This table is taken from the WHO GMP text and describes four classifications A - D.

The different grades are defined in terms of the maximum permitted number of particles per cubic metre of air (at two different sizes). The WHO text also contains values for microbial contamination of the various grades. They have been included in the text for information and are not intended to represent specifications.

We will talk later about how these characteristics are measured. They are attained by means of a ventilation system in which air is passed through a series of filters. The generally accepted design is for two pre-filters and a HEPA (high efficiency particulate air) filter at the outlet into the room. The air inlet is usually located at a high level in the room, whereas the extract is at a low level.

The filtered air supply must be maintained at positive pressure to the surrounding areas. Airflow patterns must be designed so that they do not distribute particles into the area where the product is exposed. An alarm system, in the case of air supply failures, should be installed. During the inspection, you have to evaluate records and results from the manufacturer in which it is established and proven, that a claimed grade is achieved.