Osmium tetroxide is one of the more unpleasant chemicals used in a histology laboratory, although it is still a common reagent for fixing electron microscopy preparations. Fortunately it is an expensive reagent, so it is used for light microscopy only when other, less expensive, alternatives are not satisfactory. This invariably involves the fixation of fats in some way, as it is the only fixative which can fix fats so that they can be demonstrated in paraffin sections. The times this needs to be done is usually quite low, but that in itself can lead to poor safety practices, primarily due to a lack of familiarity with handling the chemical.
It is very strongly advised that osmium tetroxide always be used in a fume hood. This should be for the whole procedure and not just for the preparation of the solutions. This is so that care may be taken to ensure that the fumes are not inhaled. Osmium tetroxide causes a very distinctive, piercing and unpleasant headache, and it can also dissolve in tears around the eyes and in the exposed mucous membranes of the nose and mouth. It can then fix the underlying tissues and may blacken them in the process.
Osmium tetroxide is usually purchased sealed into glass vials containing one gram. It is sold this way because it sublimes, producing toxic fumes in the process. The vial is scored and broken open and the osmium tetroxide dissolved in a volume of water necessary to make the concentration desired. Place the broken vial into a sealed container so that any residual osmium tetroxide is contained. The solution should be immediately capped so as to inhibit the emission of fumes. The solution should only be uncapped to remove the necessary amount of solution for use, then immediately recapped. The solution removed for use should also be placed into a capped container with any other solutions being added to prepare the required fixative.