Safe from what exactly???
If you are worried that it had lead paint on it before and it absorbed some, use a lead check swab.There are numerous other tests for all sorts of other chemicals it may have also been exposed to, which a lab will be glad to test for.
Quite simply, wear the proper PPE (Personal Protective Equipment) applicable when working on it.
- This generally means safety glasses and gloves at all times; some even say long sleeve shirt.
- And use a respirator when it comes time to sand, grind or saw it.
For more information:
Read "I'm interested in building kitchen cabinets out of reclaimed barn wood. Is this safe?" a Q&A answered by Ryan Flegal.