Smoke studies are one of the best tools you have for deciding where your particle probes actually need to go. The problem is, after the fog clears and the videos get filed, translating what you saw into permanent probe placements can feel like guesswork. It doesn’t have to be.
Why Smoke Studies Matter For Probe Placement
EU GMP Annex 1 now expects every fixed probe to be linked back to real contamination risk, not historical convenience. ISO 14644-1:2015 calls for representative sampling locations, and both standards point to airflow visualization as a way to justify those decisions. A smoke study done under dynamic conditions shows you exactly how air behaves when operators are present, doors open, and barriers move; your HVAC drawings alone can’t tell you that.
Running The Study The Right Way
For smoke study results you can actually use, set up the line exactly as it runs in routine production. Keep equipment in place, operators at their stations, and doors operated the way they normally would be. Watch what the air does at working height, that’s where product is exposed and where your probes need to collect meaningful data.
Look for three things during the study:
- Smooth, uniform downflow from HEPA filters versus turbulence or swirling.
- Bounce-back from floors or equipment and recirculation forming behind trolleys, screens, or other obstacles.
- Ingress of ISO 7 or ISO 8 air into your ISO 5 core when RABS doors, glove ports, or room doors open.
Turning Observations Into Probe Locations
Once the study is done, mark every problem zone on your room layout. Note wakes behind equipment, unstable airflow over the product path, and anywhere lower-grade air can enter when barriers open. These marks become candidate probe locations.
When you’re ready to place probes:
- Stay close to the real risk. The probe inlet should be within about 30 cm (12 inches) of the critical point, like a fill needle, open vial, or stopper bowl.
- Match height and flow direction. Keep the inlet at the same working height as the critical operation and point an isokinetic probe into the local airflow direction.
- Avoid misleading spots. Don’t place probes in corners where air short-circuits to an extract grille, far from the work surface, or where operators must lean over and routinely block them.
Scoring Risk To Prioritize Probes
Not every spot flagged by smoke needs a fixed probe. Use an ICH Q9-style risk tool to rank each candidate: describe the contamination scenario, rate likelihood, impact, and detectability (low, medium, or high), and let the matrix guide you. High-impact, medium-to-high-likelihood locations with good detection capability are priority spots for fixed probes; lower-risk background areas may only need portable or periodic monitoring.
Practical Rules For Common Features
For operator positions, place probes downstream so they capture “worst realistic” air after passing the operator. Near doors and airlocks in Grade B rooms feeding Grade A cores, position probes close to where smoke showed ingress. Along conveyors or transfer paths, probe where product is exposed and changes direction or accumulates. If trolleys or carts create wakes that reach critical areas, either reroute them or place background probes to capture their effect.
Bringing It Together
Smoke studies give you what no blueprint can: real-world proof of how air moves through your filling line. When you mark problem areas on the layout, score each candidate location for risk, and apply a few practical placement rules, you end up with probe positions you can defend to any inspector, and, more importantly, positions that actually protect your product.