Some commercial varieties of salt have small amounts of sodium ferrocyanide added to prevent the caking (or clumping) that can occur under various conditions. When humidity is high, for example, a thin layer of moisture forms on the surface of the salt crystals causing some of the salt to dissolve in this layer thereby forming brine. If the relative humidity drops, the water then evaporates and the brine solution recrystallizes between the salt crystals, causing them to aggregate into clumps. By adding ferrocyanide, the solubility of salt in water decreases so the salt is less likely to dissolve in the moisture coating the crystals. All to say that ferrocyanide reduces the amount of recrystallization.
But of course, any mention of cyanide conjures up images of poison so the very presence of ferrocyanide in salt sounds scary. Which is precisely why producers would rather list it on a label as “yellow prussiate of soda,” an old-fashioned term first coined in reference to Prussia, the country where it was originally synthesized. There is, however, no need to be terrified of ferrocyanide because the cyanide in this compound is tightly bound to an iron atom preventing it from being released in the body. And even if it were released, it would be irrelevant because the amount is way too little to cause any harm. Not to mention the fact that ferrocyanide itself is remarkably non-toxic.
For those who still, for some reason or other, prefer to steer clear of a salt with ferrocyanide, there are some salts with other anti-caking agents. Regular Windsor salt, for example, uses calcium silicate, whereas its kosher salt version uses yellow prussiate of soda. Apparently, however, ferrocyanide is the most effective of the anti-caking agents. So really, just don't worry about it.
@Joe Schwarcz
