The issue here is totally manmade. Cotton is a very pesticide intensive crop, and in the old days, lots of those pesticides were As based. This resulted in relatively high levels of As in the soils, and rice is very good at uptake of heavy metals (such as As) which then concentrate in the grain, so rice grown in soils previously used for cotton fields tend to have elevated levels of As. You are correct that As is always found in rice, but as has been known for centuries, “the dose makes the poison,” and this elevated level is the concern.
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