Unless you happen to work in a cashew plant or pick cashew apples, it’s unlikely you’ll encounter completely raw cashews. The so-called raw cashews sold in natural food stores are not exactly raw, but instead are steamed. It is the case that the double shell surrounding the raw cashew, which is technically a seed and not a nut, contains urushiol, a resin that can create significant skin rashes, and can be toxic when ingested.
Urushiol is the same chemical found in poison ivy, and it is present on the leaves of the cashew tree as well as in the raw cashew shell. Processing raw cashews can be a laborious and nightmarish ordeal, and people who work in cashew processing plants tend to exhibit greater allergies to cashew shells over time. There is a high incidence of skin rashes among people who either harvest or process raw cashews. Greater sensitivity to urushiol can lead to extreme allergic reaction when raw cashews are ingested, and anyone allergic to poison ivy could potentially have a fatal reaction to eating true raw cashews.
This is why we don’t eat strictly raw cashews. Even the “unroasted’ varieties are steamed to release urushiol from the nut and make it safe to eat. Certainly, those raw cashews sold as raw have been processed to remove urushiol, so there is no danger in consuming them. As nuts and seeds go, safely prepared cashews actually cause very few allergies, especially when compared to nuts like walnuts or legumes like peanuts.
The cashew tree is a New World food, and it’s certainly a testament to the ingenuity of New World races that we even eat “raw” cashews. At some time, pre-dating written history, the people of Brazil figured out that the fruit surrounding the cashew “nut” could be eaten, but the shell could not. Also, early Brazilians were able to understand that the nut could be used when steamed or cooked. This may have been a trial and error process, with many people getting ill from error testing, but it ultimately brought us to the enjoyment of one of the most popular nuts, now grown in many places throughout the world.
Though cashew nut oil from the shells is not safe to consume, it does have uses. It may be distilled and used to line brakes to provide friction, or may make up one of the resins in epoxy finishes and coatings. Touching these extractions from raw cashews may create rashes, but this is less common, since the oils and resins made from them undergo extensive processing.