Isolator Airflow Visualisation: How to Plan and Judge a Smoke Study

What a smoke study proves

Air velocity measurements tell you how fast the air leaves the filter. Particle counts tell you how clean it is at the sampling point. Neither shows whether the air that reaches an open vial has just passed over an operator’s sleeve, a moving part, or the edge of a stopper bowl. Airflow visualisation does — and that is why regulators treat it as a separate, essential part of qualifying an aseptic barrier.

EU GMP Annex 1 (2022) expects airflow patterns to be visualised to show that air does not move from less clean areas into the critical zone, and that unidirectional airflow protects the product, both at rest and under operating conditions with interventions. The studies should be documented — in practice that means a video record linked to the study protocol — and repeated after changes that could affect airflow.

Unidirectional or turbulent?

Where open product and sterile components are exposed, isolators normally provide unidirectional airflow over the critical zone. Annex 1 gives a guidance range of 0.36–0.54 m/s at the working position for unidirectional airflow systems, unless another value is scientifically justified in the contamination control strategy. Some closed isolators, and transfer chambers, use turbulent (mixed) airflow; there the study shows that air is well mixed and that no part of the chamber is stagnant. The study design follows the design of the isolator.

Planning the study

Decision	Practical approach
Fog source	Inside an aseptic isolator, use a fog generator producing fine droplets of water for injection or ultrapure water. It is visible, neutral in buoyancy and leaves no residue. Chemical smoke and glycol fogs leave residues and are better kept out of aseptic equipment.
Introduction point	Release fog gently just below the filter face or at the point being studied, at low velocity, so the fog follows the air rather than its own jet. A wand or perforated tube lets you release it across the whole width.
Lighting and background	A dark, matte background and a light sheet or strong side lighting make the fog visible. Glass and stainless steel reflect; plan camera angles that avoid glare.
Camera	Fixed camera positions that show the critical zone from the front and the side, plus close-ups. Record the time, the conditions and the activity on screen or in a log, so each clip can be linked to the protocol.
Conditions	At rest (equipment running, no operator activity) and in operation (machine running at production speed with interventions).
Interventions	Every routine (inherent) intervention and each foreseeable corrective intervention: loading stoppers, clearing a fallen vial, adjusting a guide, taking an EM sample, connecting a transfer port.

What to look for

A study passes when the fog shows that air over open product and sterile surfaces is first air — it comes straight from the filter — and leaves the critical zone without returning. In practice:

• Smooth, parallel flow from the filter down over the containers, needles and stopper track.
• No eddies or stagnation over the critical zone; fog released there clears within seconds.
• No air passing over the operator’s sleeve or glove before it reaches open product. Interventions are designed so the arm is downstream of the product, or the product is protected while the arm is in the zone.
• No reflux from lower-grade areas — for example through a mousehole or an opened transfer port — into the critical zone.
• Return paths clear. Air leaves through the low-level returns as designed; the returns are not blocked by equipment or components.

Common findings and fixes

Finding	Typical fix
Eddy behind a guard, a sensor or the stopper bowl	Reposition or reshape the part, perforate the guard, or move the containers out of its wake.
Air passes over the glove before the product	Change the glove port position or the intervention method; use tools that keep the hand downstream; stop the line during the intervention.
Turbulence where flow meets a flat surface	Perforated work surfaces or a gap that lets air pass through and down to the returns.
Outflow at a mousehole reverses during machine movement	Review the pressure differential and the exhaust balance; check behaviour with doors opening in the room.
Slow clearance in a transfer chamber	Increase the air change rate or move the inlet and outlet to remove dead corners.

When to repeat it

Repeat the study after any change that could affect airflow: new format parts, moved equipment, changed fan settings, new interventions or a modified glove port layout. It is also good practice to include airflow visualisation in periodic requalification. Because smoke studies are a training tool as well as a qualification test, many sites show the videos to every operator who works at the isolator.

Smoke studies complement the other barrier tests: the chamber leak test, glove integrity testing and the HEPA filter integrity test carried out at FAT and SAT.