Laminar Air Flow Workbenches (Horizontal and Vertical LAFW)

Laminar flow hoods are used to control airborne contamination of sterile products during their extemporaneous preparation. The direction of air flow may be horizontal or vertical. Horizontal flow hoods are most commonly used, with the more costly vertical flow hoods being reserved for agents that may produce an environmental hazard (e.g. cytotoxic agents, radioactive agents, antimicrobial agents).

Horizontal Flow Hood	Vertical Flow Hood

In horizontal LAFW, room air is drawn into the hood through a prefilter to remove relatively large contaminates such as dust and lint. Then the air is filtered through a high efficiency particulate air (HEPA) filter removing 99.97% of all particles 0.3 microns or larger. Parallel air streams bathe the work area with a velocity of 80-100 ft/min which is sufficient to provide the area free of particles and microorganisms and prevent room air from entering the work area.

In vertical laminar airflow workstations, the filtered air enters at the top of the work area and moves downward. In some models the air moves downward all the way through the work area before it is returned to the room air. In other models the air moves downward initially but then turns inside the work area and exits from the hood through the opening at the front of the hood.

Laminar airflow workbenches used in sterile compounding must be ISO Class 5. They are effective only when properly used. Interruption of the air flow will interfere with the effectiveness of the hood. Downstream contamination occurs when any object comes between the HEPA filter and the sterile product, interrupting the parallel flow and creating dead space. Cross-stream contamination may occur due to rapid movements of the operator in the hood. Backward contamination may be caused by turbulence created by objects being placed in the hood, by fast traffic passing the hood, or by coughing, sneezing, etc. by the operator.

It should be remembered that the hood does not produce sterilization, but merely prevents contaminants from settling onto the surface of the sterile product. Any movement of greater velocity and different direction than that of the hood's air flow will create a turbulence that reduces the hood's effectiveness. Contamination may be minimized by working at a smooth, steady pace at least 6 inches into the hood.

Maintaining laminar airflow workstations is essential. Changing prefilters and HEPA filters, routine cleaning of the hood, and any other maintenance should be done according to the manufacturer’s recommendations and time schedules. A quality control standard operating procedure (SOP) should be developed and followed to ensure that maintenance is done when required and to document that maintenance has indeed been done. These steps should be documented.