Instant/rapid ice packs that are activated by force—when used for medical/first aid purposes.
These items are allowed in carry-on or checked baggage—when carried for medical purposes such as the treatment of sporting injuries.
These products contain ammonium nitrate (a hazardous material) and are activated by force such as squeezing them or striking them. They are often used for first aid during sporting events.
Quantity limits: Included in, not in addition to, the quantity limits for medicinal and toiletry articles: The total aggregate quantity per person cannot exceed 2 kg (70 ounces) or 2 L (68 fluid ounces). The capacity of each container must not exceed 0.5 kg (18 ounces) or 500 ml (17 fluid ounces).*
See the regulation: 49 CFR 175.10(a)(1)
*When in carry-on baggage any liquid/gel ice products are further limited to 100-ml (3.4 oz) containers at the TSA security checkpoint if not already frozen solid.
Tip: Normal ice packs (those that must be first put in a freezer) are not hazardous materials and have no quantity limits in checked baggage.