Dry ice is regulated as a dangerous good on every commercial flight it travels on. Not because of what it does to a payload, which is simply cool it, but because of what it does to the air around it. As dry ice warms, it turns straight to carbon dioxide gas, and in the sealed hold of an aircraft that gas has nowhere to go. Enough of it displaces the oxygen a crew member needs to breathe. That single fact puts a block of frozen CO2 in the same regulatory category as flammable liquids and corrosives, with its own classification number, its own packaging rules, and its own paperwork trail.
None of this applies to the substance sitting in a warehouse. It applies the moment it is tendered to a carrier for transport, which is where most of the friction in a cold chain shipment actually happens.
UN1845 and the paperwork it triggers
Under the IATA Dangerous Goods Regulations, dry ice carried by air is assigned UN1845, Class 9, proper shipping name Carbon Dioxide, Solid or Dry Ice. Class 9 is the miscellaneous category for goods that are hazardous in ways the other eight classes do not capture, and dry ice sits there because the hazard is asphyxiation, not fire or corrosion. Every air waybill covering the shipment has to declare the UN number, the shipping name, and the net weight of dry ice in kilograms, not the weight of the payload it is cooling. Below a set quantity per package, a simpler acceptance checklist covers the shipment. Above it, the shipper has to file a full Shipper's Declaration for Dangerous Goods, a separate signed document that most standard commercial invoices do not include and that gets checked before the shipment is ever loaded.
Packaging built to vent, not seal
A dry ice shipment cannot travel in an airtight box. As the charge sublimates, the CO2 gas needs somewhere to escape, or pressure builds inside the container until it ruptures. Every insulated shipper rated for dry ice is built with vent holes or a loose-fitting lid for exactly this reason, and the packaging still has to meet the performance standard set for the class, which covers strength and insulation as well as venting. A shipper who reseals a dry ice box to stop condensation or tampering, without realizing the vents are load-bearing safety features, has built a pressure vessel by accident.
Labelling and declarations on the box itself
The package needs the UN1845 marking, the proper shipping name, the net weight of dry ice, and a Class 9 hazard label affixed to the outside, visible without opening anything. This is separate from and in addition to whatever product labelling the payload itself carries. A ground handler or a security screener checking the package against the air waybill is checking that these two documents, the one on the box and the one in the paperwork, say the same weight. A mismatch, even an honest rounding error between the two, is enough to stop the shipment at acceptance.
Training and the paperwork failure mode
Anyone who classifies, packs, marks, labels, or accepts a dangerous goods shipment for air transport needs dangerous goods training specific to their role, refreshed on a recurring cycle rather than taken once. That covers the shipper's packing staff, the freight forwarder preparing the air waybill, and the airline's own ramp and acceptance staff. This is why paperwork errors ground shipments more often than packaging failures do: a carrier's acceptance desk has no discretion to wave through a declaration that is missing a UN number, an incorrect net weight, or a signature. The aircraft leaves without the shipment, and every hour the payload then sits waiting for the next flight is an hour drawn from the same dry ice charge that was sized for the original schedule.
Passenger aircraft versus freighters
A passenger aircraft carries a lower quantity limit of dry ice per package, and per aircraft, than a dedicated freighter, because the same cargo hold that carries belly freight sits closer to a crew compartment with a shared air supply. Some carriers decline dry ice on passenger-configured aircraft altogether. This matters most on routes served only by passenger service, where a shipment sized for a large dry ice charge may simply not have a lane available and needs either a smaller charge split across multiple packages, a different coolant such as a gel pack for a less demanding temperature band, or a routing onto a freighter-only lane through a different hub. Planning the dry ice quantity against the actual aircraft type on the booked routing, not a figure copied from a different lane, is the difference between a shipment that clears acceptance and one that does not.