Encyclopedia
Fiber
3.6. Fiber-particle with a length-to-diameter ratio of not less than 3 : 1. Respirable fiber - a fiber longer than 5 µm and thinner than 3 µm
Source: "SanPiN 2.2.3.757-99. 2.2.3. Enterprises in certain industries of industry, agriculture, communications, transport. Work with asbestos and asbestos-containing materials. Sanitary rules and norms" (approved by the Chief State Sanitary Doctor of Russia on June 28, 1999)