 Cadmium makes up about 0 @.@ 1 mg kg − 1 ( ppm ) of Earth 's crust . Typical background concentrations in other environmental media are : atmosphere < 5 <unk> m − 3 ; soil < 2 mg kg − 1 ; vegetation < 0 @.@ 5 mg kg − 1 ; freshwater < 1 <unk> L − 1 ; seawater < 50 <unk> L − 1 ; sediment < 2 mg kg − 1 . Compared with the more abundant 65 ppm zinc , cadmium is rare . No significant deposits of cadmium @-@ containing ores are known . <unk> ( CdS ) , the only cadmium mineral of importance , is nearly always associated with <unk> ( <unk> ) . This association is caused by <unk> similarity between zinc and cadmium , with no geological process likely to separate them . Thus , cadmium is produced mainly as a <unk> from mining , <unk> , and refining <unk> ores of zinc , and , to a lesser degree , lead and copper . Small amounts of cadmium , about 10 % of consumption , are produced from secondary sources , mainly from dust generated by recycling iron and steel scrap . Production in the United States began in 1907 , but not until after World War I did cadmium come into wide use . 
