Start with the claim, not the acronym
- HEPA and ULPA filters
- HEPA and ULPA describe high-efficiency particulate filters, but each term should be connected to a named test method or classification framework. EPA describes HEPA as a pleated mechanical air filter and references the Department of Energy definition of high efficiency particulate air. IEST-RP-CC001 covers HEPA and ULPA filter units and provides a basis for agreement between customers and suppliers.1,3
In many U.S. indoor-air discussions, HEPA is shorthand for removing at least 99.97 percent of particles at 0.3 micrometer, with that size used as a worst-case MPPS reference. However, not every high-efficiency filter qualifies as a HEPA filter. This shorthand also does not establish whether a ULPA-class element, installed device, cleanroom module, or semiconductor tool interface was tested under the appropriate standard and flow condition.1,2,3
ULPA comparisons require closer attention to the test method. ISO 29463-1 classifies high-efficiency filters using performance determined through the ISO 29463 test series, which includes methods for filter media, leakage, and filter elements. The practical comparison is not simply HEPA versus ULPA. It must also identify the standard, filter format, airflow, particle counter, and acceptance endpoint used.4,5,6
| Question | Why it matters | Evidence to request |
|---|---|---|
| Is the claim HEPA, ULPA, or cleanroom cleanliness? | Each frame can point to a different standard, sample path, and report language. | Named standard, filter class target, tested configuration, and pass-fail endpoint |
| Is the sample media, an element, or a device? | Flat media, framed filters, installed modules, and ducted systems can have different bypass and seal risks. | Fixture drawing, seal approach, airflow, pressure drop, and upstream/downstream sampling plan |
| Does the claim include particles below 100 nm? | Nanoparticle behavior can sit outside ordinary cleanroom classification and MERV-style particle bands. | Added 5 to 100 nm detection plan, instrument range, dilution, background, and counting statistics |
| Will the result support certification language? | A laboratory test report is not automatically a product listing or certification. | Clear report wording separating test evidence, standards alignment, deviations, and certification limits |
MPPS is the method anchor
High-efficiency filter testing usually begins with the most penetrating particle size, or MPPS. ISO 29463-5 specifies methods for determining filter efficiency at MPPS and provides guidance for filters with an MPPS below 0.1 micrometer. For that reason, ULPA-class testing may require measurement planning at the nanometer scale.1,6
ISO 29463-2 is also relevant because high-efficiency filter testing depends on controlled aerosol production, suitable measuring equipment, and appropriate particle-counting statistics. When downstream particle counts are low, the report should document background control, challenge stability, counter range, sampling times, and the treatment of low-count uncertainty.5,9
- Use an ISO 29463 or EN 1822 frame when the claim is high-efficiency filter classification, MPPS penetration, or HEPA/ULPA element evidence.4,6
- Use paired upstream and downstream particle measurements to calculate penetration or efficiency under the stated flow and fixture condition.5,9
- Record pressure drop beside efficiency because a lower-penetration filter can also create a higher airflow resistance in the tested configuration.4,9
- Document deviations when a purifier, duct module, wafer-tool filter, or nonstandard housing cannot be represented as a standard filter element.3,4,9
Where 5 to 100 nm detection fits
Detection from 5 to 100 nm should be treated as a nanoparticle measurement extension, not a substitute for the named HEPA or ULPA standard. ARE Labs can measure particle-size distributions in the nanometer range using fast mobility sizing. Its filtration efficiency service compares upstream and downstream CPC or OPC measurements and reports pressure drop.9,10
This extension is most relevant when a claim, process risk, or customer specification depends on particles below 0.1 micrometer. ISO 29463-5 specifically includes testing and classification guidance for filters with an MPPS below 0.1 micrometer. In contrast, ISO 14644-1 states that its particle-concentration classification range for cleanrooms does not cover ultrafine particles below 0.1 micrometer.6,7,10
| Use case | Method concern | Added evidence |
|---|---|---|
| ULPA element or media development | MPPS or penetration may sit in the nanometer-scale region for the tested media. | 5 to 100 nm particle-size window, upstream/downstream counts, and penetration by size |
| Cleanroom supply module or fan-filter unit | Installed seals, bypass, and flow distribution can differ from media-only performance. | Fixture-specific filtration efficiency, pressure drop, and optional nanometer-scale downstream detection |
| Semiconductor wafer manufacturing support | Sub-100 nm particles can matter to process-control decisions even when ordinary cleanroom classification is reported separately. | Nanoparticle concentration trend, background subtraction, device state, and particle-size distribution |
| Air-treatment device with a high-efficiency filter | Fan speed, housing leakage, and re-entrainment can change installed performance. | System-level upstream/downstream measurements plus device particle emissions context when needed |
Cleanroom and semiconductor context
Cleanroom projects add another layer of evidence. ISO 14644-1 classifies air cleanliness in cleanrooms and clean zones by airborne particle concentration, using threshold particle sizes from 0.1 micrometer to 5 micrometers. Particles below 0.1 micrometer fall outside that classification range.7
ISO 14644-3 provides test methods for cleanroom and clean-zone performance parameters, with procedures that vary by room type and occupancy state. Cleanroom classification and filter testing are therefore related but distinct. Cleanroom testing evaluates air cleanliness in the controlled environment, while filter testing evaluates a filter, filter media, or device path under defined conditions.4,7,8
- For cleanroom classification, define the room or clean zone, occupancy state, particle-size thresholds, sampling locations, and ISO 14644 reporting basis.7,8
- For a HEPA or ULPA filter element, define the sample, seal, flow, aerosol challenge, MPPS or size range, and pressure-drop endpoint.4,5,9
- For semiconductor or wafer-tool questions, decide whether the result needs a sub-100 nm particle count, a filter penetration curve, a cleanroom particle map, or a device-emissions check.7,9,10
- For installed air-treatment devices, separate media capture from housing bypass and particles introduced by fans, motors, ionizers, or process-air interactions.9,10
How to scope the test path
A practical HEPA versus ULPA study begins with the decision the report must support. A product team may need to compare media, qualify a supplier, measure filter-element penetration at MPPS, troubleshoot a cleanroom, evaluate installed device performance, or screen semiconductor contamination-control performance. Each objective affects the fixture, aerosol, instrument set, replicate plan, and reporting language.3,4,5,6,9,10
- Send the filter format, dimensions, media type, gasket or seal design, intended airflow, target standard, and claim language before the method is selected.3,4,9
- State whether the endpoint is overall efficiency, penetration by size, local leak context, pressure drop, cleanroom particle classification, or 5 to 100 nm nanoparticle behavior.4,6,7,10
- Choose counters and dilution around the expected concentration range because low downstream particle counts require attention to background and counting statistics.5,9
- Decide in advance whether the report should avoid certification wording and instead present standards-aligned evidence, deviations, and method limits.3,4,9