What Are DOP and PAO?
Two aerosol agents have dominated HEPA filter in-place testing across different eras of nuclear air cleaning practice. Understanding what each is — and why one replaced the other — is essential background for any nuclear health physicist, facilities engineer, or air cleaning system operator responsible for HEPA filter qualification.
Why DOP Was Replaced: Carcinogenicity and OSHA
DOP/DEHP was the original aerosol challenge agent for HEPA filter testing from its development in the 1940s military filtration programs through the 1990s. Its replacement by PAO resulted from a convergence of toxicological evidence and regulatory pressure.
The Health Risk Evidence
The International Agency for Research on Cancer (IARC) evaluated DEHP (DOP) and classified it as Group 2B — Possibly Carcinogenic to Humans based on sufficient evidence of carcinogenicity in experimental animals (primarily hepatocellular adenomas and carcinomas in rodent studies at high doses) and inadequate evidence in humans. This classification, while not establishing definitive human carcinogenicity, was sufficient to drive precautionary action in nuclear facilities — environments with already stringent chemical exposure standards.
OSHA established a Permissible Exposure Limit (PEL) of 5 mg/m³ for DOP as an oil mist. In the context of HEPA filter testing — where technicians work in close proximity to the aerosol generation equipment and downstream sampling zones — maintaining exposures below PEL with DOP requires significant engineering controls.
PAO as a Direct Technical Equivalent
The reason PAO (specifically Emery 3004, the accepted nuclear standard formulation) is an effective DOP replacement is its physical similarity as an aerosol test medium:
- Identical target MMAD: Both generate aerosols with mass median aerodynamic diameter (MMAD) of approximately 0.3 μm — the Most Penetrating Particle Size (MPPS) for HEPA media
- Similar refractive index: Critical for photometer calibration; DOP and PAO have comparable refractive indices, meaning existing photometers calibrated for DOP produce equivalent readings with PAO aerosol
- Same generation method: Both are generated using Laskin nozzle aerosol generators at 20–30 PSI compressed air supply — no equipment change required
- Same forward scattering properties: The light scattering behavior used by forward scattering photometers is essentially equivalent between DOP and PAO at 0.3 μm
DOP vs PAO: What Changes, What Stays the Same
| Parameter | DOP (DEHP) | PAO (Emery 3004) | Changed? |
|---|---|---|---|
| Test aerosol particle size (MMAD) | 0.3 μm | 0.3 μm | No change |
| Acceptance criterion (nuclear) | ≤0.03% penetration | ≤0.03% penetration | No change |
| Upstream challenge concentration | 10–100 μg/L | 10–100 μg/L | No change |
| Probe scan speed | 2 in/sec (5.08 cm/s) | 2 in/sec (5.08 cm/s) | No change |
| Probe aperture | 1 inch (25 mm) | 1 inch (25 mm) | No change |
| Probe-to-filter distance | 1 inch (25 mm) | 1 inch (25 mm) | No change |
| Generation equipment (Laskin nozzle) | 20–30 PSI | 20–30 PSI | No change |
| Test instrument (photometer) | Forward scatter photometer | Forward scatter photometer | Verify calibration |
| IARC carcinogenicity | Group 2B | Not classified | Improved (safer) |
| OSHA PEL | 5 mg/m³ | None established | Improved (safer) |
| Regulatory acceptance (NRC) | Legacy | Accepted | PAO accepted |
| Flash point | 215°C | >150°C | Both non-flammable at test conditions |
| Aerosol fluid cost | Lower | Similar | Slight PAO premium |
Other Aerosol Alternatives: DEHS and More
While PAO (Emery 3004) is the dominant alternative in nuclear applications, other aerosol agents are used in different contexts:
| Aerosol Agent | Full Name / CAS | MMAD | Primary Use | Nuclear Accepted? |
|---|---|---|---|---|
| PAO (Emery 3004) | Polyalphaolefin · 68037-01-4 | 0.3 μm | Nuclear, defense, semiconductor | ✅ Yes (NRC accepted) |
| DEHS | Di-Ethyl-Hexyl-Sebacate · 122-62-3 | 0.3 μm | Hospital, cleanroom, ISO 14644 | ⚠️ Facility-specific |
| PEG-400 | Polyethylene glycol · 25322-68-3 | 0.3–0.5 μm | Healthcare, pharmaceutical cleanrooms | ❌ Not standard for nuclear |
| Corn oil | Natural triglyceride mixture | Variable | Some cleanroom applications | ❌ Not acceptable for nuclear |
| DOP (DEHP) | Di(2-ethylhexyl) phthalate · 117-81-7 | 0.3 μm | Historical nuclear and military | ❌ Superseded by PAO |
For nuclear applications subject to NRC RG 1.52 and ANSI N509/N510, use PAO Emery 3004 as the test aerosol. This is the formulation specifically validated as a DOP replacement for nuclear HEPA testing. DEHS may produce slightly different photometer readings and requires facility-specific validation if used in a nuclear context. Maintain lot documentation for the PAO fluid in your QA records.
Nuclear HEPA Filter In-Place Test Procedure (ANSI N510 / IEST-RP-CC001)
The following procedure applies to in-place efficiency testing of installed HEPA filters in nuclear air cleaning systems per ANSI N510 and IEST-RP-CC001. This is a condensed field procedure guide — always refer to the actual standard document and your facility's specific procedures for definitive requirements.
Acceptance Criteria by Application
| Application | Standard | Max Penetration | Min Efficiency | Notes |
|---|---|---|---|---|
| Nuclear power plant (GDC-41) | NRC RG 1.52 / ANSI N510 | ≤0.03% | ≥99.97% | At MPPS (0.3 μm); applies per 10 CFR 50 App. A |
| Nuclear D&D, hot cell, decommissioning | ANSI N509 / N510 | ≤0.03% | ≥99.97% | Same criterion as NPP; portable cart in-place test |
| Hospital AII / surgical suite (ASHRAE) | ASHRAE 170 / 241 | ≤0.03% | ≥99.97% | H13 HEPA minimum; H14 for bone marrow transplant |
| HEPA H13 (EN 1822, commercial) | IEST-RP-CC001 / EN 1822 | ≤0.05% | ≥99.95% | Less stringent than nuclear; not for NRC-regulated systems |
| HEPA H14 (EN 1822, commercial) | IEST-RP-CC001 / EN 1822 | ≤0.005% | ≥99.995% | Used for oncology, pharma GMP, semiconductor |
| ULPA U15 (EN 1822) | EN 1822 | ≤0.005% peak / ≤0.001% avg | ≥99.999% | Semiconductor fab; not standard for nuclear |
H13 per EN 1822 requires ≥99.95% efficiency (≤0.05% penetration). The NRC criterion for nuclear power plants is ≤0.03% penetration (≥99.97%). This is a meaningful difference: a filter that passes the commercial H13 standard at 99.96% efficiency would fail the NRC ANSI N510 in-place test at 0.04% penetration. All nuclear air cleaning HEPA filters for NRC-regulated systems must meet the ≤0.03% nuclear criterion, not the H13 commercial standard.
Test Instruments: Photometer vs Optical Particle Counter
Two primary instrument types are used for HEPA in-place testing. The choice affects test speed, sensitivity, equipment cost, and regulatory documentation.
For nuclear in-place scanning per ANSI N510, the forward scattering photometer is the primary method. The continuous analog readout is ideal for the 2 in/sec scan traverse — the tester can observe instantaneous penetration in real time and identify leak locations precisely. The OPC requires a stationary dwell time at each sample point, making comprehensive filter face scanning slower and less practical for in-place work. Use the photometer for in-place scanning; use the OPC for overall efficiency verification and particle size characterization when required by the facility's QA program.
NRC Regulatory Requirements for HEPA Testing
Nuclear Regulatory Commission requirements for HEPA filter in-place testing are specified in Regulatory Guide 1.52 Rev. 4 and the associated standards (ANSI N509, ANSI N510). Key regulatory requirements:
| Requirement | NRC Specification | Source Document |
|---|---|---|
| Test aerosol | PAO (Emery 3004) or equivalent at 0.3 μm MMAD | NRC RG 1.52 Rev.4 / ANSI N510 |
| Challenge concentration | 10–100 μg/L upstream | ANSI N510 Section 9 |
| Probe scan speed | ≤2 in/sec (5.08 cm/s) | IEST-RP-CC001 / ANSI N510 |
| Acceptance criterion | ≤0.03% penetration at MPPS | NRC RG 1.52 Rev.4 |
| Initial test trigger | After installation in system housing | ANSI N509 Chapter 7 |
| Maintenance test trigger | After any maintenance affecting filter integrity | NRC RG 1.52 / ANSI N509 |
| Surveillance test frequency | Annually at minimum | NRC RG 1.52 Rev.4 |
| Post-event test trigger | After flooding, fire, chemical exposure, high ΔP event | ANSI N509 Chapter 7 |
| Documentation retention | Per facility QA program / 10 CFR 50 App. B | 10 CFR 50 Appendix B |
| Tester qualification | Per facility radiation protection program | Facility RWP / QA program |
Technical Glossary
- DOP — Dioctyl Phthalate (DEHP)
- Di(2-ethylhexyl) phthalate (CAS 117-81-7). Original HEPA test aerosol; IARC Group 2B carcinogen. Replaced by PAO in nuclear and most other applications. Legacy term "DOP test" is still used colloquially even when PAO is the actual aerosol.
- PAO — Polyalphaolefin (Emery 3004)
- Synthetic hydrocarbon (CAS 68037-01-4). Current standard aerosol for nuclear HEPA in-place testing. Non-carcinogenic, no OSHA PEL. Identical 0.3 μm MMAD and photometer response to DOP. The name "Emery 3004" refers to the specific formulation accepted by NRC.
- DEHS — Di-Ethyl-Hexyl-Sebacate
- An alternative DOP replacement used in cleanroom and hospital HEPA testing. Lower viscosity than PAO; slightly faster evaporation. Used in ISO 14644-3 cleanroom qualification. Requires facility-specific validation if substituted for PAO in nuclear applications.
- MPPS — Most Penetrating Particle Size
- The particle size at which HEPA filter efficiency is at its minimum (penetration is at its maximum). For HEPA media, MPPS is approximately 0.2–0.3 μm. The 0.3 μm DOP/PAO test aerosol is specifically chosen to challenge the filter at or near its MPPS.
- MMAD — Mass Median Aerodynamic Diameter
- The aerodynamic diameter at which 50% of the total aerosol mass is above and 50% below. Target MMAD for HEPA in-place test aerosols (both DOP and PAO) is 0.3 μm.
- Laskin Nozzle
- A standard aerosol generation device used for HEPA filter testing. Produces a polydisperse aerosol (range of particle sizes) by passing compressed air through submerged nozzles in the aerosol liquid (PAO). Named after inventor Irwin Laskin.
- Forward Scattering Photometer
- An instrument that measures the intensity of light scattered in the forward direction by aerosol particles. The primary instrument for HEPA in-place scanning per ANSI N510. Provides continuous analog output expressed as % penetration relative to the upstream reference concentration.
- In-Place Test (vs Factory Test)
- An in-place test is performed on the installed HEPA filter in its housing — verifying not just the filter medium but also the frame seals, gaskets, and housing-to-filter interface. A factory filter efficiency certificate (EN 1822, IEST-RP-CC001 scan) tests only the filter medium on the manufacturer's test bench, not the installed configuration.
Engineering FAQ
Why was DOP replaced by PAO for HEPA filter testing?
DOP (DEHP, CAS 117-81-7) was classified by IARC as Group 2B (possibly carcinogenic) based on rodent carcinogenicity studies. OSHA established a PEL of 5 mg/m³ for DOP as oil mist. The availability of PAO (Emery 3004) — with essentially identical aerosol particle size (0.3 μm MMAD), photometer response, and generation method, but without the carcinogenicity classification — provided a practical direct substitute. NRC accepted PAO as an equivalent replacement, and the nuclear industry transitioned during the 1990s–2000s.
Does switching from DOP to PAO change the acceptance criterion?
No. The acceptance criterion for nuclear HEPA in-place testing remains ≤0.03% penetration (≥99.97% efficiency) regardless of whether DOP or PAO is used as the challenge aerosol. The test aerosol change is purely a health and safety substitution — the physical test methodology, probe scan speed (2 in/sec), upstream concentration range (10–100 μg/L), and acceptance threshold are unchanged. The term "DOP test" is still commonly used in the industry even when PAO is the actual aerosol being used.
What is the correct probe speed for HEPA in-place scanning?
Per IEST-RP-CC001 and ANSI N510, the probe should be traversed at no more than 2 inches per second (5.08 cm/s). This maximum speed ensures sufficient dwell time for the photometer to respond to localized leaks. At 2 in/sec with a 1-inch probe aperture, the probe sweeps approximately 120 inches per minute — a typical 24×24 inch filter face can be scanned in approximately 90–120 seconds including overlapping passes. Faster speeds reduce the probability of detecting pinholes.
When does NRC require nuclear HEPA in-place testing?
NRC RG 1.52 Rev. 4 and ANSI N509/N510 require testing: (1) after initial installation; (2) after any maintenance that could affect filter integrity (opening housing, replacing filters, modifying housing); (3) at minimum annually during surveillance; (4) after any event that may have challenged filter integrity — flooding, fire, chemical exposure, or a high differential pressure event (such as a downstream blockage that caused reverse pressure). Test results must be maintained in the facility QA records per 10 CFR 50 Appendix B.
What documentation does IAS provide with HEPA systems for nuclear applications?
Every IAS-NC700-HI and IAS-NC500-HI-CUSTOM system ships with a complete HEPA documentation package: (1) Factory DOP/PAO in-place test certificate — filter tested in housing per IEST-RP-CC001, ≤0.03% penetration verified; (2) HEPA filter EN 1822 H13/H14 efficiency certificate from the filter manufacturer; (3) HEPA filter serial number, lot number, and installation date; (4) Maintenance schedule specifying required in-service surveillance test frequency. This package directly supports your NRC surveillance documentation requirements. Contact: (650) 646-5199.
Can an H13 HEPA filter fail the NRC nuclear acceptance criterion?
Yes — this is a critical distinction. EN 1822 H13 requires ≥99.95% efficiency (≤0.05% penetration). The NRC nuclear criterion per ANSI N510 / RG 1.52 is ≤0.03% penetration (≥99.97%). A filter that tests at 99.96% efficiency (0.04% penetration) meets the H13 commercial standard but fails the NRC nuclear criterion. For NRC-regulated nuclear air cleaning systems, verify that your HEPA supplier's test certificates demonstrate ≤0.03% penetration per ANSI N510 — not just EN 1822 H13 compliance. IAS-NC700-HI ships with nuclear-criterion-verified HEPA. Contact: (650) 646-5199.
- NRC Regulatory Guide 1.52 Rev. 4 — Design, Testing, and Maintenance Criteria for Post Accident Engineered-Safety-Feature Atmosphere Cleanup System Air Filtration and Adsorption Units of Light-Water-Cooled Nuclear Power Plants (2012)
- ANSI/ASME N509 — Nuclear Power Plant Air-Cleaning Units and Components (1996)
- ANSI N510 — Testing of Nuclear Air Treatment Systems (1989)
- IEST-RP-CC001 — HEPA and ULPA Filters — Institute of Environmental Sciences and Technology
- IEST-RP-CC007 — Testing Unidirectional Air Flow Room Components
- EN 1822-1 through EN 1822-5 — High Efficiency Air Filters (EPA, HEPA, ULPA) — European Standard
- IARC Monographs Vol. 77 — Di(2-ethylhexyl) phthalate (DEHP) classification Group 2B
- OSHA 29 CFR 1910.1000 Table Z-1 — DOP as oil mist, PEL 5 mg/m³
- ISO 14644-3 — Cleanrooms and Associated Controlled Environments — Test Methods
- 10 CFR 50 Appendix A (GDC-41) — Nuclear Air Cleaning Systems Design Criterion
IAS Systems Ship with DOP/PAO Test Certificates
Every IAS-NC700-HI and IAS-NC500-HI-CUSTOM ships with factory DOP/PAO in-place HEPA test certificate (≤0.03% penetration per IEST-RP-CC001), filter manufacturer EN 1822 certificate, and full NRC compliance documentation package.
