Background Introduction
The safety level of a cleanroom depends on its internal air movement. Regulatory authorities are conducting increasingly rigorous reviews of airflow pattern visualization studies and first-pass air, yet industry practices remain inconsistent.
While smoke visualization continues to be the definitive tool for verifying laminar flow, this method often suffers from high variability and subjective interpretation.
Furthermore, there are no unified standards for smoke generator specifications; smoke output requirements and airflow characteristics vary across companies. Despite the critical importance of interpreting first-pass air, manufacturers face substantial challenges in addressing limitations of existing equipment.
Terminology and Definitions
Airflow pattern: The flow morphology and distribution of air. Clean area: A room or defined space for pharmaceutical manufacturing where airborne particulate and microbial concentrations, as well as temperature, humidity, pressure and other parameters are controlled. Unidirectional flow: Air flowing in a consistent direction at a stable, uniform and sufficient velocity, which continuously sweeps particles away from critical operating or testing zones. First-pass air / initial airflow: Filtered airflow that encounters no obstructions before contacting exposed products and product-contact surfaces, and thus has a low risk of contamination prior to reaching the critical zone.
Airflow pattern test (smoke study): A contamination control verification method that introduces neutrally buoyant tracer particles to visualize airflow patterns within a clean environment. Tests are performed under static (no personnel) and dynamic (with operations/interventions) conditions to verify the protective effect of first-pass air on critical zones, and confirm the absence of contamination risks such as eddies, dead zones and reverse flow.
Visualization Test Methods for Unidirectional Flow Equipment
Test Procedures
1.Conduct airflow test design research, including selection of smoke output and smoke tube length, testing of target equipment, ambient airflow velocity and direction, determination of test height, and selection of shooting background and angles.
2.Complete preparations for preliminary airflow tests and carry out trial simulations.
3.Develop a detailed test protocol according to equipment performance and actual operating conditions. The protocol shall include, but is not limited to: test scenarios, acceptance criteria for each scenario, tracer selection, smoke release positions, smoke emission directions, determination of smoke flow rate, camera angle setup, video processing requirements, and protocol/report approval requirements. General principles: the smoke outlet shall release smoke horizontally or at an upward inclination ≤15°. Capture footage covering the smoke outlet as well as smoke trajectory (origin and destination of smoke). Record video covering the full path from the smoke source through the entire airflow field. Video clips shall use a wide-angle lens to cover the area under smoke study. Each video recording shall last no less than 30 seconds and fully capture all phases of interventions from start to finish.
4.Conduct formal testing in accordance with the validated and approved test protocol, evaluate test results and obtain approval from relevant departments.
Air Velocity Measurement
1.Air velocity measurement complying with relevant standards shall be completed and accepted prior to airflow visualization testing. Unidirectional flow systems shall deliver uniform air velocity within the working zone in the range of 0.36 m/s ~ 0.54 m/s (guidance value). Air velocity measurements shall be correlated with airflow visualization studies.
Airflow Visualization Testing
1.Visual airflow observation method: The visual airflow observation method is recommended for airflow pattern visualization testing. Tracer particle properties enable direct visual identification of airflow direction or flow fluctuations induced by interventions, which shall be documented via recording devices such as video cameras. Tracer particles shall not contaminate the clean area and shall faithfully follow airflow trajectories.
2.Test tracers: Neutrally buoyant substances shall be prioritized. Where conditions do not permit, dry ice, cooled water for injection, purified water or alcohol-based tracers may be adopted. Tracer selection requires assessment to confirm residues will not impair the environment or equipment. Residue removal procedures shall be established where necessary. Tracers shall not be excessively buoyant (floating upward) or overly heavy (sinking downward).
Smoke Release Positions
Smoke tube layout shall not interfere with airflow or routine operations. Fixed single or multiple smoke tubes are preferred; movable smoke tubes shall be avoided except for mandatory mobile scenarios.
1.Static testing: a) Smoke may be released 15 cm ~ 30 cm below HEPA filters to observe overall airflow trajectory; b) Smoke may be released at the filter centerline, or additional release points may be added; c) Smoke tube positioning shall demonstrate airflow travelling from higher-risk zones toward lower-risk zones.
2.Dynamic testing: Smoke may be released at an appropriate elevation above the operating surface to observe local airflow morphology and track airflow variations during operations. Dynamic airflow pattern visualization can assess the impact of dynamic conditions on aseptic processes. Dynamic airflow visualization tests shall be performed under actual or simulated dynamic production conditions. Intrinsic production operations (including mechanical equipment movements) and interventions shall be simulated, to provide documented evidence (e.g., smoke test recordings) that the adopted airflow scheme does not introduce contamination risks. An additional purpose is to guide personnel on aseptic operations and optimize process workflows via visualization of airflow in critical working zones.
Smoke Emission Direction
The smoke outlet should be oriented perpendicular to airflow or inclined toward the HEPA filter to minimize initial smoke deviation. Smoke tubes shall not be placed excessively close to equipment sidewalls or protective doors. If proximity is unavoidable, smoke shall be released away from wall panels to prevent smoke rebound.
Smoke Flow Rate
1.Smoke output shall be adjusted according to practical conditions.
2.Smoke density shall not be excessive; the background must remain distinguishable to facilitate observation of airflow trajectories.
3.Smoke density shall not be insufficient; airflow wake shall be observable extending beyond the unidirectional flow equipment or reaching air return openings.
Shooting Range and Angles
For static testing, filming scope shall match smoke coverage. Segmented filming is permitted for large areas, provided the full production line is covered. Camera angles shall fully record airflow from smoke release points to air return openings.
For dynamic testing, footage shall capture overall equipment operation together with personnel interventions, with identical coverage requirements as static testing. Camera angles shall fully record airflow from smoke release points to air return openings, alongside footage of personnel and equipment operations.
Auxiliary Facilities
1.Appropriate lighting is recommended to improve smoke visibility.
2.Backdrops with strong colour contrast may be used to enhance recording clarity. Backdrops must not alter the original structure or airflow patterns of facilities; they should preferably be placed remotely or outside the filming zone.
Miscellaneous Considerations
Implement numbering and identification management for systems/equipment before filming.
Define operating actions, intervention sequences and material loading quantities for each workstation.
Confirm smoke generator placement, smoke emission directions and camera arrangements.
Verify air velocity remains within acceptable limits.
Confirm differential pressure, measuring instruments and equipment operate normally.
Test Frequency
Initial testing: Airflow pattern visualization testing shall be performed after on-site installation of unidirectional flow equipment.
Retesting: Periodic retesting is recommended; intervals shall be defined based on risk assessment and statutory specifications, with a reference interval of 2 years aligned with pharmacopoeia requirements.
Retest evaluation shall be triggered if any of the following occur: a) Major maintenance of unidirectional flow equipment (e.g., filter replacement); b) Changes to production processes and operations; c) Modifications to operational parameters.
Summary of Common Deficiencies
Airflow pattern testing conducted without dynamic conditions
This is the most prevalent deficiency. Tests fail to simulate real production activities and cannot prove that unidirectional flow is maintained under actual conditions including personnel movement, equipment operation and material transfer. One example is failure to operate online monitoring systems during testing.
Class A airflow patterns fail to demonstrate unidirectional flow
Video recordings reveal turbulent flow, stagnant zones, reverse flow or upward/lateral flow instead of steady, downward unidirectional flow.
Airflow pattern testing fails to simulate all critical operations and interventions
Key production steps such as material transfer, equipment assembly, personnel operations and abnormality handling are omitted, leading to incomplete risk evaluation.
Inadequate test methodology or poor visualization effect
Insufficient smoke volume, improper placement of smoke sources or unclear video recordings prevent effective observation and evaluation of airflow patterns.
Airflow pattern testing fails to cover all critical zones or equipment
Zones with risks of airflow obstruction and turbulence are not evaluated, including surrounding equipment, boundary areas and locations with physical obstacles.
Other Related Deficiencies
1.Independent evaluation is not performed for each unit; critical zones are omitted. The dynamic performance of one laminar flow hood cannot serve as evidence for adequate unidirectional flow in others. Every laminar flow unit requires independent assessment, including through-wall openings and joints between laminar flow zones.
2.Tests are not executed per approved procedures; guidance documents and regulatory standards are adopted in place of formal written protocols or SOPs.
3.Obvious turbulent flow occurs when doors are opened during pattern testing.
Conclusion
Smoke testing can assess the rationality of air distribution. Proper air distribution accelerates achievement of required environmental temperature, humidity and cleanliness classification, and mitigates adverse impacts of environmental contaminants on products. It supports the formulation of aseptic operation SOPs, planning of transfer routes for aseptic tools, materials and products, and validates the rationality of aseptic operations. The method may also be used to verify compliant air distribution in non-laminar flow zones.
