The occupation on a mechanized mining unit with the highest dust exposure, based on results of respirable dust samples collected by MSHA, is classified as the designated occupation (DO) by MSHA. In addition to being sampled by MSHA, each DO is also sampled bimonthly by the mine operator. The samples are then submitted to MSHA for analysis to determine compli ance with the applicable dust standard. MSHA lowers the dust standard below 2 mg/m3 if the silica content of the sample exceeds 5% by weight in dust samples. For operations on reduced dust standards, MSHA inspector samples from 2004–2008 show that 20% of miner operator samples and 10% of bolter operator samples exceeded their applicable reduced dust standard [MSHA 2009]