[nashalo]

It seems very weird to me that borax is so widely used in contact with humans in a variety of settings like ocular care and anti foot odor, because I use it as a flux in silversmithing and there are warning labels everywhere about the flux that it's reprotoxic, carcinogenic and mutagenic. Anyone knows why is that? Are my labels overcautious?

[ramchip]

I would guess there's more than borax in it:

> Traditionally borax was used as a flux for brazing, but there are now many different fluxes available, often using active chemicals such as fluorides[9] as well as wetting agents. Many of these chemicals are toxic and due care should be taken during their use.

https://en.wikipedia.org/wiki/Flux_(metallurgy)#Brazing_and_...

[autojoechen]

From the toxicity section (https://en.wikipedia.org/wiki/Borax#Toxicity) it seems like the concern is most about ingesting borax in large quantities or breathing in the dust. And also being cautious on potential health affects:

  Since people are already exposed to boric acid naturally through their diets and water, Health Canada advised that exposure from other sources should be reduced as much as possible, especially for children and pregnant women. The concern is not with any one product, but rather multiple exposures from a variety of sources.

[ethbr0]

The more likely effect seems to be the result (again, as boric acid) on fertility.

https://en.m.wikipedia.org/wiki/Boric_acid#Toxicology

"Although it does not appear to be carcinogenic, studies in dogs have reported testicular atrophy after exposure to 32 mg/kg bw/day for 90 days. This level is far lower than the LD50."

The base toxic dosage (on the order of multiple grams per kilogram bodyweight) is the reason it's typically labeled as very safe.

[nashalo]

The labels and general knowledge passed from trainer to trainee insist on the reprotoxicity of the flux, which as you point appears to be at exposure levels far below toxic levels. I wonder if the safe label of boric acid for feet is made looking only at the general toxicity of the product, leaving aside potential reproductive issues that might arise with repeated use of small quantities?

[refurb]

Borax flux is a mixture of boric acid and ammonium chloride and then you are dumping it into molten metal where it likely combines with metals and gives off gases.

Kind of different than just boric acid itself?

[nashalo]

At least in silversmithing I don't recall flux to contain any ammonium chloride it seems to be a solution of borax alone.

[julienmarie]

Borax is used in most sturgeon french caviars to preserve it. I guess in small quantities.