A través de la Bee-L aparece la teoría de Bernhard Heuvel apicultor que postea en la Beesource sobre el modo de acción del ácido oxálico "vaporizado". Según este colega el oxálico vaporizado "condensa" sobre superficies húmedas como las patas de las varroas que forman sendos cristales así como se ve a través de un microscopio.
El colega plantea que en las fotografías se ve un "canal abierto" que probablemente se hace cargo de mantener humectadas las patas de las varroas. Por ese canal "penetraría" (¿contra flujo?) el ácido oxálico que condensa y luego se diluye en la punta de sus patas.
Algo así explica más abajo ... en ingles.
The mite has some sticky pads (empodium) that look like this: (my own microscope pictures)
Detail of the sticky pad at the end of the mite's foot:
Note that there is an open channel going from the pad up into the leg. That channel is open constantly and provides moisture of the mite's body fluid to the pad, so it stays sticky.
A beekeeper I know, Gerhard Brüning, made some impressive pictures of the mites after an oxalic acid treatment:
His description in German: http://www.varroamilbe.ch/bericht3.pdf
It seems that the vaporized oxalic crystals condense where it is moist, and that is at the sticky pads of the mites. All the pads have a buildup of oxalic acid crystals and the mites drop off the bees.
Those crystals disappear after about 2 hours and the mites are even able to crawl back up upon a bee.
But: it seems the oxalic acids dilutes into the mite through that open channel right into the mite where it causes damage to the inner organs.
So there are two effects: the immediate effect is the dropping (preferably through a screen bottom board out of the hive) and the second effect is the destroying of the inner organs of the mites.
From the drop I'd guess that'll be one to three days after the treatment, probably there are longterm effects, too.
Bees don't get damaged much, although they too have a sticky pad at their feet. But there is a difference: bees can close that channel between the pad and the foot. There is a ring-like structure that can close down that channel. It is most probable - but not proven yet - that this is why bees don't get hurt much by the oxalic acid vapor.
Anyway, for us it is important to make the vapor circulating inside the hive for long enough, to make it condense at the mite's sticky pads.
I reckon, dosage doesn't really matter much other than providing enough crystals to build up with type of vaporizer you have. The varrox vaporizer has been tested intensively and was found to be very effective. Other devices were not half as good. Temperature and other parameters will play into it. If you burn the oxalic acid and not vaporize it, there will be no buildup of crystals, just smoke.