A Laminar Airflow Hood (LAFH)/ Laminar Aseptic Hoods, or a workbench, is a primary engineering control device which provides the following services during aseptic compounding:

1. Clean air to the critical sites (immediate aseptic compounding area),
2. Constant flow of air out of the work area to prevent the entry of room air, and
3. Outward flow of air from the hood that suspends and removes contaminants which have been introduced in the work area by personnel.

A High Efficiency Particulate Air (HEPA) Filter is the most important part of a LAFH. The air within the room is taken into this filter and passed through a pre-filter which removes the gross contaminants (lint, dust, etc.). The air is then blown at a uniform velocity through the hood and HEPA filter in a unidirectional (laminar flow) manner over the critical sites (immediate aseptic compounding area).

HEPA filter is a particulate filter which traps the airborne particles and microbes; but allows the gases to pass through.

HEPA filter should be fitted either at or near to the clean room inlet. A pre-filter is fitted upstream of the HEPA filter, thus extending the final filter’s life. A fan is also fitted which pumps the air through the filter.

The filter medium used in HEPA filters is made up of pleated fiberglass paper. Parallelly arranged pleats not only increase the filter surface area but also the air flow through the filter (figure). This parallel arrangement of filter medium also allows the filter to retain a compact volume. In the traditional type of HEPA filters, aluminium foil was used as spacers, which are no longer used in the modern mini-pleat type of filter (now widely used).

The mini-pleat filters have a shallower depth in construction than the traditional HEPA filter. The filter material is sealed to an aluminium frame within the filter (figure). One side of the filter is protected with a coated mild steel mesh.

HEPA filters provide:

1. A high air flow rate,
2. A high particulate holding capacity, and
3. A low-pressure drop across the filter.

HEPA filters remove larger, medium, and smaller particles from the air by inertial impaction, direct interception, and by Brownian diffusion, respectively. The HEPA filters are least efficient in removing particles of about 0.3μm size. However, its efficiency of removing particles is affected by air velocity and filter packing. It removes larger and smaller particles with more efficiency.

The LAFH works on the principle that twice-filtered laminar layers of aseptic air continuously sweep the work area within the hood to prevent the entry of contaminated room air.