Every perishable shipper eventually hits the same question: what actually goes in the box to keep the product cold? The three most common answers are dry ice, gel (cold) packs, and phase change materials (PCMs), and picking the wrong one is one of the fastest ways to get spoiled product, angry customers, and refund requests.
This guide breaks down how each coolant works, what temperature range it’s actually good for, how long it lasts, what it costs to use, and the shipping rules you need to know before you tape the box shut.
Quick Answer
Use dry ice for products that must stay frozen solid (meat, seafood, ice cream) since it runs at about -78.5°C / -109.3°F. Use gel packs for products that need to stay cold but must not freeze, like produce, dairy, or cosmetics, typically in the 32-50°F range. Use phase change materials when you need tightly controlled, repeatable temperatures over longer transit times, such as pharmaceuticals, vaccines, or lab samples in the 2-8°C refrigerated range or deep-frozen ranges around -20°C. Dry ice is treated as a Class 9 hazardous material with labeling requirements and per-package quantity limits when shipped by air, but most US ground carriers treat dry ice used as a refrigerant as not regulated as a hazardous material for ground service, with simpler package-marking rules instead.
How Each Coolant Works
Dry ice is solid carbon dioxide. Instead of melting into a liquid, it sublimates directly into CO2 gas, which is why shippers love it (no leaking mess) but also why it needs ventilation: a sealed cooler full of sublimating dry ice can build up gas pressure and displace oxygen. It’s the coldest common option by a wide margin and is the only practical choice for keeping something genuinely frozen during a multi-day shipment.
Gel packs are sealed pouches filled with a water-based gel, often water thickened with a polymer or combined with a small amount of salt, that you pre-freeze or pre-chill before packing. They melt and warm gradually, absorbing heat as they change from solid to liquid, and they’re reusable: freeze them again for the next shipment. They’re best thought of as a refrigerator substitute, not a freezer substitute.
Phase change materials (PCMs) are engineered substances, some water-based, some paraffin or salt-hydrate based, that are formulated to melt and solidify at a specific, narrow target temperature rather than at 32°F like plain water. Because the material absorbs and releases heat right at that set point, PCM packaging can hold a shipment inside a tight temperature band (say, 2-8°C for vaccines) far more precisely and for longer than a generic ice pack, which is why pharma and biotech cold chains rely on them.
Cost, Duration, and Handling Compared
Dry ice sublimates at roughly 5-10 lbs per 24 hours in a well-insulated cooler, so a two-day shipment typically needs at least 20 lbs to stay effective, and the exact amount depends on insulation quality, ambient temperature, and whether you’re using blocks or pellets (blocks sublimate more slowly). It’s inexpensive per pound and widely available from gas suppliers and some grocery/hardware stores, but it isn’t reusable, needs to be sourced close to your ship date, and can’t be shipped in a fully sealed airtight container because of gas buildup.
Gel packs generally hold their cooling effect for roughly 12-48 hours depending on ambient conditions and insulation, which makes them a solid fit for one- to two-day ground shipments of refrigerated (not frozen) goods. The upfront cost per pack is higher than dry ice, but they’re reusable dozens of times, so the cost per shipment drops fast for anyone shipping regularly. They’re also unregulated for transport, non-toxic, and don’t require special labeling.
PCM packaging (panels, pouches, or bladders built into an insulated shipper) is the most expensive option upfront, both the PCM material and the purpose-built insulated box or liner it ships in, but it’s engineered for consistency: passive PCM shippers are designed and lab-tested to hold target temperatures for extended periods, often days, without any active refrigeration. For regulated shipments like pharmaceuticals or clinical samples, that repeatability, plus qualification data you can hand to an auditor, is the whole point of paying more.
Shipping Rules You Need to Know
Dry ice is classified by DOT and IATA as UN 1845, Class 9 (miscellaneous hazardous material), under the proper shipping name ‘Carbon dioxide, solid.’ That classification primarily governs air transport: packages flown on a passenger aircraft are generally capped at a small net quantity per package (commonly around 2.5 kg / 5.5 lbs, with higher limits allowed on cargo-only aircraft), and packages above the excepted-quantity threshold must carry the proper shipping name, net weight of dry ice, a Class 9 hazard label, and dangerous-goods paperwork. Ground shipping within the US works differently: major carriers generally treat dry ice used as a refrigerant as not regulated as a hazardous material for ground service at all, regardless of quantity, so there’s no Class 9 label or hazmat declaration required, though you should still mark the package as containing dry ice, note the net weight, and let the driver know at pickup. Rules vary by carrier and by ground vs. air service, so always confirm the specific carrier’s current dry ice policy before you ship, and never ship dry ice in a fully airtight container.
Gel packs and most PCMs are not regulated as hazardous materials for transport, so they don’t require special labels, declarations, or carrier approval. The tradeoff is that they can’t get anywhere near as cold as dry ice, so they’re the wrong tool if your product needs to arrive frozen solid rather than just ‘cold.’
Tips / Common Mistakes
Don’t seal dry ice in an airtight cooler or box; the sublimating CO2 gas needs somewhere to vent, or pressure can build and the container can bulge or burst. Use an insulated box that allows some airflow and never handle dry ice bare-handed (it causes frostbite-like burns almost instantly).
Matching the coolant to the product’s actual failure mode matters more than picking the ‘strongest’ option. Produce and dairy packed in dry ice can freeze and turn to mush; meanwhile, frozen seafood packed with only gel packs can thaw and spoil in transit. Know whether your product needs to stay below freezing or just stay cool, and choose accordingly.
Oversize your coolant for worst-case transit time, not average transit time. A carrier delay of even one extra day is a common cause of spoiled shipments, and it’s cheaper to add a bit more dry ice or an extra gel pack than to reship a spoiled order.
If you ship the same product on a recurring basis, get a real box/coolant qualification test done (a simple data-logger run through a typical shipping cycle) rather than guessing. This is standard practice for pharma and food shippers and quickly shows whether your current setup has enough margin.
Explore more: more packaging materials guides.
Dry ice vs gel packs vs phase change materials FAQs
Can I use dry ice and gel packs together in the same box?
Yes, and it’s a common combination: dry ice for the coldest zone of the package and gel packs to buffer areas that shouldn’t freeze directly, though you should still follow the ventilation and marking rules that apply to whatever quantity of dry ice you’re using and the mode you’re shipping by.
How long does dry ice last in a shipping cooler?
In a well-insulated cooler, dry ice typically sublimates at about 5-10 lbs per 24 hours, so a two-day shipment generally needs a minimum of 20 lbs, adjusted up for hot weather or a less-insulated box.
Are phase change materials worth it for a small business shipping food?
Usually not for basic frozen or refrigerated food shipping, dry ice and gel packs are cheaper and simpler. PCMs earn their cost when you need a tightly controlled temperature band over long transit times, which is more common in pharma, lab, and specialty biotech shipping than everyday food e-commerce.
Is dry ice regulated as a hazardous material for ground shipping?
Generally no. Most US carriers treat dry ice used as a refrigerant as not regulated as a hazardous material for ground service, so it doesn’t need a Class 9 label or hazmat declaration the way air shipments do, though you still need to mark the package as containing dry ice and state the net weight. The Class 9 labeling and per-package weight limits (like the roughly 2.5 kg passenger-aircraft threshold) are air-transport rules, so always check your specific carrier’s current ground policy before shipping.
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Photo by Wander Fleur on Unsplash.