>>Hogyan őszülhet meg teljes hosszában néhány óra alatt

mert megőszülhet? ;-)
(egyébként igen, egy igen ritka betegség egyik részeként... mondjuk a cukorbetegség egyik fajtája mellet, néha igen gyors lefolyással... mondjuk sokszor teljes szőrtelenedés követi, szintén gyorsan...)
egyébként meg:
"Historically, there are many anecdotes about “overnight graying,” caused by stressful situations. This has been dubbed “telogen effluvium,” but the evidence for its actual occurrence is spotty. Henry IV supposedly went gray overnight after escaping the St. Bartholomew’s Day Massacre in 1572, and Marie Antoinette supposedly went gray overnight before her execution (but with that wig, how would you know? Moreover, if her hair only had one day to grow before her head was chopped off, how do you know she went gray right then?). Evidence is pretty much non-existent on the stress-related sudden graying. There is a rare condition called “diffuse alopecia areata” (use that in a sentence, see where it gets you), which causes large quantities of pigmented hair to suddenly fall out. If you’ve got gray hair hiding on your head that doesn’t fall out, you might look like you’ve suddenly gone gray, when you’ve really suddenly gone bald."