L. 91-173, as amended by Pub. Active workings means any place in a coal mine where miners are normally required to work or travel.
Concentration means a measure of the amount of a substance contained per unit volume of air.
MRE instrument means the gravimetric dust sampler with a four channel horizontal elutriator developed by the Mining Research Establishment of the National Coal Board, London, England. Sampling device approvals issued by the Secretary of the Interior and Secretary of Health, Education, and Welfare are continued in effect. L. 91-173 as amended by Pub.