Leak Testing HEPA Filter Housings: DOP, PAO and Challenge Aerosols

When a HEPA system is installed, the filter element is only part of the picture. If the housing, gasket, frame or access doors leak, air can bypass the filter and compromise the performance you expected from the system. That is why integrity testing matters on site, not just filter selection on paper.

For mechanical contractors and facility managers, understanding DOP, PAO and other challenge aerosols helps you specify, install and maintain the system with confidence. The key point is simple: a well-sealed housing is just as important as the HEPA filter itself.

Why installed HEPA systems need integrity testing

A factory test certificate tells you the filter element met its rated performance under controlled conditions. Once that filter is fitted into a housing, the system has new potential leak paths that were not part of the original test environment.

Those leak paths can include:

• •Gasket interfaces between the filter and housing
• •Damaged frames or distorted holding clamps
• •Access doors or service panels that do not seal properly
• •Penetrations around probes, dampers or duct connections
• •Installation issues such as uneven seating or overtightening

Integrity testing checks the complete installed assembly, not just the filter media. That is especially important where the air stream must pass through the HEPA stage without bypass.

For larger commercial and specialist applications, site requirements should be confirmed with the mechanical services engineer or building owner, and aligned with the relevant project specification and standards such as AS 1668.2:2024 where applicable.

DOP, PAO and challenge aerosols explained

The terms DOP and PAO are often used in the same conversation, but they are not exactly the same thing. Both relate to aerosol challenge testing, which introduces a controlled test aerosol upstream of the filter to look for leakage downstream.

DOP

DOP stands for dioctyl phthalate, a traditional test aerosol used in filter leak testing. It has been widely referenced historically, but there are important handling and safety considerations, so modern testing practices often use alternatives depending on the site, equipment and procedures in place.

PAO

PAO stands for polyalphaolefin. It is commonly used as a replacement challenge aerosol in many HEPA integrity testing procedures. The exact test method depends on the equipment and the application, but the principle is the same: introduce the challenge aerosol upstream and measure whether any escapes downstream.

Other challenge aerosols

Some testing programmes use other approved challenge aerosols or photometric methods suited to the site and the filter system. The important point is not the brand name of the aerosol, but whether the method is appropriate for the installed system and carried out by a competent technician.

If you are planning a new installation or upgrading an existing system, it can help to discuss the housing design and filter selection together. Total Air Filtration can supply suitable products from our range at our products or assist with custom air filters where the application calls for a specific size or configuration.

Why the housing seal matters as much as the filter element

A HEPA filter can only do its job if the air is forced through the media. If air finds an easier route around the filter, the system may still be running, but the filtered air stream is no longer what the installation was designed to deliver.

This is why housing integrity deserves the same attention as the filter rating. In the field, leakage often comes from small but important details:

• •Gasket compression that is too light, uneven or damaged
• •Poor alignment between the filter and the clamping frame
• •Worn seals on doors, access covers or scanning sections
• •Service access left open or reinstalled incorrectly
• •Unsealed modifications made after commissioning

For mechanical contractors, this means installation quality is not just about fitting the filter correctly. It is also about ensuring the housing is clean, square, undamaged and prepared for a reliable seal.

For facility managers, it means routine visual checks should include the condition of the housing, not only the filter face. If there is dust tracking around the frame, rattling panels or evidence of distortion, that is worth investigating before the next integrity test.

What a proper leak test can show

A well-executed challenge aerosol test can help identify whether leakage is occurring through the filter media, the gasket line or the housing itself. That distinction matters because the fix is different in each case.

A test may reveal:

• •A damaged filter that needs replacement
• •A filter seated incorrectly in the housing
• •A gasket issue that can be corrected by reseating or replacing the seal
• •A housing fault that may need repair or rework
• •A service access issue that is preventing a proper seal

This is one reason integrity testing is best approached as part of the whole system, not as a standalone filter exercise. If a scan test or other approved method shows a leak, the next step is to identify where the bypass is occurring and correct that point before returning the system to service.

If your site also needs ongoing maintenance support, filter servicing and commercial filter servicing can help keep the system moving in the right direction.

Practical points for contractors and facility managers

Good results start before the test technician arrives. A little preparation can make testing smoother and findings easier to interpret.

Useful checks include:

• •Confirm the correct filter type, size and rating for the housing
• •Inspect the housing for dents, warping, corrosion or damaged seal surfaces
• •Make sure the filter is seated fully and evenly
• •Check that access doors, clamps and fasteners are secure
• •Remove dust and debris from sealing surfaces before installation
• •Ensure any test ports or sampling points are fitted as intended

Inspection frequency depends on the system, site conditions and dust load. Follow the filter and equipment manufacturer instructions, and monitor pressure drop where gauges are fitted so you know when the system is changing.

If you are measuring replacements or checking whether a filter will fit a particular frame, our guide on how to measure an air filter can help avoid sizing mistakes that lead to poor sealing.

HEPA ratings and why the exact filter matters

For HEPA filters, the performance level is commonly described under EN 1822. H13 filters are rated at greater than or equal to 99.95% efficiency at the most penetrating particle size, while H14 filters are rated at greater than or equal to 99.995% at the most penetrating particle size.

That rating tells you about the filter element itself. It does not tell you whether the installed housing is sealed properly. A high-performing filter in a poor housing can still leave you with an installation that does not perform as intended.

It is also worth remembering that MERV, EN 779 and ISO 16890 do not convert directly. In general commercial work in Australia, G3/G4/F5-F9 are still commonly referenced EN 779 classes even though ISO 16890 has superseded that system. For any site-specific interpretation, check the project documentation and talk with the mechanical services engineer or building owner.

Getting the housing, filter and test method aligned

Leak testing works best when the filter specification, housing design and test method are all considered together. That is particularly important on critical systems, but it also helps on standard commercial installations where reliable performance and easier maintenance matter.

If you are planning a new system, upgrading an existing one or troubleshooting a suspected bypass issue, Total Air Filtration can help you choose the right filter or build to the job. We are a family-owned air filter manufacturer and supplier in Hillcrest, Brisbane QLD, established 2009.

For product advice, service support or a site-specific enquiry, call 07 3800 3448 or request a quote. You can also contact us to discuss the application or explore industries we supply.