There are thousands of clean rooms in the micro-electronics, pharmaceutical, medical, laser, and fibre optics industries. Standards of air quality and monitoring and control vary.
Typically, control is at the micron level. However, there is a shift towards sub-micron measurement and monitoring. For example, Nanoelectronics demands measurements at 40nm and even 20nm.
In biological clean rooms, viruses are typically in the nanoparticle size range and other biological agents such as enzymes, proteins etc. can be present in sizes below 100nm.