toxicity of genuine maganese blue

Thank you so much for your quick and thorough answer, Koo! ​

I believe the main issue with this pigment is soluble barium. My understanding is that PB 33 is two different molecules that are coprecipiated/bound. I could look this up in Wikipedia but it’s not that important here except when it comes to the exact color of the pigment. Apparently, the proportions can be varied some to yield a more bluish and darker pigment and a paler and more greenish pigment. Old Holland is the only paint line I know of that offered two manganese blues (a deep and a regular) — and that is in the oils line only.

Barium sulfate is used ​as a contrast agent in medicine due to its very low solubility. However, soluble barium compounds are considered quite poisonous, as far as I know. Fortunately, unlike arsenic (which reacts with hydrogen in the air to form arsine), they do not become airborne when they’re in paint form.
I also don’t believe soluble barium is as toxic (especially to children) as soluble lead and some forms of soluble mercury. However, I am not certain about that. I haven’t, for example, heard of any lab tech dying from spilling several drops of a soluble barium compound on a work glove, unlike with a mercury compound. However, before that scares people away from the use of vermilion — genuine vermilion, when produced well (a pure product), is highly insoluble — to the point wherein it was used as a food coloring in the 19th century. Unfortunately for customers, adulteration with red lead led to people becoming quite ill sometimes from those treats. The insolubility of mercuric sulfide makes it vastly less dangerous to paint with, versus the particular mercury compound that killed that researcher.
The biggest problem with lead white is that it is a combination of two soluble lead molecules. Carbonates aren’t tremendously soluble, which is why people taking magnesium and calcium supplements often get a citrate or some other form that’s more readily absorbed. Adults also don’t absorb much lead carbonate when it is ingested but it is very dangerous for children/fetuses — the developing brain, as any amount of lead strongly disrupts neural connection formation. I also remember a documentary with Alan Alda about lead in gasoline (tetraethyl lead) that said lead is 1,000 times as attractive to human cells (at least one type of them), versus the calcium it mimics, and causes them to vibrate to death. Once the cell has died from this overexcitement, the lead atom passes to the next cell. Some lead is excreted and some is stored in bone and periodically comes out, goes back in after doing more damage, or is excreted. Those with low calcium level face more damage, even though the lead is so much more attractive to the body than calcium. The bottom line here is that lead (which is always in an at least semi-soluble compound form in fine arts materials) should be kept away from kids/pregnant women at all costs and should be treated with great care. That includes digging around in the soil of old buildings, et cetera. The demolition of buildings in inner cities leads to elevated lead levels (lead poisoning) in inner-city youth.

Solubility is something often overlooked when comparing pigments. Many artists declare, for instance, that they won’t use “toxic” cobalt pigments. Well, some cobalt pigments have the cobalt tightly bound in a mixed metal matrix that leaves it largely (or completely insoluble on a practical level). That makes such cobalt compounds non-toxic for professional artists’ usage in non-sprayed non-dust paint form.
I am also dubious about the belief that manganese blue can’t be manufactured without harming the environment. I have little doubt that it’s possible to create a workflow that will yield the pigment without environmental pollution. The question then involves: “Does industry feel it will benefit enough from the pigment to justify the higher cost of the non-polluting production method?”
As pigments aren’t produced for artists’ materials in volume, the answer to that, so far, appears to be “no.” Phthalo blue is extremely cheap and for most industrial purposes, seen as lightfast enough (except for at least one of the sulfonated versions, which is BW6). It lacks the chroma and the lightfastness of PB 33 but only watercolorists in particular are going to miss that strongly. I say “only” glibly because I am writing from the point of view of industrial large-scale manufacturing, which cares not about the fine arts. Personally, I think manganese blue is one of the most important watercolor pigments there is — irreplacable.

It’s a beautiful pigment in oils, too, but affected by the yellowing of oil. I can’t say it’s true but one test elsewhere showed that adding 5% zinc white (PW 4) prevented the color shift from cyan blue to greenish. My guess is that the zinc oxide disrupts the cross-linking of the drying oil molecules which prevents the yellowing. The same person found the same result with cobalt violet light (PV 14 and PV 49). He/she did not test PV 47. However, I have painted with manganese blue and cobalt violet lights and haven’t had dramatic color shift from those ground in paler oils such as poppy. Therefore, this issue might be related to linseed-ground paint exclusively and/or related to linseed-ground paint that has an abundance of oil-holding filler (which exacerbates the yellowing by reducing the pigment level). This is all very speculative but it does seem that Old Holland’s manganese blue (which I have a tube of in oil) and Blockx’s now-discontinued (which I also used, which was ground in poppy) show a brightness difference out of the tube, relating the oil coloration. Budget-brand manganese, like Permalba, seems to have an even greener cast, out of the tube. The issue with adding the zinc, of course, is the embrittlement of the paint film. If one has found that one’s manganese blue and/or cobalt violet light has color shift issues and wants to add zinc to combat that, my advice is to only paint on a fully-stable support, such as aluminum (primed/prepared suitably, of course). Aluminum expands and contracts less than other common rigid supports.
While I am rambling a bit about the addition of white to a pigment to preserve its color… one conservation article said that genuine vermilion (mercuric sulfide) does not darken when mixed with some quantity of lead white (the typical carbonate type, not the sulfate). I don’t know what the minimum amount of lead white is that is required to prevent the blackening of the vermilion but that would be something worth investigating, although cadmium red has made vermilion obsolete.

Vermilion does blacken rapidly. I have two abstract paintings, one painted with Harding’s Chinese vermilion and one painted with Holbein’s Chinese variety. Both, despite being very different shades out of the tube (Harding’s is made with an inferior process that yields brownish dull color — something warned about in one of the 19th-century industrial paint manuals) — both have blackened noticably. They aren’t black but they have darkened significantly. They weren’t mixed with lead white.
For both the lead white and zinc white color preservation tactics, mixing the white component in while grinding the pigment may be best, to ensure that it’s fully distributed. For the vermilion, removing all of the residual sulfur is probably a good idea, to avoid having it react with the lead.
Back to the original question… I wouldn’t avoid using manganese blue in artwork so long as the usual precautions for toxic heavy metal pigments are followed. 
Arsenic is a special case in terms of handling precautions due to the off-gassing problem. I asked a chemist recently if cobalt arsenate (the original cobalt violet light) is a risk for producing arsine (a problem with the copper arsenates like emerald green and Scheele’s), as I haven’t heard of it blackening nor losing its color in some other way on the pigment level when used in artists’ paints. He is an organic chemist so perhaps he doesn’t know but he was still concerned about the possibility of arsine being formed. I have looked at the literature and haven’t found any detailed information whatsoever about this pigment’s stability. One hobby chemist who made some as a demonstration said he believes it is tightly bound enough to not react. My guess, based on the complete lack of writing about the pigment blackening, fading, changing color, et cetera — is that it’s the only stable arsenic pigment used in art. However, I would really like to know. Holbein was the last to produce it. Its catalogue from the 1980s still showed it.