average concentration
For the purpose of this subchapter, the term “average concentration” means a determination which accurately represents the atmospheric conditions with regard to respirable dust to which each miner in the active workings of a mine is exposed (1) as measured, during the 18 month period following December 30, 1969 , over a number of continuous production shifts to be determined by the Secretary and the Secretary of Health and Human Services, and (2) as measured thereafter, over a single shift only, unless the Secretary and the Secretary of Health and Human Services find, in accordance with the provisions of section 811 of this title , that such single shift measurement will not, after applying valid statistical techniques to such measurement, accurately represent such atmospheric conditions during such shift.