Insulated lunch boxes usually contain an outer layer made of a tough plastic fabric like vinyl, nylon or polyester that can be hard to stain or tear. The bag's inner layer is usually made from a water-resistant material – plastic, aluminum, vinyl and foil liners are common and help keep food fresh and dry. An inside middle layer of insulating foam is typically made from materials like durable polyurethane, polyethylene plastic or thermal batting made out of polyester fibers.
While insulated, soft-sided lunch boxes or bags can often be best for keeping food cool, packing at least two cold sources, such as gel packs or ice can help.
Lunch boxes also can be made from silicone, glass and metal. While these materials are also tough and durable, if they don't have interior insulation, adding an ice or gel pack to keep food cool will help keep food safe to eat. Reusable gel packs are often made of materials like hydroxyethyl cellulose (a thickening agent made from plants), sodium polyacrylate (a type of salt) or vinyl-coated silica gel that will not contaminate the food if the pack breaks.