The 1970s ban on mustard oil in food by the EU, US and Canada is so inexplicable that it must have been politically motivated. The ban hinges on a component called erucic acid, which can cause heart damage at high doses in rats and piglets. Erucic acid is present at lower concentrations in rapeseed oil (canola in the US), which is of course widely-consumed - and farmed - in the West. (Rubs chin...wonder whether Monsanto has the patent for that.)
It's an important flavour in Indian desi cooking, and although it is a "marmite" ingredient, many of us get hooked on the flavour, which hovers in the background rather like sesame oil.
To the best of my knowledge, the mustard oil on the shelves of British Indian grocers that is euphemistically labelled "for external use only" or "massage oil" is food grade.