Radon
An invisible, odorless radioactive gas from uranium in soil and rock that can build up in homes and raise lung cancer risk. NJDEP recommends testing all homes and fixing levels of 4 pCi/L or more.
New Jersey example
Made-up example: a buyer's contract allows a radon test. A New Jersey certified company must place and pick up the device, since the buyer, seller and agents can't do any part of it. A 2 to 7 day test reads 6 pCi/L, above the 4 pCi/L level. A buyer who would rather test after closing can have the seller set aside escrow funds for mitigation. The most common system averages about $1,300.
Official source
Radon Testing and Mitigation: The Basics (NJDEP Radon Section)
This is a plain-English summary. Rules and dollar amounts change, so check the official source or a professional before you file or sign anything.