A clean room is a space designed in order to reduce the density of airborne particles. An airborne particle is a solid object small enough to stay in suspension in the air. Typically, their sizes range from 0.1 to 25 Ám.
There's no current israeli standard about that matter, but the most frequently reffered are the american Federal standard 209 (which latest version is the "e" version), the IES-RP-C006 and the Bristish Standard 5295. A new ISO standard is emerging lately.
The Federal Standard 209e describes several degrees of cleanliness, based on the concentration of 0.5 Ám and larger particles per cubic foot of air. The classes range from 1 (0.1 is sometimes reffered) to 100,000, while the "natural" concentration almost never falls under several millions.
Conditions of class 1,000 to 100,000 are obtained by a non unidirectional patent of absolute (or HEPA) filtered air which constantly dilutes the particles inside or emitted within the clean room. For class 100 or better, a laminar flow is required, which means that one of the surfaces of the room will provide the maximum possible percentage of filter coverage in order to create a constant sweep of the particles within the room.
Beginning after WWII, the concept of clean room took a particular importance for the micro-electronic, pharmaceutical, optic and spatial industries. The first clean rooms were built by adapting existing installations, but the concept of modular clean rooms using pre-built partitions proved to be more efficient by offering better performance, minimizing the work to be performed on site and allowing easier performing of changes or upgrades.
The two important aspects in a modular clean room are the enveloppe itself which must be made of neutral materials that won't release or absorb particles and may need to present some additional characteristics such as being antistatic or chemical proof... Flat horizontal surfaces are to be avoided as much as possible.
The second aspect is the air conditioning system, or HVAC, which must be designed to supply enough absolute (or HEPA) filtered air in order to reach the cleanliness required, sufficient overpressure of the clean areas compared to the surrounding zones and conditions of temperature, humidity as required by the process and the comfort of the workers.