you know, chrom is a metal. but in the process of tanning, nobody puts a piece of chrom on the leather and waits. in this process liquids are used - but metal doesn't dissolve in liquids. --> if somebody says that chrom is used, he means a compound which contains chrom. --> the liquids used by tanning are containing a dissolved chrom compound.
after the process of tanning, the leather is washed with fresh water; the water takes up the chrom from the leather (and bring it in the rivers and seas
).
don't worry, if you can wear a material on the skin (clothes of leather) you also can put your food in it.
i'm sorry if my english expression isn't perfect (i learn it in school, but this doesn't really prepare for the real use of this language), i hope you can understand what i mean