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Osmium(VIII) oxide or osmium tetroxide is a volatile chemical compound with the formula of $\ce{OsO4}$. It is highly toxic for supposedly irritating mucous membranes, but I'm not too sure if it is that or because it is a really strong oxidizer.

PubChem rates it as corrosive and highly toxic and according to Wikipedia, the toxic effects of $\ce{OsO4}$ are as follows:

$\ce{OsO4}$ is highly poisonous. In particular, inhalation at concentrations well below those at which a smell can be perceived can lead to pulmonary edema and subsequent death. Noticeable symptoms can take hours to appear after exposure.

$\ce{OsO4}$ 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 $\pu{2 µg/m3}$

How is $\ce{OsO4}$ toxic in nature? What makes it so?

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    $\begingroup$ You seem highly interested in understanding what makes compounds toxic or reactive. The following few links might help with the first question: ccohs.ca/oshanswers/chemicals/oxidizing/oxiziding_hazards.html nature.berkeley.edu/~dnomura/pdf/Lecture6Mechanisms3.pdf organic.lu.se/education/Kemiteknik&Bioteknik/EnvirChem/… $\endgroup$
    – Buck Thorn
    May 25, 2021 at 7:52
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    $\begingroup$ Osmium tetroxide might be particularly damaging because it acts catalytically. $\endgroup$
    – Buck Thorn
    May 25, 2021 at 10:05

2 Answers 2

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Metallic osmium is harmless but it reacts with oxygen at room temperature, forming volatile osmium tetroxide. Any other osmium compounds are also converted to the tetroxide if oxygen is present. This makes osmium tetroxide the main source of contact with the environment and human body. Exposure to osmium tetroxide cause harm to the skin, eyes, and respiratory system.

So, why makes osmium tetroxide so harmful? (TL;Dr: Because to its oxidization ability)

There is a paper1 which studied the effect of osmium tetroxide in human body and how it can be a lethal bioweapon. The following is the text from the paper:

Osmium tetroxide is highly volatile and penetrates skin readily, and is very toxic by inhalation, ingestion, and skin contact. Airborne low concentrations of osmium tetroxide vapor can cause lung congestion and skin or eye damage.

[...]

Osmium tetroxide is a rapid, indiscriminative oxidizer that does not distinguish between organic tissue and inorganic materials. An inhalational toxicity study with rabbits proved futile, because of the rapid reduction of $\ce{OsO4}$ by the skin, hair, mucous membranes, etc., as well as by the chamber walls. Inhalation, ingestion, contact with skin and with mucous membranes may all result in severe consequences. Due to its high vapor pressure (7 mmHg at 20 °C), most exposures are to vapor. These can cause severe chemical burns to the eyes, skin and respiratory tract. Very short-term contact with the vapor may cause lacrimation, accompanied by cough, headaches and dizziness. $\ce{OsO4}$ may cause irreversible blindness by turning the cornea black. Symptoms may not be noticeable until several hours after the exposures. Another severe delayed effect following inhalational exposure is acute lung injury, which may be followed by non-cardiogenic pulmonary edema Direct contact with osmium tetroxide solution will turn the skin black (severe chemical burns due to strong oxidizing properties). Painful burns or contact dermatitis may result, depending on the concentration.

The paper compares the LD50 value of osmium tetroxide with other chemical warfare agent and thus proves how lethal it can be as a bioweapon:

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The paper also notes that osmium tetroxide can rapidly reduced to relatively inert compounds by e.g. ascorbic acid or polyunsaturated vegetable oils (such as corn oil which contains double/triple bonds. $\ce{OsO4}$ can attack those bonds and can convert to non-toxic vicinal diols).

Reference

  1. Makarovsky, Igor & Markel, Gal & Hoffman, Azik & Schein, Ophir & Finkelstien, Arseny & Brosh-Nissimov, Tal & Tashma, Zeev & Dushnitsky, Tsvika & Eisenkraft, Arik. (2007). Osmium Tetroxide: A New Kind of Weapon. The Israel Medical Association journal : IMAJ. 9. 750-2.
  2. McLaughlin AIG, Milton R, Perry KMA. (1946). Toxic manifestations of osmium tetroxide. Br J Ind Med 3:183-186.
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    $\begingroup$ I upvoted, of course! Maybe ruthenium tetroxide will pop up next! ;-) $\endgroup$
    – Ed V
    May 26, 2021 at 18:30
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http://www.chm.bris.ac.uk/motm/oso4/oso4h.htm https://www.epa.gov/sites/production/files/2014-09/documents/osmiumtetroxide_interim_ornl_jun_2008c.pdf https://www.michigan.gov/documents/mdch-osmium_tetroxide_fs_109244_7.pdf

Osmium tetroxide is an incredibly strong oxidant: it is stronger as an oxidant than the halogens and also oxidizes halides back to their halogen form. When it comes into contact with skin, it will readily reduce into osmium dioxide, causing irritation and leaving a noticeable black residue.
It is also notable for its volatility. Osmium Tetroxide will sublime, which makes it incredibly easy for vapors to be inhaled through the nose or come in contact with the eyes. This allows it to potentially oxidize and irritate vital organs the heart and lungs, as well as stain the cornea of the eyeball. Those actions, in turn, will lead to the symptoms you describe.

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  • $\begingroup$ I am new to StackExchange, so please do not hesitate to give feedback/criticism to my answer! :D $\endgroup$
    – MilanG
    May 25, 2021 at 22:33
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    $\begingroup$ You should turn urls into links, in text or bibliography. $\endgroup$
    – Mithoron
    May 25, 2021 at 23:41

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