Chlorpropham
About
Chlorpropham is a germination inhibitor approved for use on potatoes in Germany, Austria, and Switzerland. This video implies that it is also in use in the United States.
It is marketed under the names Beet-Kleen, Bud Nip, Chloro IPC, CIPC, Furloe, Sprout Nip, Spud-Nic, Taterpex, Triherbide-CIPC, and Unicrop CIPC.
Questions
Chlorpropham's LD50 (the dose necessary to cause death 50% of the time) in rats is roughly 2 parts in 1000 -- which for a 100-lb human would be about 3 ounces (of solid chlorpropham). How does this compare with other more familiar substances? How much chlorpropham is in an average treated potato? The EXTOXNET article also says absolutely nothing about accumulation over time, which could significantly lower the danger threshold for ingestion.
It may be that it really is "safe", for any meaningful definition of "safe" (e.g. no more likely to kill you than driving 2-3 hours per day), but if this claim is being made, it is apparently being made in the absence of information which would demonstrate its veracity.