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, etc.) can be used to supplement the dry ice.