Do old LEGO bricks contain lead, cadmium, or other hazardous elements?
No, you should not be concerned about lead.  LEGO has always used lead-free colors in their elements, even back in the beginning.

However, not all LEGO-compatible bricks are lead free.  For example, Mega Bloks, which are made in China, suffered from the poor quality control that led to massive recalls over lead-tainted products a few years ago.


  David Clerk, the magazine's publisher and the executive director of
  Les Editions Protégez-Vous, said that after consulting with Health
  Canada, the magazine hired an independent lab in Quebec to perform
  what he called a "total lead test" on 32 toys, including the Maxi
  blocks.
  
  Essentially, the process involves scraping off a sample of the toy's
  plastic, dissolving it in acid and then analyzing the solution.
  
  When the results were returned by a lab, which Clerk said he could not
  identify because of a confidentiality agreement, a yellow Maxi block
  was the only toy that exceeded the 600-parts-per-million limit for
  lead set by Canada and the United States. Blue and red Maxi blocks
  showed no lead at all.
  
  For confirmation, the magazine tested a second yellow block. It
  contained 1,180 parts per million of lead, nearly double the initial
  result.