ISO is the International Standards Organization. It has created the ISO 4644-1 Cleanroom Standards that describe the allowed number of particles, the allowed size of particles and HEPA filtered air flow changes per hour meet ISO-4, ISO-5, ISO-6, ISO-7, and ISO-8 standard. It relies on measurements per cubic meter. It corresponds to the USA based Fed Standard 209E which relies on measurements per cubic foot. Fed Standard 209E corresponding classes are 10, 100, 1000, 10k and 100k.