Dry ice is solid carbon dioxide, held at around -78.5°C at normal atmospheric pressure. Warmed, it never passes through a liquid stage: it sublimates, converting straight from solid to CO2 gas. That single property is why pharma couriers, biologics shippers and food distributors use it as a shipping coolant. There is no meltwater to leak, no liquid to slosh, and no refrigeration unit to power along the route. A charge packed around a payload holds a headspace far colder than almost any shipment needs, which is both the appeal and, on the wrong lane, the problem.
It is one entry in the cold chain knowledge base, alongside the other coolants and packaging formats shippers weigh against it for a given lane.
Cooling by sublimation, not melting
Sublimation is an endothermic change: the CO2 absorbs heat from its surroundings as it converts to gas, and that conversion is the entire cooling mechanism. There is no compressor, no battery and nothing to fail electrically. The trade-off is that the coolant is also the thing being consumed. A well-insulated shipper with a generous charge holds a deep-frozen product for several days; the same charge in a thin foam box burns off in hours. Sublimation rate scales with surface area and insulation quality, not a fixed shelf life, so the packaging design matters as much as the amount of dry ice loaded.
Safe handling and ventilation
Dry ice is cold enough to burn skin on contact, so handlers use insulated gloves or tongs, never bare hands. It also needs somewhere for the CO2 gas to go. Pack it into a sealed, airtight container and the gas has nowhere to vent: pressure builds and the container can rupture. Every insulated shipper built for dry ice has vent holes or a loose-fitting lid for exactly this reason. The same gas is why dry ice needs a ventilated space to work in and travel through. In an enclosed vehicle, cargo hold, walk-in cooler or lift, sublimating CO2 can displace enough oxygen to become an asphyxiation risk, which is a large part of why air carriers treat it as a hazardous material rather than an ordinary consumable.
The dangerous-goods rules for air freight
Under the IATA Dangerous Goods Regulations, dry ice carried by air is UN1845, Class 9, proper shipping name "Carbon dioxide, solid" or "Dry ice." The air waybill has to carry the UN number, the shipping name and the net weight of dry ice in kilograms, and the package itself needs the same UN1845 marking, the shipping name, the net weight and a Class 9 hazard label affixed. The packaging has to meet Packing Instruction 954 and stay vented, the same requirement covered above.
Per package, {{dry_ice_iata_max_per_package}} kg is the threshold above which a full Shipper's Declaration for Dangerous Goods is required; below it, the simpler acceptance checklist most air carriers use covers the shipment instead. States and individual operators layer their own variations on top of that baseline, and a passenger aircraft will typically carry far less per package than a dedicated freighter, so plan around whatever the carrier's dangerous goods desk confirms for that specific routing, not a figure copied from a different lane. None of this applies to road or sea freight: IMDG and national ground hazmat rules cover dry ice separately, with their own limits and marking.
The lanes it's built for
Dry ice is built for frozen and deep-frozen lanes: -20°C frozen freight, and the -70°C ultra-low range that clinical trial samples, certain vaccines and some biologics need. On those lanes it is cheap, widely available on short notice, and needs no charging infrastructure at the destination beyond a fresh supply of dry ice itself. For genuinely cryogenic cargo, below roughly -150°C, a sublimating solid cannot get there at all: that band belongs to a liquid nitrogen dry shipper, a different format entirely. Between those extremes, dry ice covers a defined hold time rather than an open-ended one; long multi-leg routes plan for a recharge at each handoff, not a single charge lasting the whole transit.
The cases it doesn't cover
It is the wrong choice for anything held at 2-8°C. The headspace around a dry ice charge sits close to -78.5°C, and refrigerated product that touches or sits too close to it freezes, which for a temperature-sensitive biologic is as much a failure as an excursion the other way. Phase change materials and gel packs hold that 2-8°C band directly without the freeze risk, and they are the default choice for that lane instead.
Dry ice also sublimates away completely, so it has no place in a reusable, closed-loop shipper programme: those run on PCM or gel charges that survive a wash-and-recharge cycle, not a coolant that turns to gas before the box comes back. And on short or lower-value lanes, the dangerous-goods paperwork, training and carrier surcharges attached to a Class 9 shipment can outweigh what the product is worth to move, especially across a multi-carrier interline routing where every leg needs its own acceptance check. On those lanes, passive packaging built around PCM or gel, carrying none of the same dangerous-goods overhead, is usually the cheaper way to hit the same target temperature.