In Great Britain, the arbitrary standards laid down by the National Coal Board in 1949 as representing comparative dust-free conditions in coal mines. These are as follows: stone dust, 450 particles/cm₃(size range, 0.5 to 5.0 mu m); anthracite, 650 particles/cm₃(size range, 1 to 5 mu m); and coal, 850 particles/cm₃(size range, 1 to 5 mu m).