“A HEPA filter can be the correct grade and still leak after it has been installed.”

A filter’s manufacturer rating describes the filter element under defined test conditions. It does not, by itself, show that the installed system has no leaks around the filter, its seals or its housing.

DOP testing is the common name for an in-situ integrity test that checks the installed HEPA or ULPA filtration system for leakage.

This guide explains what the term means, how the test works, when it may be needed and what the resulting report should tell you.

Quick Answer: What is DOP testing?

DOP testing is an in-situ filter integrity test used to find leaks in an installed HEPA or ULPA filtration system. A suitable test aerosol is introduced upstream of the filter. A calibrated instrument then checks downstream for penetration through the filter media or leakage around the frame, seals and housing. The term “DOP test” remains in common use even when the aerosol used is PAO, Ondina or another suitable substance rather than the original DOP compound.

Explore Velocity Safety’s DOP, PAO and HEPA filter integrity testing service.

Key Takeaways – DOP and HEPA Filter Integrity Testing

  • DOP originally referred to dioctyl phthalate, a substance historically used as the filter-test aerosol.
  • “DOP test” is still used as an industry name for in-situ HEPA and ULPA filter leak testing.
  • A modern test may use PAO, Ondina or another suitable aerosol; the substance used should be identified in the report.
  • The test examines the installed filter system, including potential leakage at seals, frames and housings.
  • It is different from a manufacturer’s test of the filter element’s efficiency or classification.
  • There is no single pass/fail limit or testing interval that should be applied to every filter installation.
  • Where filtration forms part of an exposure-control system, its testing should fit the relevant risk assessment and examination programme.

Jump to:

What Is DOP Testing?

DOP testing checks an installed HEPA or ULPA filter system for leaks. It is commonly used where filtered air is important to contamination control, worker protection or the performance of a process.

A test aerosol is introduced into the airflow before the filter. The engineer checks the downstream side using an appropriate calibrated instrument to find any aerosol passing through or around the installation.

Depending on the system and test method, leakage may be found at:

  • The filter media
  • The seal or gasket between the filter and its frame
  • The filter frame or its seating
  • Joints and penetrations in the housing
  • Other routes that allow air to bypass the filter

This is why a new, correctly classified filter can still fail an in-situ integrity test if it has been damaged or installed incorrectly.

A DOP or PAO integrity test should not be presented as a complete manufacturer efficiency-classification test. It answers a different question: is the installed filtration system leaking?

What Does DOP Test Stand For?

DOP originally stood for dioctyl phthalate, the substance historically used to produce the aerosol challenge for this type of test.

The name has remained familiar across the industry even though another suitable aerosol is often used today. You may also hear people describe the procedure as a dispersed oil particulate test, PAO test, HEPA integrity test or installed filter leak test.

Those terms can point to the same general purpose, but they do not identify the exact aerosol, instrument, test method or acceptance criteria. Those details should be confirmed for the particular system.

Is DOP Testing the Same as PAO Testing?

They usually refer to the same type of in-situ filter integrity assessment, but DOP and PAO are not the same substance.

PAO means polyalphaolefin. It can be used to create the upstream aerosol challenge. Other suitable test aerosols, including Ondina products, may also be used according to the equipment and procedure.

If a contractor or site specification asks for a “DOP test”, confirm:

  • Which aerosol will actually be used
  • Which equipment and method are appropriate
  • Whether the filter can be challenged safely upstream
  • How the downstream side will be accessed and checked
  • Which acceptance criteria apply

The name of the test alone is not enough to define its complete scope.

How Does a DOP or PAO Filter Integrity Test Work?

The exact procedure depends on the filter system and applicable method. A typical in-situ aerosol test follows these stages:

Stage What Happens Why It Matters
Pre-test review The engineer identifies the filter, airflow path, access points, test method and acceptance criteria. Confirms that the test can be performed safely and interpreted correctly.
Upstream challenge A suitable aerosol is introduced before the filter under controlled conditions. Provides the challenge needed to detect penetration or bypass leakage.
Downstream check A calibrated instrument is used to scan accessible areas or take measurements as required by the method. Locates leakage through the media, seals, frame or housing.
Result and report Results are compared with the agreed criteria, with defects and remedial actions documented. Provides a traceable result for maintenance and validation records.

Not every installation permits the same downstream scan or sampling arrangement. The engineer should identify any limitations in the report rather than imply that an inaccessible part of the system has been fully examined.

What Are the HEPA Filter Integrity Test Acceptance Criteria?

There is no single percentage that should be quoted as the universal pass/fail limit for every in-situ HEPA or ULPA installation.

The applicable criterion depends on the filter class, the installation, the test method, relevant standard and the facility’s requirements. For example, a cleanroom validation programme may refer to BS EN ISO 14644-3, while a different industrial filtration application may require another appropriate method.

The criterion should be agreed before the test and stated on the report. The result should then make clear whether each tested filter system met that criterion and where any leakage was found.

A filter’s factory efficiency rating and an installed-system leakage result are related, but they should not be treated as interchangeable measurements.

When Should You Book a DOP Test?

An integrity test may be appropriate:

  • After installation: To check that a new filter and its housing have been installed without unacceptable leakage.
  • After filter replacement: To identify damage or a poor seal before relying on the replacement.
  • After repairs or relocation: Where work could have affected the filter, frame or housing.
  • After suspected damage or a performance change: Where there is a reason to question containment or filtration.
  • During a planned validation or examination programme: At an interval appropriate to the system, application and risk.

Do not apply “every six months” or “every 6–24 months” to every HEPA filter without considering its application. The required interval may come from the site’s validation programme, manufacturer instructions, risk assessment, contract specification or other applicable guidance.

Where a HEPA filter forms part of a local exhaust ventilation system used to control exposure under COSHH, its integrity test may be coordinated with the wider LEV thorough examination and test. That does not make a DOP test identical to a full LEV examination.

What does COSHH actually require?

COSHH Regulation 9 requires relevant exposure-control measures to be maintained and engineering controls to be thoroughly examined and tested at the applicable intervals. It does not prescribe a DOP test for every HEPA filter. Where filter integrity is important to the control measure, an appropriate integrity test may form part of the evidence that the system continues to perform as intended.

What Equipment Can Be DOP or PAO Tested?

Velocity Safety provides in-situ integrity testing for suitable HEPA and ULPA filtration systems, including applications such as:

  • Cleanrooms and controlled environments
  • Negative pressure units and air scrubbers
  • HEPA-filtered dust extraction equipment
  • Filters within suitable LEV systems
  • Spray booths and process extraction systems
  • Laboratory and healthcare filtration systems
  • Suitable portable extraction units

Whether a particular unit can be tested using an aerosol challenge depends on its design, airflow, safe upstream injection and downstream access. An L-, M- or H-class label alone does not establish that a specific portable extractor contains an accessible HEPA filter suitable for this method.

If you are unsure about your equipment, provide its make, model, filter details and photographs when enquiring. Velocity Safety can confirm the practical test arrangement.

What Problems Can an Integrity Test Find?

An installed filter may leak because:

  • The filter media has been damaged during handling or use
  • A gasket is missing, compressed unevenly or incorrectly seated
  • The filter is the wrong size for its housing
  • The frame or housing has been damaged
  • A joint or penetration allows air to bypass the filter
  • Repairs or filter replacement have disturbed the installation

Finding the location matters. The appropriate remedy may be to reseat the filter, repair a seal or housing, or replace the filter. The system should then be re-tested as appropriate before the result is treated as resolved.

What Should a DOP Test Report Include?

A useful report should identify what was actually tested and how the result was reached. Depending on the system and method, it may include:

  • The site, equipment and filter identification
  • The test date and location
  • The aerosol used
  • The instrument and its calibration details
  • The test procedure and agreed acceptance criteria
  • The upstream challenge conditions or concentration
  • Downstream readings or scan results
  • Any leakage location and its measured result
  • Pass or fail outcome and any test limitations
  • Recommended remedial action and re-test result, where applicable

The report should not simply state “passed” without making the tested equipment, method and acceptance criterion clear.

Final Takeaway

A DOP test checks whether an installed HEPA or ULPA filtration system has unacceptable leakage. It can find faults in the filter media and around the seals, frame and housing that a filter’s manufacturer rating cannot rule out.

The correct aerosol, test method, pass/fail criterion and interval depend on the installation. Establish these before testing and retain the report as part of the system’s maintenance or validation records.

Need an installed HEPA or ULPA filter tested?

Velocity Safety provides DOP and PAO filter integrity testing for suitable cleanroom, extraction, negative pressure and other filtered systems. Tell us what equipment you have, where it is located and whether the filter is newly installed, due for periodic testing or suspected of leaking.

We can confirm the available test approach and provide a quotation.

📞 Call: 01254 933 486
📩 Email: info@velocitysafety.co.uk
🌐 Or contact Velocity Safety online.

Arrange DOP / PAO Filter Integrity Testing

Frequently Asked Questions

What does DOP stand for?

DOP originally referred to dioctyl phthalate, the substance historically used to create an aerosol for filter testing. The phrase “DOP test” remains common even when a different aerosol is used.

Is DOP testing the same as PAO testing?

They commonly describe the same general type of in-situ filter integrity test, but DOP and PAO are different substances. Ask which aerosol and test method will actually be used.

Is a DOP test the same as testing HEPA filter efficiency?

No. An in-situ integrity test looks for leaks in the installed filter and its surrounding assembly. A manufacturer’s efficiency or classification test measures the filter element under a defined procedure.

Is DOP testing legally required in the UK?

There is no blanket legal rule requiring a DOP test for every HEPA filter. Where filtration is part of an engineering control for hazardous substances, COSHH requires appropriate maintenance and examination of the control. An integrity test may be needed to demonstrate that the particular system remains effective.

How often should a HEPA filter be DOP tested?

There is no universal interval for every installation. Follow the applicable validation programme, risk assessment, manufacturer instructions and relevant standards. Testing should also be considered after installation, replacement, damage or repairs that could affect integrity.

Does a new HEPA filter need an integrity test?

Where the installation needs verified containment or cleanroom performance, testing after installation can reveal damage or leakage around the filter that a factory classification would not detect.

What happens if a filter fails?

The leak location and result should be recorded. The filter may need reseating or replacement, or the seal or housing may need repair. The affected system should be re-tested as appropriate after the remedial work.

Will I receive a report?

Yes. Velocity Safety supplies a traceable report identifying the tested system, procedure, results, outcome and any defects or recommended remedial action.

Resources & Further Guidance