Like DDT. My dad tells me when he was in WWII in the south pacific that the planes would fly over and dump DDT on the islands...and the soldiers, locals, food, and everything else in the way.
What about asbestos? People used to walk through the stuff neck deep sometimes, with NO protection. Only relatively recently have they figured out that it causes cancer among other things.
Things like this may never be "discovered" about DNP because of the tiny percentage of people taking it. If 1/2 the population were using it though...who knows.