Back when I was tangentially involved with the cashew business (during college vacations), I never quite worked out why we had to wear goggles to extract the nut from its hard shell. The story was that you would go blind if you didn't, but I thought that was silly. It's a nut - how is it going to make you go blind? Anyway, the Oxford Companion to Food answers my question:

"The nut is not easy to extract. The tissue between its two layers of hard shell contains strongly irritant substances, cardol and anacardic acid. This tissue has medical uses, including burning off warts, an indication of its great corrosive power. The usual way of treating the nut is to roast it whole, driving off the irritants and making the shell brittle enough to crack without crushing the contents".

Oh.