10% formalin is classified as an "aviation regulated liquid, n.o.s.," UN 3334. See the US Department of Transportation (DOT) letter of clarification for more information.
The DOT's classification "includes any material which is not covered by any of the other classes, but which has an anesthetic, narcotic, noxious or other similar property such that, in the event of spillage or leakage on an aircraft, extreme annoyance or discomfort could be caused to crew members, preventing the correct performance of assigned duties." (172.102©(2)(A35))