Particles (contamination) in the air tend to either float around. Most airborne particles will slowly settle, with the settling rate dependent on their size.

A well-designed air handling system should deliver both “fresh” and “recirculated” filtered clean air into the cleanroom in such a way and at a rate so that it flushes the particles from the room. Depending on the nature of the operations, the air taken out of the room is usually recirculated through the air handling system where filters remove the particulates. High levels of moisture, noxious vapours or gases from processes, raw materials or products cannot be recirculated back into the room, so the air in these cleanrooms is often exhausted to atmosphere and then 100% fresh air is introduced into the facility.

Rooms occasionally experience high levels of airborne particulates during routine operation, such as in a sampling room or dispensary. In these cases, the room needs to be cleaned quickly between operations to prevent cross-contamination.

The volume of air introduced into a cleanroom is tightly controlled and so is the volume of air that is removed. Most cleanrooms are operated at a higher pressure to the atmosphere, which is achieved by hiving a higher supply volume of air into the cleanroom than the supply of air being removed from the room. The higher pressure then causes air to leak out under the door or through the tiny cracks or gaps that are inevitably in any cleanroom.

As a rule of thumb, within a facility the room you need to be the cleanest operates at the highest or the lowest pressure.

A good air handling system makes sure that air is kept moving throughout the cleanroom. The key to good cleanroom design is the appropriate location of where the air is brought in (supply) and taken out (exhaust).

Supply air and exhaust (return) air

The location of the supply and exhaust (return) air grilles should take the highest priority when laying out the cleanroom. The supply (from the ceiling) and return air grilles (at a low level) should be at the opposite sides of the cleanroom, to facilitate a “plug” flow effect. If the operator needs to be protected from a high potency product, for example, the flow should be away from the operator.

For sterile or aseptic process that need Grade A air, the airflow typically mimics a plug flow from top to bottom and is unidirectional or “laminar”. Careful consideration should be made to ensure that the “first air” is never contaminated before it comes into contact with the product.