A cleanroom is environmentally controlled space. More precisely, it is a more than a room that is clean, a cleanroom is defined in the ISO standard 14644-1 as:
“A room in which the concentration of airborne particles is controlled, and which is constructed and used in a manner to minimize the introduction, generation, and retention of particles inside the room and in which other relevant parameters, e.g. temperature, humidity, and pressure, are controlled as necessary”
A cleanroom has a controlled level of tainting that is indicated by the quantity of particles per cubic meter at a predetermined molecule measure. By comparison, the surrounding air outside in a typical urban area contains 35,000,000 particles for each cubic meter in the size range 0.5 μm and bigger in measurement, relating to an ISO 9 cleanroom, while an ISO 1 cleanroom permits no particles in that size range and just 12 particles for each cubic meter of 0.3 μm and smaller.