Are there any health concerns with letting children use old LEGO kits? I have a hoard of LEGO kits that passed from me to my nephew, and now to my son. Some of the kits that are included with the set date back to the early 1970's. Are there any health risks with passing the older kits down to my son? Have there ever been recalls or warnings published about the chemical make-up of older bricks?
The most important thing is to make you keep any swallowable magnet out of reach.

This is potentially lethal if two magnets are swallowed and attrach each other from different part of the intestine, as unfortunately did happen to a kid a few years back with some other toy. That's why LEGO has completely redesigned train magnets to be completely encased in the buffers, which are a way too big element to swallow.