A phase change material cools a shipment by melting at a temperature built into the formulation itself, tuned to sit inside a band such as 2-8°C or a controlled frozen range. Packed as a solid, it absorbs heat as it turns to liquid and holds close to that melting point for as long as any of it remains solid. Nothing sublimates and nothing boils off: the same charge reconditions and rides along on the next trip.
Dry ice is solid carbon dioxide, held near -78°C at normal atmospheric pressure. Warmed, it never passes through a liquid stage: it sublimates straight from solid to CO2 gas. That property removes any meltwater risk from the box, but the coolant is consumed while it works, and a fresh charge has to be sourced for every trip. Both travel inside an insulated shipper, the coolant side of passive packaging, and neither depends on a power source to hold its band.
Temperature band is the deciding factor
Band, more than anything else, decides which coolant belongs in the box. A PCM formulated to melt around 4 to 6°C holds a 2-8°C shipment inside its target range without ever pulling it below freezing. Formulated closer to -20°C, the same mechanism covers a controlled frozen lane. Dry ice sits far below either range: a headspace near -78.5°C is built for deep-frozen freight and the -70°C ultra-low band that some vaccines, biologics and clinical trial samples need. Below roughly -150°C, genuinely cryogenic cargo, neither coolant reaches that far; that territory belongs to specialist cryogenic equipment instead. Once the band is fixed, most of the other differences between the two coolants follow from it rather than the other way round.
Dangerous goods paperwork
Dry ice carried by air is a classified dangerous good: UN1845, Class 9, with its own hazard label, shipping name and net weight declared on the air waybill, packed to a defined packing instruction and left vented so the CO2 gas has somewhere to go. Handlers need dangerous-goods training, packages need the right markings, and carriers add their own acceptance checks and surcharges on top of the base rule. A shipment routed across more than one carrier repeats that acceptance check at every handoff, since each operator applies its own limits on top of the underlying rule. A PCM pack carries none of that. It is not a hazardous material, needs no venting and adds no special handling code to a shipment, which is why parcel networks and small clinics can use it without the training a dry ice shipment demands.
Mass loss and hold time
Dry ice's cooling mechanism is also its limit: the charge sublimates away completely over the trip, so hold time is a countdown that starts the moment the box is packed. Sublimation rate scales with surface area and insulation quality rather than a fixed shelf life, so the same charge burns off faster in a thin box than a well-insulated one. A long multi-leg route plans for a recharge at each handoff rather than one charge lasting the whole transit. A PCM pack loses no mass in transit. It holds at its melting point until the last of it turns liquid, then simply stops cooling. It does not vent gas and does not need topping up mid-route, though it does need reconditioning back to a solid before it can protect another shipment.
Freeze risk to the payload
Dry ice is the wrong choice for anything held at 2-8°C. The headspace around a charge sits close to -78°C, and refrigerated product that touches or sits too near it freezes, which for a temperature-sensitive biologic is as much a failure as an excursion the other way. A PCM formulated above 0°C cannot pull the payload colder than its own melting point, by design, which is the main reason it displaces dry ice on any lane where freeze damage is the greater risk.
Cost and availability
Dry ice is cheap and available on short notice almost everywhere a courier or freight forwarder operates, provided a supplier sits nearby and no cargo hold restriction stands in the way. That availability is uneven: a major city has same-day suppliers, while a rural site or a remote clinic may have to plan a delivery a day or more ahead. PCM packs cost more per unit and need a conditioning room to bring them to the right temperature ahead of packing, but they carry no dangerous-goods surcharge and no per-trip sourcing problem, since a reusable pack spreads its cost across many trips rather than one.
Neither coolant is the default answer. A deep-frozen or ultra-low shipment moving through a network with dangerous-goods handling in place, with no refrigerated 2-8°C product anywhere near it, is a dry ice job. A 2-8°C shipment, a freeze-sensitive biologic, or any lane running through handlers without hazmat training is a PCM job. The choice follows the band and the payload, not a preference for one coolant over the other.