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.