What is the M descriptor?

Дозирование жидкости в стеклянные пробирки
ISO 14644-1 describes any particle with an equivalent diameter of ≥0.5µm as a ‘macro-particle’. Air cleanliness is expressed in terms of the M descriptor–the numerical value of the concentration of macro-particles per m3.  When consideration of these particles is required, counting and sizing is expressed using the M descriptor in the following format–ISO M (a; b); c.

For example, ISO 5 at rest concentration of 20 particles/m3 @ ≥5.0µm is conveyed as: 

ISO 5 (20; ≥5.0µm); at rest/operational; LSAPC Annex 1 Grade A and Grade B and have been defined by the new M descriptor using ISO 14644-1.