Clean benches are designed to protect biological specimens by bathing the work area with HEPA filtered air that is free of particulate contamination. They were developed as an adjunct to clean room technology (the need to protect the work from contamination). In recent years, the use of the clean bench has spread from research and manufacturing to other fields such as aerospace, bioscience, pharmaceutical production and food processing.
Laminar flow clean benches are work benches or similar enclosures, which have their own filtered air supply. Laminar airflow is airflow in which the entire body of air within a confined area moves in a unidirectional velocity along parallel flow lines. In such cases, the fluid flow is free of macroscopic fluctuations, which occur when the Reynolds Number is less than 2000. (The Reynolds Number is the ratio of inertial to viscous forces in a pipe or duct.)
A common component in clean benches is the HEPA (high efficiency particulate air) filter. The HEPA filter removes particulates, generally called aerosols, such as microorganisms, from the air. However, the HEPA filter does not remove vapors or gases. HEPA filters are made of boron silicate microfibers formed into a flat sheet by a process similar to making paper. Depending on its quality, a HEPA filter can trap from 9,997 to 9,999 of every 10,000 particulates of a diameter greater than 0.3 microns. For most industrial use, the 99.97% performance is acceptable. Pharmacy and laboratory applications, however, require a 99.99% filtration performance level.
Aside from the HEPA filter rating, clean benches are also rated according to their Air Cleanliness Class. In Federal Standard No. 209E (last revised Sept.11, 1992) the United States Government provides requirements for three classes of air cleanliness. Classifications are based on particle counts taken at a location within the clean bench, which will yield a particle count of air as it approaches the work location. It is important to note that government classifications reflect performance when the bench is "at rest", or free of materials or work activity within the hood. These classes are Class 100, Class 10,000 and Class 100,000.
In Class 100, particle count is not to exceed a total of 100 particles per cubic foot of a size 0.5 micron and larger.
In Class 10,000, particle count not to exceed a total of 10,000 particles per cubic foot of a size 0.5 micron and larger, or 65 particles per cubic foot of a size 5.0 micron and larger.
In Class 100,000, particle count not to exceed a total of 100,000 particles per cubic foot of a size 0.5 micron and larger, or 700 particles per cubic foot of a size 5.0 micron and larger.
All clean benches direct air in one direction only, although the design of the bench determines whether the air is directed in a vertical or horizontal direction. However, this direction is always away from the specimen and towards the worker. As such, clean benches provide no protection for workers. Clean benches also do not protect the worker from aerosols or fumes generated by the procedure.