Clean Rooms Defined

To merit the term "clean room," an internal area must be supplied with filtered air.

The types of filter generally used for this purpose are not classifiable as sterilizing filters, and would not pass the stringent tests applicable to bacteria-retentive sterilizing filters. They are capable of removing the great majority of microorganisms. When coupled with other filters, and with recirculation of previously filtered air to dilute the challenge of air from uncontrolled sources, they can be extremely effective in the maintenance of asepsis.

Clean rooms must be maintained at higher differential air pressures than adjacent uncontrolled areas. This prevents the quality of air achieved by filtration and dilution being compromised by any unsealed or openable junctures with lower standards in adjacent areas. The result of the pressure differentials is a constant outward air flow at unsealed points, and outflow for limited periods when doors are opened. Microorganisms are unable to move against pressure differentials, and in the opposite direction to air flows. The limits for pressure differentials and air flow values normally used in clean-room design and control are based largely on history rather than scientific data.

The inability of microorganisms to "swim against the tide" is also relevant to the use of laminar flow air to protect specific areas in sterile pharmaceutical manufacture. Filtered air is accelerated across the area, and is protected so as to prevent back flow, mixing and turbulence. Air moving in this manner is described as laminar flow (or unidirectional) air.

Laminar flow air serves two purposes. First, it provides a protective directional air movement, preventing microorganisms entering the protected area. Second, it is able to "sweep away" microorganisms already in the area. This second attribute is somewhat arguable, and whereas there is no doubt that laminar flow air (such as in air showers) can clean microorganisms from surfaces, etc., it is rarely used as the sole means, and is never used to clean microorganisms from surfaces and materials unless they have been precleaned by other means.

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