MSHA - Code of Federal Regulations - 30 CFR 27.40
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MSHA - Title 30 CFR
30 CFR § 27.40
Test to determine resistance to dust.

Components, subassemblies, or assemblies, the normal functioning of which might be affected by dust, such as coal or rock dust, shall be tested in an atmosphere containing an average concentration (50 million minus 40 micron particles per cubic foot) of such dust(s) for a continuous period of 4 hours. The component, subassembly, or assembly shall function normally after being subjected to this test.

Note: Dust measurements, when necessary, shall be made by impinger sampling and light-field counting technique.