Cold packs for shipping work by cooling the air and the contents of the shipping box to keep foods fresh during the shipping period. Dry ice packs keep items frozen during shipping, while gel packs keep items refrigerated. Cold packs work best in insulated containers. The packs start out very cold, transferring the low temperatures to the contents of the containers, while the insulation holds the cold in and keeps heat out.
Gel ice packs are lightweight and therefore commonly used in shipping. The packs are available ready-to-use or in a dry form that requires hydration before packing. The gel has a higher freezing point than water to freeze more quickly and stay frozen longer to keep foods fresher better than regular ice. The gel inside the ice packs is nontoxic and not harmful if the pack breaks during transit.
Dry ice is frozen carbon dioxide. All packages containing dry ice must be labeled, because dry ice is a hazardous substance. Whether using dry ice or gel packs, perishables need to be placed in an insulated container for shipping. With dry ice, a Styrofoam container at least 1.5 inches thick is necessary. A minimum of 5 pounds of dry ice is necessary for every 24 hours of transit. Alternatively, a minimum of 1 pound of gel packs is necessary for every cubic foot in the shipping box.