"The toxicity of oxalic acid is quite low; minimal lethal dose for humans is
considered to be about 5 g for an adult. For example, an excessive consumption
of rhubarb (more than 100 g) can represent a tenth of the minimal lethal dose
. On the other hand, one of these plants, Rumex crispus L., has caused fatal
poisoning through ingestion of the plant material. Among the pathological
findings were centrolobular hepatic necrosis and birefringent crystals in the
liver and kidneys [2, 3]."
Plant Foods for Human Nutrition 51: 99–107, 1997
"Oxalic acid is found in high concentrations in some plants consumed by ruminants and may cause renal toxicity. To determine whether exposure to oxalic acid affects the capacity of the rumen of sheep and goats to degrade the compound, 20 animals (10 sheep and 10 goats) were dosed with free oxalic acid by gelatin capsule twice daily for 3 weeks at one of five levels (0·0, 0·3, 0·6, 0·9 and 1·2 mmol/kg live weight (M) per day). Rumen samples were collected by stomach tube in the week prior to the start of dosing and in each week of the 3-week experiment. Oxalic acid degradation capacity was measured by adding uC-labelled oxalic acid to rumen fluid in vitro and capturing evolved 14CO2. Rates of degradation increased with increasing level of administration (2·30, 4·71, 6·74, 9·83 and 13·90 mmol of oxalic acid degraded per I rumen fluid per day for doses 0·0, 0·3, 0·6, 0·9 and 1·2 mmol/kg M per day, respectively; P < 0·001). Rates of degradation increased during the dosing period (P < 0·001) with the largest increases occurring in the 1st week of dosing. Goats showed a greater response than sheep, with a higher mean oxalic acid degradation capacity (9·04 v. 5·95 mmol of oxalic acid degraded per I rumen fluid, P < 0·05). Oxalic acid administration did not influence plasma calcium concentration or cause renal function impairment as measured by plasma creatinine concentrations. The experiment demonstrated adaptation in the rumen to potential toxins in the host diet and suggests that the rumen micro-organisms of goats may have been more adapted to degrading oxalic acid than sheep."
AJ Duncan, P Frutos, SA Young - Animal Science, 1997 - cambridge.org
Foods high in oxalate (100–900 mg per serving) include:
"Acute oral toxicity : Acute toxicity estimate : 378.79 mg/kg
Acute inhalation toxicity : 4 h Acute toxicity estimate : 5.3 mg/l"
"Acute oral toxicity (LD50): 375 mg/kg [Rat]."
The last two citations are both Material Safety Data Sheets. These documents are written by lawyers which explains the ridiculous number of significant figures shown. This data overall suggests the LD50 for adult humans would be someplace around 20 grams even thou someone died from only eating five grams. However LD50s are notorious for giving data with far outliers. The problem is acute metabolism can vary widely from one test animal or one human to the next. The result is a lethal dose to one rat may be two or three or more times the lethal dose to the next rat. The same is true of humans. The reasons can range from slight differences in liver function to how well the particular individuals kidneys are working. So, I do not find the first citation that someone died from a dose of five grams either surprising nor alarming.
The first MSDS also lists an inhalation hazard. This hazard has been widely talked about in the bee community. The lethal dose listed is 5.3 mg/liter of air breathed over a period of four hours. A person at rest will breath about 7 liters of air per minute. So, to breath a lethal dose you would need to breath in 7 l/min times 60 min/hour times 4 hours times 5 mg/l. That translates to breathing in a total of 80 grams of oxalic acid. As you only treat a double with two grams you would need to treat an awful lot of hives to breath in 80 grams of oxalic acid escaping thru cracks in the hive. However, far less than that will give you the worst cough you have ever had in your life (unless you have worked around chlorine gas). I suppose that cough could even be lethal if you happened to have heart issues as the cough could trigger a heart attack. The good news is very inexpensive and highly effective breathing protection is easy to find at any of the big box lumber and hardware stores. I find that even with my full beard I have yet to detect the slightest amount of oxalic acid particles getting past and I know it is impossible to get a good seal between the respirator and my face with the beard. I conclude that the compressed beard is a decently effective filter that stops the particles in the air that leaks by due to the poor seal. I also conclude that the bee keeper is not running any significant risk of harming him or herself if they use decent sense and spend the few bucks on breathing protection. All you need is a real good dust mask althou one rated for acid gasses was the one I picked. It was $20.
So what does all this mean in terms of using it on bees? The normal dose for a full sized hive by vaporization application is 2 grams. It rapidly gets spread thru the whole hive. I have done experiments with a double deep with three empty honey supers on top and using the Provap applicator a two gram dose will cause fog to come out of cracks under the top cover. It is also coming out any cracks around the bottom board in much larger amounts than at the top based on visual density of the fog. So, it gets spread thru the whole hive in seconds. If I treated with honey supers on, as is common practice in Europe, perhaps one gram at most might end up in the supers dissolved in honey. I am assuming three medium supers and two full depth boxes for the brood chamber. Let's assume it is a modest honey year and those three supers end up yielding 75 pounds of honey because they are not stuffed full. I now have one gram of oxalic acid in 75 pounds of honey. So, even if you turn out to be an outlier and extremely sensitive to oxalic acid you need to eat 375 pounds of honey in one sitting to get to the five gram lethal dose. That is physically impossible. Or perhaps I treated that hive with oxalic acid ten times and put ten grams in my honey. Now the person has to eat 35 pounds of honey to get his five gram lethal dose. That is still physically impossible. I conclude the Europeans are not dumb and running any measurable risk of producing toxic honey by treating with honey supers on.
In fact oxalic acid is a normal component of a human's diet. As it is a normal component of the human diet I think we can safely not worry very much about cumulative effects of smaller daily doses in spite of the fact that cumulative smaller doses of many chemicals are very harmful or even deadly. The third citation above lists a number of foods which contain from 0.1 grams to 0.9 grams of oxalic acid per serving. Even thou some of these single servings contain nearly 20% of a lethal dose we do not hear about anyone dying from eating too much spinach or chocolate or beets. I will be more than happy to eat two helpings of spinach if someone else will eat my helping of beets. I am very willing to bet that five grams is not my lethal dose based on the amount of spinach I have eaten at one sitting a number of times. Besides, I am pretty sure the carcinogens in the mushrooms I eat will get me before the oxalic acid I eat gets me. After all, the damage from the carcinogens is cumulative in contrast to oxalic which either kills you or is metabolized and eliminated. Or, maybe the raw beans I eat will kill me first? Life sure is dangerous.