Carbon dioxide, solid, 2.5 kg (5.5 lbs) or less, when used to pack perishables
Quantity limit: 2.5 kg (5.5 lbs) per package and per passenger.
Airline approval is required.
Packages must NOT be air tight and must allow the release of carbon dioxide gas.
When in checked baggage, the package must be marked "Dry ice" or "Carbon dioxide, solid" and marked with the net quantity of dry ice, or an indication that it is 2.5 kg (5.5 lbs) or less.
See the regulation: 49 CFR 175.10(a)(10)
Tip: Additional non-hazardous ice packs (blue ice, gel packs, ammonium nitrate instant cold packs, etc.) can be used to supplement the dry ice. However, for carry-on baggage, if the product contains any liquid or gel in excess of 100 ml (3.4 oz) per container, the TSA security rules require that the product be in the frozen state (i.e., solid) when the passenger goes through security screening.