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DOP Testing: Everything You Need to Know

You may have heard the term “DOP Test” used before, but you’re left feeling unsure of what DOP testing entails. In this article we explain the what, why, where, when and how, of these critical checks.

 DOP Tests are done to check the integrity and efficiency of HEPA (High Efficiency Particulate Arrestance) air filters. These filters are used in a variety of industrial equipment, such as negative air units, fume hoods and powerful vacuums. They protect people and the environment from harmful substances that may be present in the air, like bacteria, hormones, or asbestos, by filtering out a minimum of 99.97% of particulate that is up to 0.3 microns in size. Because HEPA filters play such a key role, it is essential that they work properly and are tested regularly to determine compliance.

 DOP stands for Dispersed Oil Particulate, and DOP tests blast controlled clouds of oil particles at HEPA filters to “challenge” them. Traditionally, Di-Octyl Phthalate has been used in the tests, but because of concerns about its carcinogenic qualities, Poly Alpha Olefin, which is non-carcinogenic, is now commonly being used instead. For this reason, DOP Tests are sometimes referred to as PAO Tests. Other liquids, such as paraffin oil, DEHS may also sometimes be used. In a DOP Test, the following steps will typically be taken.

First, there is a visual inspection of the equipment containing the HEPA filters, with a check for cracks, holes and any other damage or issues. The equipment is switched on, then the DOP Testing equipment is prepared. This equipment comprises of an aerosol generator, which is usually pneumatic or thermal.

The next step involves releasing a dense cloud of aerosol particles into the upstream air flow of the HEPA filters, the distance from the filter being critical to the testing accuracy. A sample of the air and aerosol mixture is taken to set the baseline defined by the ISO, VC or EN standard being applied. Then the media face and seal of the filters can be scanned, using a photometer, to assess whether there is any leakage. The maximum permissible leak is 0.01% or 0.03% dependant on the standard applied. A pass provides assurance that the air is clean and free of harmful contaminants.

The DOP Test is relatively quick and repeatable. It confirms that all components of the HEPA filters – the seals, housing, joints and actual filter media, are functioning correctly. If a filter fails the test, the problematic parts can be identified and replaced.

HEPA filters need to be checked regularly to ensure they are performing to industry standards. At the very least, DOP Tests, according to ISO, should be done every 24 months and Bio-safety cabinets are done at 6 monthly intervals; though recommendations vary by country. If equipment is moved or used in a new project, additional tests need to be run to prove no damage was done and that the unit is still in compliance.

It is important to ensure that the DOP Test provider’s equipment conforms to accepted standards, is well-serviced, and is correctly calibrated. It is also necessary to check that the technicians performing the tests are suitably qualified and experienced.

DOP Tests have been used worldwide since the 1950s and are listed as a first choice for filter checking in ISO 14644-3: 2005 standards. Businesses should not be tempted to skip DOP Tests; they ultimately save money and provide protection, including that of company reputations, in the long run.


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