Was there ever cadmium in LEGO bricks?
I asked Gary Istok on your behalf in this Brickset Forum thread, who is an expert on LEGO's history. He said the following: 


  When LEGO replaced the Cellulose Acetate bricks circa 1963 with ABS
  plastic for non-trans parts, and polycarbonate for the trans ones,
  they had a problem with the red and yellow parts.  For some reason
  (I'm not a chemical engineer)... red and yellow ABS parts were more
  difficult to color to TLG standards.  So Bayer and Borg-Warner (the
  chemical companies that produced the ABS and other plastic pellets
  worldwide) added Cadmium, a heavy metal, as an additive to those 2
  LEGO colors to help in the coloring process.
  
  Although Cadmium will not leach out of the LEGO bricks of that era (kids could safely chew on those LEGO parts), the longer term landfill
  issues of Cadmium laced LEGO getting into the ground, did become an
  issue with environmentalists.  So TLG had their plastics makers
  Borg-Warner (for USA, Canada, Britain, Ireland and Australia) and
  Bayer (for continental Europe and Asia) work on removing the cadmium
  from the red and yellow elements of that era.  I don't know the
  chemical specifics of that action, but by 1973 LEGO parts were Cadmium
  free... and have remained so.
  
  Next question.... can you tell Cadmium parts (1963-72) from Cadmium free parts?  Generally yes... but it's easier for red bricks, which
  are a darker red color with Cadmium.  Yellow LEGO parts with Cadmium
  are also a darker color, but the difference between Cadmium and
  Cadmium free yellow parts is less noticeable... especially with used
  or dirty yellow elements.