3.2 Total Airborne Non-Viable Particle Counting & Cleanroom Certification Standards

Cleanroom Facility Certification Overview

Sterile compounding facilities rely on robust physical engineering controls to maintain an environment free of airborne contaminants. Certification of primary engineering controls (PECs) and secondary engineering controls (SECs) is legally required to verify that air cleanliness, air exchange rates, pressure differentials, and HEPA filter integrity meet compendial standards. USP <797> requires independent certification against the chapter's requirements and, where applicable, the manufacturer's specifications. The 2022 revision dropped the old reference to the CETA certification guide, although certifiers still widely use CETA (Controlled Environment Testing Association) application guides and ISO 14644 test methods.

Mandatory Recertification Frequency

Facility certification is required:

1. At least every 6 months (semiannually) for all sterile compounding facilities.
2. Following relocation or movement of any PEC.
3. After major repairs, redesigns, or construction within the compounding cleanroom suite or HVAC systems.
4. After replacing a HEPA filter or making any other change that could affect airflow or air quality. A HEPA change also triggers microbiological air and surface sampling. Routine prefilter changes usually do not, because the downstream HEPA filter stays intact (USP FAQ).

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Total Airborne Non-Viable Particle Counting

Unlike viable sampling, which identifies living microbes, total airborne particle counters use light-scattering optical sensors to detect the concentration of all airborne particulates (dust, lint, dead skin cells, aerosol droplets) $\ge 0.5\,\mu\text{m}$ in size.

ISO 14644-1 Cleanroom Particulate Thresholds

HEPA Filter Integrity Testing (Aerosol Challenge)

High-Efficiency Particulate Air (HEPA) filters are the critical barrier protecting sterile compounding spaces. A standard cleanroom HEPA filter must demonstrate minimum particulate capture efficiency of 99.97% on particles $0.3\,\mu\text{m}$ in diameter (the Most Penetrating Particle Size, or MPPS).

Aerosol Photometer Leak Testing Protocol

1. An upstream challenge aerosol—typically Polyalphaolefin (PAO) or Emery 3004—is introduced into the air plenum upstream of the HEPA filter.
2. An aerosol photometer with a calibrated scanning probe is swept across the entire downstream face of the filter media, perimeter frame seals, and gasket joints.
3. Pass/Fail Threshold: Any measured downstream aerosol concentration exceeding 0.01% of the upstream challenge concentration ($> 0.01\%$) constitutes an unacceptable leak requiring repair (silicone caulk patching within allowable surface limits) or immediate filter replacement.

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Dynamic Airflow Smoke Pattern Testing

Airflow visualization studies (smoke pattern testing) are required for all primary engineering controls under USP <797>. Smoke studies visually verify that:

• Airflow is unidirectional and laminar across the entire Direct Compounding Area (DCA).
• Clean, HEPA-filtered "first air" smoothly sweeps over critical sites (vial stoppers, needles, syringe hubs) without obstruction.
• No eddy currents, air dead zones, or room-air reflux occur at the boundary between the room and the cabinet interior.

When Smoke Studies Are Repeated

A dynamic airflow smoke pattern test is part of every certification for every PEC. It is also repeated whenever equipment inside the PEC is moved. For laminar airflow systems and robotic enclosures, it is done initially and at least every 6 months. In rooms whose air returns are not low on the wall, a visual smoke study must show that there is no stagnant air, and that study is repeated after changes such as moving equipment or HVAC work. USP does not generally require video, although video is a common way to document these tests, and USP's FAQ suggests it for integrated vertical laminar flow zones.

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Instrumentation: Optical Particle Counters (OPCs) and Isokinetic Sampling

Total non-viable particle monitoring relies on Optical Particle Counters (OPCs). An OPC draws air at a regulated flow rate (typically $1.0\text{ CFM}$ [$28.3\text{ L/min}$] or $50\text{ L/min}$) through a laser sensing chamber. As individual particles transit the focused laser beam, they scatter light across various angles (Mie scattering). Photodetectors capture this scattered light and convert it into electrical voltage pulses. Because the pulse height is proportional to the particle cross-sectional area, pulse-height analysis determines both particle count and size distribution.

Isokinetic Sampling Requirements

When sampling within unidirectional airflow environments (such as an ISO Class 5 laminar airflow workbench or biosafety cabinet), the particle counter probe must be an isokinetic sampling probe:

• Isokinetic Alignment: The probe opening must face directly into the oncoming airflow vector, parallel to the streamlines.
• Flow Velocity Matching: In true isokinetic sampling, the velocity of air entering the probe inlet matches the velocity of the approaching laminar airstream ($90\ \text{ft/min} \pm 20\%$, or $0.45\ \text{m/s} \pm 20\%$).
• If the probe is oriented perpendicular or backwards, turbulent eddies form at the probe lip, deflecting larger particles and yielding artificially suppressed counts.
• Sample Tubing Length: The tube from the probe to the OPC should be as short as practical and within the counter manufacturer's specification, with gentle bends. Long tubing lets larger particles ($\ge 5.0\ \mu\text{m}$) settle or stick to the walls before they are counted.

Determination of Sample Locations and Volume

ISO 14644-1:2015 sets the minimum number of sampling locations ($N_L$) from its Table A.1, based on the room's floor area. The older formula $N_L = \sqrt{A}$ came from the 1999 edition and is no longer used. USP's FAQ points to the same table and notes that USP itself sets no minimum number of samples. Locations are spread across the room at working height.

The volume sampled at each location ($V_s$) must be large enough to see 20 particles if the concentration were at the class limit for the largest particle size considered:

$$V_s = \frac{20}{C_{n,m}} \times 1000 \text{ liters}$$

There is a minimum of 2 liters and at least 1 minute of sampling per location. For ISO Class 5 at $0.5\,\mu\text{m}$, $V_s = \frac{20}{3{,}520} \times 1000 \approx 5.7$ liters. Do not confuse this with the 1,000-liter volume required for viable air samples.

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Troubleshooting Particulate Excursions

When non-viable particle counts breach ISO limits under dynamic conditions, a systematic engineering investigation must ensue:

1. Verify Instrument Integrity: Confirm OPC calibration certificate, check for zero-count filter pass, and inspect isokinetic probe orientation.
2. Inspect HEPA Media and Seals: Re-run PAO aerosol scan to rule out micro-tears in the delicate fiberglass filter medium or shrinkage in perimeter neoprene gaskets.
3. Assess Personnel Technique and Clutter: Dynamic particle spikes often stem from personnel rushing, introducing corrugated shipping boxes into the buffer room, or crowding the PEC with excessive supply items, which blocks return air grilles and disrupts laminar sweep.