A clean room is a space with a controlled level of contamination. That means it has very, very few particles, like dust, in the air. More accurately, the level of contamination is defined by the number of particles per cubic meter at a specified particle size. To give perspective, the ambient air outside in a typical urban environment contains 35,000,000 particles per cubic meter in the size range 0.5 μm and larger in diameter (ISO 9 cleanroom). An ISO 1 clean room (the cleanest environment) allows no particles in that size range and only 12 particles per cubic meter of 0.3 μm and smaller.