point where penetration exceeds 0.01% is recorded as a leak. This is the only method that proves the efficiency certified in the laboratory is actually achieved on site.
This article explains the difference between DOP and PAO, the test equipment, the scanning method, acceptance criteria and repair rules.
Why Is Leak Testing Necessary?
An H14 marking on an EN 1822 certificate does not guarantee H14 performance in the field. Leaks can appear at three distinct points:
- In the media: micro tears or pinholes caused during transport or installation
- At the media to frame joint: voids or cracks in the potting compound
- Between frame and housing: insufficient clamping, aged gaskets, distorted housing
Consider the scale: total permitted penetration for an H14 filter is only 0.005%. A single pinhole sized leak in the filter face can raise total penetration to many times that figure. Sealing can therefore matter more than the filter class itself, a point we also cover in our filter classes guide.
The Difference Between DOP and PAO
Both are liquid oils used to generate the test aerosol. The difference lies in their chemistry and safety profile.
| Property | DOP | PAO (Emery 3004) |
|---|---|---|
| Chemistry | Di-octyl phthalate | Poly-alpha-olefin |
| Health assessment | Abandoned over suspected carcinogenicity | Considered safe, current standard |
| Status | Largely discontinued | Widely used today |
| Particle size | Around 0.3 µm (cold generation) | Around 0.3 µm (cold generation) |
Although the term "DOP test" persists out of habit, the aerosol used in practice is usually PAO. The accurate term is therefore DOP/PAO test or aerosol leak test.
Test Equipment
- Aerosol generator: cold generator with Laskin nozzles, or a thermal generator. Cold generators produce a polydisperse aerosol averaging around 0.3 micron.
- Photometer: works on light scattering and displays penetration as a percentage. The most common instrument for field testing.
- Particle counter: counts particles individually by size. Preferred at low aerosol concentrations and for ULPA class filters.
- Scanning probe: a rectangular intake, typically 25-50 mm wide, traversed parallel to the filter face.
- Anemometer: for verifying face velocity and airflow.
How the Test Is Performed
1. Verify airflow
The test must be performed at the filter design airflow. Below design, leaks are masked; above it, false alarms are generated. Face velocity is measured with an anemometer first and airflow confirmed.
2. Set the upstream aerosol concentration
Aerosol is introduced uniformly on the upstream side of the filter. Typical upstream concentration is 20 to 80 µg/L. That concentration is first read on the photometer as the 100% reference, and all leak percentages are calculated against it.
3. Scan the downstream face
The probe is held about 25 mm from the filter face and the surface is scanned in overlapping strips. Scanning speed must not exceed 5 cm per second; moving faster can prevent the instrument from registering a leak.
Areas that must be scanned:
- The entire media face, in overlapping passes
- The full length of the potting line between media and frame
- The gasket or gel seal line between frame and housing
- The housing joints and weld seams themselves
4. Evaluate and record results
Every detected leak is marked on a sketch with its penetration percentage and position. The test report records the filter serial number, test date, aerosol type, upstream concentration, instrument calibration details and measured values.
Acceptance Criteria and Repair Rules
| Condition | Measured Penetration | Decision |
|---|---|---|
| No leak | Below 0.01% | Filter accepted |
| Limited leak | 0.01% - 0.05% | Consider repair, then retest |
| Major leak | Above 0.05% | Filter rejected or replaced |
General repair limits:
- A single patch must not exceed 0.5% of the filter face area
- All patches together must not exceed 3% of the filter face area
- Patching applies only to the media; structural leaks in the potting compound are not repaired
- Every repaired area is rescanned to confirm the leak has been eliminated
Leaks between frame and housing are not a repair matter but an installation matter: clamping must be adjusted, the gasket renewed or the housing corrected. Use filter frame housings and gel seal frame options for a reliably sealed installation.
Factory Test Versus Field Test
| Criterion | Factory Test (EN 1822) | Field Test (ISO 14644-3) |
|---|---|---|
| Purpose | Certify the filter class | Find leaks after installation |
| Test particle | At MPPS, 0.1-0.25 µm | Polydisperse aerosol, around 0.3 µm |
| Scope | The filter alone | Filter, frame, housing and installation together |
| Output | Class label and serial number | Leak sketch and acceptance report |
| Requirement | Mandatory in production for H13 and above | At commissioning and periodic maintenance |
The two tests are not interchangeable. A certified filter that is badly installed will fail the field test. This is why cleanroom and hospital projects require both documents. We explain how to read a factory certificate in our article on how to identify a real HEPA filter.
How Often Should Testing Be Done?
Leak testing is carried out:
- At commissioning: after every new HEPA installation, without exception
- After filter replacement: every time a new filter is fitted
- Periodically: every 6 months in ISO 5 and GMP Grade A and B areas, every 12 months in ISO 7 and 8 areas
- After an event: a sudden fall in pressure drop, a rise in particle counts, or any intervention on the housing or ductwork
A sudden drop in pressure differential deserves particular attention, as it usually indicates a tear in the media. Rising pressure as the filter loads is normal; falling pressure is not. See our article on filter pressure drop and energy efficiency for monitoring guidance.
Frequently Asked Questions
What is a DOP test and how is it performed?
A DOP test is a leak test in which a controlled aerosol concentration is fed upstream of a HEPA filter and the downstream face is scanned with a probe. The probe is held about 25 mm from the filter and traversed in overlapping strips at no more than 5 cm per second. Points where penetration exceeds 0.01% are recorded as leaks.
What is the difference between DOP and PAO?
DOP is di-octyl phthalate and PAO is poly-alpha-olefin. DOP has largely been discontinued over suspected carcinogenicity and replaced by PAO, which is considered safe. Both generate a test aerosol of around 0.3 micron and the test method is identical.
How often should HEPA leak testing be performed?
Always after every new installation and every filter replacement. Periodically it is repeated every 6 months in ISO 5 and GMP Grade A and B areas, and every 12 months in ISO 7 and ISO 8 areas. It should also be repeated immediately if pressure drop falls suddenly or particle counts rise.
Can a leaking HEPA filter be repaired?
Limited leaks in the media can be patched. A single patch must not exceed 0.5% of the filter face area and all patches together must not exceed 3%. Structural leaks in the potting compound and leaks caused by housing installation cannot be patched; the installation must be corrected or the filter replaced.
Is a field test still needed if a factory certificate exists?
Yes. The factory certificate documents the efficiency of the filter itself, while the field test verifies that filter, frame, gasket and housing are leak free together. A certified filter that is badly installed will fail the field test, so the two documents do not replace each other.
Testing and Commissioning Support
At Safe Air Filtration every HEPA filter of class H13 and above is individually tested and certified with its own serial number, and we supply both gel seal and gasket frame options suited to leak free installation. For filter selection, housing compatibility and field test planning, contact us or review our HEPA range on the HEPA filters page.
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