[azalemeth]

I think it's worth highlighting just how horribly toxic osmium tetroxide is -- it's used as a stain for electron microscopy as it is dense and binds irreversibly to lipid membranes. Quoting from Wikipedia:

> In the staining of the plasma membrane, osmium(VIII) oxide binds phospholipid head regions, thus creating contrast with the neighbouring protoplasm (cytoplasm). Additionally, osmium(VIII) oxide is also used for fixing biological samples in conjunction with HgCl2. Its rapid killing abilities are used to quickly kill live specimens such as protozoa. OsO4 stabilizes many proteins by transforming them into gels without destroying structural features. Tissue proteins that are stabilized by OsO4 are not coagulated by alcohols during dehydration.

> OsO4 will irreversibly stain the human cornea, which can lead to blindness. The permissible exposure limit for osmium(VIII) oxide (8 hour time-weighted average) is 2 µg/m3. Osmium(VIII) oxide can penetrate plastics and food packaging, and therefore must be stored in glass under refrigeration.

[admash]

Worse, osmium tetroxide is volatile and sublimes at room temperature. I was told by my lab instructor that it is said to smell sweet, but that if you ever inhale enough to be able to smell it, you will have already absorbed a lethal dose.

[kragen]

lots of historical chemists smelled osmium and survived; that's how it got its name