I just read an item from the American OSHA regarding their update which has taken 40 years to finalise regarding Silica Dust. ....The new permissible exposure limit for respirable crystalline silica – 50 micrograms per cubic meter of air averaged during an 8-hour shift – equals what NIOSH recommended in 1974. OSHA’s new PEL is half the previous limit for general industry and 5 times lower than the previous limit for construction.... http://www.safetyandheal...i=2TXW,7U7O,PBL8J,P75N,1 The HSE use: (d)which is dust of any kind, except dust which is a substance within paragraph (a) or (b) above, when present at a concentration in air equal to or greater than— (i)10 mg/m3, as a time-weighted average over an 8-hour period, of inhalable dust, or (ii)4 mg/m3, as a time-weighted average over an 8-hour period, of respirable dust; http://www.legislation.g...2/2677/regulation/2/made My question is simple, mg/m3 is which is it Micrograms or Milligrams per cubic metre. Micrograms is part of 1 million (1,000,000) units. Milligrams is part of 1 thousand (1000) units. I'm stuck as I can't find an answer in what I've read so far, I think it's milligrams. If so, will we harmonise?

Be careful reading exposures on Silica - EH40 gives different values dependent on form (Silicon Dioxide, amorphous or Respirable Crystalline) - the latter is the greater concern and has a much tighter 0.1 mg/m3 8hr TWA. This form under US OSHA rules is placing Category 1B Carcinogen labeling on a lot of building materials where sand is a constituent on - this does not currently apply under the EU classification rules.

'Micro' is abbreviated as the Greek letter mu, or sometimes as the Latin letter u which looks a bit like a mu; 'milli' as the Latin letter m.