"In years of use I have never seen this with OA vaporization."

Neither have I.  I also have way over dosed nucs with brood (two grams in a five over five deep) simply to see what the limits were before damage and failed to see any.  There are a lot of home brewed vaporizers out there and perhaps some way over heat the oxalic acid resulting in a lot of formic acid?  That could cause damage.

While we talk about oxalic acid vapor in reality there is no vapor exposure to bees or brood.  The oxalic acid desublimes to a fog of oxalic acid crystals the instant the temp drops a bit and that is within an inch or less of the sublimer as near as I can tell.  In fact in my experience a simple dust mask is adequate respiratory protection from the fog coming out of a hive during treatment.  If you were making a bunch of formic acid that dust mask would not stop it, so I am convinced my vaporizer does not make formic acid in any significant amount.

On the other hand those using a fogger with ethanol solutions of oxalic acid could well be totally destroying the oxalic acid and are just using a hard way to apply formic acid.  A very old lab method to make formic acid was to heat a solution of oxalic acid in glycerin (glycerin is an alcohol) to a bit over 100 deg C and distill out the formic acid which forms rapidly at those temps.  The tubes in a fogger are red hot so roughly 600 deg C.

Dick

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