Using the 100 ppb arsenic level as a baseline, and using selenium as a model for daily requirements and toxicity since we don't have one for arsenic, that seems

Using the 100 ppb arsenic level as a baseline, and using selenium as a model for daily requirements and toxicity since we don't have one for arsenic, that seems to suggest that up to half a kilo per day of rice should be well-tolerated? Assuming I math-ed it correctly. There is insufficient data to build an authoritative model of dietary arsenic requirements AFAIK. It is a necessary micronutrient in animal biology and evidence suggests it is similar to selenium in requirement magnitude and toxicity profile, so it is sometimes treated as comparable in the literature. Obviously, it is a rough cut since this was extrapolated from animal models -- we don't have good human models -- but in principle the RDA should be well-tolerated by definition and these levels are endemic in some parts of the world.