In the attached file the study in Selenium relates that inorganic forms that include Selenium Di-oxide, builds up in the blood to where it will become toxic over time. See this section on "SELENIUM TOXICITY IN ANIMALS".
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"In the rat, the dietary requirement for selenium is ca. 0.20 mg kg-1 (Hafeman et al., 1974). The threshold for selenium toxicity from selenite is about 0.50 mg kg-1, a mere 2.5 times the dietary requirement (Tinsley et al., 1967). Chronic dietary selenite toxicity in the rat begins at 3–4 mg kg-1 and there is almost no survival of rats fed 16 mg kg-1 Se (Harr et al., 1967)."
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"Chronic ingestion of either selenite or selenate in the rat, at the same selenium dietary levels, exhibits nearly equivalent toxicity (Brasher and Ogle, 1993; Wilber, 1993). Dietary ingestion of organoselenium compounds, however, exhibit wide differences in toxicity."
This is why the amounts noted in rodent block diets are below the daily requirment for selenium at 150 per Kg of diet, or 7.5 mcg. per 50 grams of diet. For when you look at other diets noting 0.02 mcg. daily, this is most likely due to the source being one of the inorganic higher elemental sources of Selenium that build up quickly in the blood, andn why the amounts noted in block diets don't't reflect the amount of the daily requirement.
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"Dietary ingestion of organoselenium compounds, however, exhibit wide differences in toxicity."
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1. Selenium compounds, i.e., selenite and selenium dioxide, can react with glutathione (GSH) and other thiols to form selenotrisulfides that will ultimately react to produce superoxide and hydrogen peroxide, and are toxic.
2. Diselenides, i.e., selenocystine and selenocystamine in the presence of GSH and other thiols, are reduced to selenols (RSeH), which are catalytic, produce superoxide and hydrogen peroxide, and are toxic.
3. Selenium compounds that do not react with thiols,i.e., selenate and all tested selenoethers (RSeR), do not produce superoxide or hydrogen peroxide in vitro and are not toxic per se.
4. Selenate and selenoethers are toxic in tissue culture or in vivo only after being reduced to selenite or a selenol.
5. Selenium toxicity manifests itself acutely or chronically when oxidative damage exceeds antioxidant defenses or the ability of either plants or animals to form seleno -
proteins, selenoethers, or elemental selenium (Se0) (Spallholz, 1994).
Without looking deeper into the toxicity level of the form of selenium in Brazil Nuts, to consider including this source in the diet would be risky. If you already feed a block diet it may aleady include what is a safe level to include in the diet; adding to it wouldn't be needful, nor warranted.