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In the house on my own and an article popped up on my feed with the following statement:

While the Government has insisted the chemical is safe, cadmium is recognised as a cause of lung cancer and during the Second World War was considered by the Allies as a chemical weapon.

The proceeding paragraphs are about zinc cadmium sulphide.

full article

I think this is a flawed argument, essentially inferring that any given molecule can be said to be toxic just because one of it's component parts is toxic but I can't think of a good example to back this up.

What is a good example of a common compound that is ingested often (daily?) but contains a particularly toxic element if that element were ingested in some way on it's own?

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  • $\begingroup$ Apple seeds - they contain cyanide, but not enough to actually harm you unless you harvest ONLY the seeds for ingestion.. and while most people don't eat the seeds, do you think that the companies making apple juice/cider bother with removing them? $\endgroup$ – user2813274 Jan 27 '15 at 3:14
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    $\begingroup$ @user2813274: I don't think the apple juice is a good example: the cyanide is released when the amygdalin is hydrolysed. But apple juice is sour, so HCN gas would leave the juice. When in contact with air, HCN is oxidized to HOCN (cyanate; far less toxic than cyanide). Humans do something similar: we detoxify cyanide by making thiocyanate (SCN⁻) which goes out via urine. The capacity of this pathway is limited, it covers apple seeds in "typical" amounts, very few bitter almonds (which are bitter!), and is close to its limit for people eating lots of manioc (chronic cyanide poisoning). $\endgroup$ – cbeleites supports Monica Jan 27 '15 at 10:51
  • $\begingroup$ Many antidepressants also are fluorine-substituted, and elemental fluorine kills any living thing. $\endgroup$ – DumpsterDoofus Jan 28 '15 at 0:09
  • $\begingroup$ Even almonds contain some quantity of cyanide. Toothpaste contains fluorides. Fluorine is faaaar more powerful than an acetylene torch, and can destroy your body within seconds. It's going to rip off the hydrogen from whatever it can find, and form hydrofluoric acid. $\endgroup$ – Lexicon Jan 28 '15 at 14:33
  • $\begingroup$ Dihydrogen Monoxide - Sometimes I have trouble convincing people a glass of water is full of this chemical. $\endgroup$ – Agriculturist Jan 22 '16 at 15:25
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Table salt! What would be worse than putting sodium (it can spontaneously combust if you get it wet) and chlorine (used as a war gas in WWI) all over your food?

Then there's water which always contains some $\rm OH^-$ (an active ingredient in Drano) and $\rm H^+$ (that stuff eats through metal, man). Although those aren't elements.

And hiding inside every $\rm O_2$ molecule are two deadly oxygen atoms; once freed, they'll wreak havoc on just about anything they come into contact with.

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    $\begingroup$ Haha, this is funny and true... (and this comment is absolutely unnecessary.) $\endgroup$ – Martin - マーチン Jan 27 '15 at 4:22
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    $\begingroup$ As extreme example take Barium. Toxic under all circumstances but one, $\ce{BaSO_4}$, because this one is insoluble and is used in medical imaging. $\endgroup$ – ssavec Jan 27 '15 at 10:01
  • $\begingroup$ @ssavec: which is pretty close to the difference in free Cd²⁺ vs. CdS $\endgroup$ – cbeleites supports Monica Jan 27 '15 at 10:24
  • $\begingroup$ @cbeleites: exactly, the solubility (or better bioavailability) is the key. I wanted to point out the funny fact, that the doctor could ask you to drink a glass of "milk", containing extremely poisonous metal. $\endgroup$ – ssavec Jan 27 '15 at 13:12
  • $\begingroup$ Also insert the usual joke about the chemist and his friend at the restaurant. $\endgroup$ – Faraz Masroor Jan 28 '15 at 0:51
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A few things:

1) There is a saying that "the dose makes the poison." This reflects the fact that most substances (even water!) become toxic at a high enough dosage.

2) As to your question about individual elements, surprisingly many of them are highly problematic in elemental form. But very reactive elements (Group 1, 2, 16 (some), and 17) are often not present in their elemental form in nature, and once reacted, they are quite benign (as Fred Senese noted about table salt). These are the best examples for your specific question.

3) The other problematic elements tend to be so-called "heavy metals", and they can be problematic because they can substitute for lighter metals in the same group and interfere with biological processes (generally slows them down). For example, both zinc and iron are necessary for health, but cadmium and mercury (same group and zinc) are very toxic, and the same goes for ruthenium and osmium (similar to iron). This goes to your example: these heavier metals can often be problematic regardless of the form.

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I want to clarify that, although as the other answers demonstrate many elements are harmless in some but not all possible compounds, cadmium is not like that. Cadmium is a heavy metal; heavy metals should be assumed toxic in all their forms. This is because they easily substitute for lighter elements in the same column of the periodic table (for cadmium, that would be zinc) and many of those lighter elements are critically important to normal biochemistry. (Zinc, for instance, is used in key enzymes for aerobic respiration, protein synthesis, and DNA replication. Substituting cadmium will make many if not all of those malfunction.) For the same reason, heavy metals are easily absorbed by the body (the digestive system is actively trying to take up as much zinc as possible) and not easily gotten rid of.

Some heavy metal compounds are so unreactive and/or insoluble that they're harmless, but that's the exception rather than the rule.

That said, the microorganisms mentioned in that article were probably orders of magnitude more dangerous than the cadmium compound.

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As already mentioned in Fred's answer, Sodium (and other group 1 and 2 metals) are dangerous in elemental form, but are flammable and corrosive rather than toxic. But the halogens in elemental form are, as he says, much more toxic than the corresponding anions.

Another element which is very important for life in the form of phosphates but rather toxic in elemental form is phosphorus. My high school chemistry teacher claimed that phosphorus poisoning could be diagnosed by glow in the dark vomit.

Carbon, which is another extremely common element essential for life, will do you no good at all if inhaled in the form of coal dust.


Finally, it's worth noting that toxic heavy metals like lead and mercury are often far worse in their compounds than as the free elements.

There are various reasons for this, one of which is bioavailability. Lead pipes were once commonly used and only really a hazard when they corroded and thus dissolved in the water they were carrying. (BTW corrosion products of copper are also pretty bad for you.) Even if you eat lead shot, most of it will pass through your system unchanged.

On the other hand, the organolead compounds (tetraethyl lead etc) that were once used in gasoline as an antiknock were extremely toxic required special handling procedures. The combustion products were much less toxic (though still a health hazard.)

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  • $\begingroup$ But bioavailability works the other way round as well: if the bioavailability is extremely low (lead in lead pipes covered under $\ce{CaCO_3}$ or the $\ce{BaSO_4}$ in ssavec's comment to FredSenese's answer), the actual danger can be very low. $\endgroup$ – cbeleites supports Monica Jan 27 '15 at 10:32
  • $\begingroup$ Also remember that while white phosphorous ($\ce{P_4}$) is toxic (and self-iginiting), but red phosphorous is not nearly as dangerous. $\endgroup$ – cbeleites supports Monica Jan 27 '15 at 10:52
  • $\begingroup$ @cbeleites that's a good point, I hadn't thought of that one. But it's not straightforward bioavailability of "elemental barium", because Ba reacts with water to form Ba(OH)2 which is a soluble (and therefore bioavailable) compound. Ba(OH)2 is classified as "harmful" with an LD50 of 308mg/kg in rats according to the first source I found. That's toxic but not massively so. I tend to think of group 1 and 2 metals as "not heavy metals" because the really nasty ones tend to be in the transition and posttransition blocks, though it is debatable. $\endgroup$ – Level River St Jan 27 '15 at 10:59
  • $\begingroup$ I'd even have said that that say, eating, elemental Ba is massively dangerous because of the basic reaction and the formation of hydrogen, and my guess would be that you may have so much trouble with the alkali burn that toxicity doesn't matter... Maybe I was thinking too large scale: toxicity will play a role if the amount is small enough that you have enough pH buffering capacity. Then the question is probably whether enough sulfate is availble. So not straightforward at all. Whereas BaSO4 is straightforward: not easy at all to release the Ba²⁺ once the sulfate is formed. $\endgroup$ – cbeleites supports Monica Jan 27 '15 at 11:07
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    $\begingroup$ @cbeleites I wouldn't say it was just dangerous to eat elemental Ba, I'd say it was impossible. You'd burn your mouth and stop before you'd ingested sufficient to kill yourself by toxicity. The question asks for "toxic" elements, and you'd really need barium in a more, ahem, "palatable" form to be able to ingest enough to cause any internal organ damage. $\endgroup$ – Level River St Jan 27 '15 at 11:20
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Soap is another good example, if you include corrosive materials. And while you're not ingesting it, you're likely absorbing some amount via your skin on a regular basis.

One of the necessary components of soapmaking is lye, or sodium hydroxide, which is so alkaline that it will eat through flesh. It also has an exothermic reaction with water, to the point that it can be explosive if you pour the water onto lye instead of the lye into the water (the former mixes it too fast, while the latter allows for a slower mix and more time for things to settle a little bit).

However, due to the chemical reaction that lye undergoes with fatty acids, you're left with an inert and safe compound.

Additionally, sodium hydroxide is often used on pretzels before baking, but the (much more subtle) reaction with the dough renders it nonhazardous, as the end result is a change in the pH of the surface of the dough, which gives you that nice brown, chewy exterior. (Sodium bicarbonate or sodium carbonate can be used for this purpose, as well.)

Borax is another one, though human exposure is generally skin or indirect contact. Boric Acid, one of the chemical components of Borax, is toxic in both acute exposure (LF50 = 1-20g/kg in humans, depending on age), and chronic exposure (as little as 32mg/kg for chronic lethal dose). However, Borax, itself, is a salt of Boric Acid, and so, doesn't carry the same toxicity unless broken down with something like hydrochloric acid. In fact, Borax is a great detergent for all sorts of household cleaning purposes (laundry, dishes, all-purpose cleaning).

Baking soda (sodium bicarbonate) -- most commonly used as a leavening agent in baked goods -- is the product of a reaction of sodium chloride, ammonia, and carbon dioxide in water. Its cousin, washing soda (sodium carbonate) can be produced using sodium hydroxide. Which one you get from that initial reaction depends on how much carbon dioxide you add to the mix.

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  • $\begingroup$ I don't think soap is a very good example, since the finished product (at least nowadays) doesn't really contain any sodium hydroxide in any meaningful sense: it may well contain some sodium ions, but there's no hydroxide left to speak of, as what isn't consumed in the saponification process is washed away afterwards. That said, back in the old days before modern industrial chemistry, some cheap crude soaps did often contain considerable amounts of left-over NaOH, making them somewhat caustic and unpleasant to use. $\endgroup$ – Ilmari Karonen Jan 27 '15 at 19:54
  • $\begingroup$ @IlmariKaronen - the same can be said of any reaction involving compounds as opposed to elements. Compound reactions will usually result in one or more other compounds, plus one or more elemental thing (often a gas), and sometimes heat. In the case of soap, the hydrogen is released. However, that doesn't change the fact that it's required in the soap making process and is caustic on its own. Also, it's not the industrial process of making soap that gives less leftover lye, but the use of crystalline lye instead of potash, resulting in more accurate measuring of the lye content. $\endgroup$ – Shauna Jan 28 '15 at 16:19
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The obvious example is roxarsone, an arsenic based growth hormone commonly found on the shelves in chicken. We are assured that it is non toxic. I guess the body handles toxic elements differently if they are bound to other elements?

http://www.iatp.org/documents/arsenic-widespread-in-chicken-testing-finds

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  • $\begingroup$ Welcome to chemistry.se! If you have questions about how to beautify your posts, have a look at the help center. Do you want to know more about this site, please take the tour. $\endgroup$ – Martin - マーチン Jan 28 '15 at 6:19
  • $\begingroup$ Salvarsan (or arsphenamine), a medicine for syphilis, is also an arsenic based compound. $\endgroup$ – Lexicon Jan 30 '15 at 8:50
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This may be one of the simplest molecules to answer the question.

Barium sulphate is widely used, in the form of an emulsion in water, for the "barium meal" X-ray procedure for diagnosing problems in the digestive tract. Barium is highly toxic, as are most of its compounds, but the sulphate is insoluble in water and stomach acid, so does not pass into the bloodstream and kill the patient.

Barytes, the mineral, is slightly impure barium carbonate (oftem pink colour) and is found in most parts of the world where mineralisation is present, e.g. in and adjacent to veins of metals such as lead and copper ores. It is safe to handle, and obviously has not been dissolved by groundwater over millions of years, demonstrating that it is insoluble.

But ingest almost any other barium compound, and you die. Handle with extreme care!

Oh, and as a quick edit in response to another post above, barium is indeed a heavy metal. I agree that almost all heavy metals (exceptions such as gold and platinum) are toxic, however there will probably be a few of their compounds which, like barium sulphate, can't pass into the body fluids due to lack of solubility, and as such will be non-toxic.

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