Integrity Testing of HEPA Filter | DOP Testing | PAO Testing

Integrity Testing of HEPA Filter | DOP Testing | PAO Testing

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This article is about the Integrity Testing of HEPA Filter | DOP Testing | PAO Testing. 

DOP testing is Dispersed Oil Particulate testing, also known as filter integrity testing, is the process in which the integrity of your HEPA (High-Efficiency Particulate Air) filter is challenged through introducing particulates and measuring the output. To read more about HEPA filters, Click Here…

DOP substance is oil-based and Dioctyl Phthalate has been used. Dioctyl Phthalate is a liquid chemical that produces mono or polydispersed test aerosol of sub-micron particles, generated to challenge the integrity of HEPA filters.

Concerns regarding the carcinogenic properties of this substance have prompted the use of other substances such as Poly Alpha Olefin. Nowadays, DOP has been replaced with PAO (Poly Alpha Olefin). Poly Alpha Olefin is a non-carcinogen liquid that is used in HEPA filter integrity testing.


To ensure that the HEPA filter installed in the area is not damaged during operation so that there is no leakage from the filter or any other joint gaskets, frames, etc.

Equipment Needed:

  • DOP Smoke generator
  • Aerosol photometer (duly calibrated with national traceability)


  1. Introduce the DOP cloud/aerosol as a challenge agent into the upstream side of the HEPA filter system with a range of 20- 80% concentration.
  2. Detect the concentration in the photometer and set the photometer upstream concentration to 100%.
  3. Once 100% setting is established, turn the instrument knob downstream.
  4. Keep the scanning probe one inch or 25mm below the HEPA filter grill, and start scanning the HEPA filter face area and fitment of the HEPA filter with frame.
  5. If any leak is observed, then take out the grill and reset it. Assure that the leakage is coming through the fitment part or HEPA filter itself.
  6. If fitment leakage is there, tightening the HEPA filter fitment with a pressure plate can arrest it and if the HEPA filter leakage is still there, the joint can be sealed by silicone sealant, but it should not exceed 3% of the total surface area.
  7. If downstream leakage still exceeds 0.01%, replace the HEPA filter with new.
  8. The results shall be recorded in the datasheet.

Points to be Taken Care of:

  1. The upstream concentration shall be within the range of 20 -80%.
  2. The scanning speed should not exceed 10 ft/min. 
  3. The downstream penetration should not be more than 0.01% of the upstream concentration.
  4. In case if the sealed area is more than 5% and/or any individual sealed area is more than 1 in2 then the filter should be rejected and replaced with the new one.
  5. Cleanroom HEPA filters should be tested once a year and HEPA filters of LAF should be tested every 6 months.

So, friends, this was all about Integrity Testing of HEPA Filter | DOP Testing | PAO Testing. HEPA integrity test is done during PQ (Performance Qualification) phase.

Now, tell me why upstream concentration shall be within the range of 20 -80%, why not 100%?

Let me know your answer through your comments. If you have any questions or concerns, let me know in the comment section below.

Thank you everyone for your precious time.

Stay Home! Stay Safe!


To read more HVAC related articles, Click Here…


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