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.
The LEGO group pride themselves on their low product recall rate, which I believe generally only effects parts that break too easily or similar - often this is related to Primo or DUPLO components that are aimed at very young children.
I'm not aware of a recall due to the chosen materials - as noted in this question the LEGO Group switched to their current material (ABS) in 1963, however the exact composition may vary from modern bricks.
It probably wouldn't hurt to take a look at the advice for cleaning the bricks in warm, dilute soapy water or with a mild bleach.
Certainly any effects would have to be low level and based on long term exposure, as my kids haven't shown any adverse effects to using my collection from the late 70's onwards.
Just one further point of note: The original rubber bands from the Technic sets are prone to deterioration - so watch out for them giving way.
A health risk with old, well used hand me down bricks is the possibility of broken pieces and foreign objects like staples, glass shards, old batteries, (loose magnets!) and stuff being mixed in with the elements. It's always a good idea to check through them before you hand them over to the next generation of builders.
Nothing really worse than the newer kits, quite frankly.
Below 3 years old, they tend to explore what the world has to offer by putting small bricks in their tiny noses. for these reasons, an adult need to be there all the time brick toys are in use. If the bricks happen to be the adult`s toy, rigorously stowing them away when not playing is in order.
Old bricks might just be plain dirty. If your LEGO sets have been collecting dust on a shelve for twenty years, you might want to clean them off first.