I called the American Association of Poison Control Centers (1-800-222-1222) and asked about putting rhubarb in a smoker. The reply was there is no information in their data set about rhubarb vapors. I know that Randy (thanks Randy) has some good information about using Oxalic acid on his website. It is just my way of thinking but if I use rhubarb in the smoker then each new batch of bees gets treated for mites two or three times in their lifetime, plus get some benefit of the antibiotics. 
As far as the human safety side of it this site (http://www.rhubarbinfo.com/poison ) had this to say:  From an MSDS (Material Safety Data Sheet) for Oxalic acid, LD50 (LD50 is the Median Lethal Dose, which is the dose of a drug or chemical predicted to produce a lethal effect in 50 percent of the subjects to whom the dose is given) in rats is 375 mg/kg. So for a person about 145 pounds (65.7 kg) that's about 25 grams of pure oxalic acid required to cause death. Rhubarb leaves are probably around 0.5% oxalic acid, so that you would need to eat quite a large serving of leaves, like 5 kg (11 lbs), to get that 24 grams of oxalic acid. Note that it will only require a fraction of that to cause sickness.

Cheers

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