A discrete-particle-counting, light-scattering instrument is used to determine the concentration of airborne particles, equal to and larger than the specified sizes, at designated sampling locations.
Small numbers refer to ISO 14644-1 standards, which specify the decimal logarithm of the number of particles 0.1 µm or larger permitted per m3 of air. So, for example, an ISO class 5 cleanroom has at most 105 particles/m3.
Both FS 209E and ISO 14644-1 assume log-log relationships between particle size and particle concentration. For that reason, zero particle concentration does not exist. The table locations without entries are nonapplicable combinations of particle sizes and cleanliness classes, and should not be read as zero.
Because 1 m3 is about 35 ft3, the two standards are mostly equivalent when measuring 0.5 µm particles, although the testing standards differ. Ordinary room air is around class 1,000,000 or ISO 9.