Cashew Shells Are Toxic

The familiar cashew nut is a product of a surprisingly hazardous process, owing to its botanical relationship with poison ivy and poison sumac. The nut grows inside a hard, double-layered shell that contains a potent, toxic (Review) oil. This oil is composed of anacardic acid and urushiol, the same chemical irritant found in poison ivy that causes an allergic skin rash known as contact dermatitis. Direct contact with the shell's oil can lead to severe skin burns and irritation, which is why cashews are never sold to consumers in their shells like other nuts.

To render them safe for consumption, cashews must undergo a heating process, either through roasting or steaming. This treatment neutralizes the toxic oils, ensuring the nut inside is harmless. Even cashews marketed as "raw" have been steamed to remove any urushiol residue before they reach store shelves. This essential processing step was discovered by the Portuguese in the 16th century, who then introduced the Brazilian native plant to India and Africa. Initially, the trees were planted primarily to prevent soil erosion.

The complex and often manual labor required for shelling has a significant human cost. In many processing facilities, particularly in India and Vietnam, workers shell the nuts by hand. If performed without protective gloves, the constant exposure to the caustic shell liquid can cause painful burns and skin damage on the workers' hands. This darker side of the cashew industry highlights the intensive effort required to transform this toxic-shelled seed into the popular snack we enjoy today.