I was reading an interview with Jake Adelstein, a Yakuza expert, who says this:
Also the tattoos. Like, you can see on the cover of this book and the back of this book - see, one of the things they tend to do is get these full-body tattoos which mark them as members of the Yakuza. And the tattoos themselves are carcinogenic and they prevent your skin from sweating.
The "stopping the skin from sweating" sounds like the Goldfinger gold paint/skin breathing Urban Legend, but is there any truth in the claims, particularly regarding Japanese ink and any potential nasties it might contain.