Wet Cartons, Dry Vials: Condensation Inside a 2-8 °C Pharma Shipper

A distributor of vaccines opened a validated shipper at a clinic in Singapore and found the outer carton dark with water, the leaflet stuck to itself, and the vials dry and in spec at 4.6 degrees. The temperature record was perfect. The customer still rejected the delivery, because a wet box reads as a failed cold chain to anyone not holding a logger. The water had condensed out of the air inside the box, not leaked from a pack.

Condensation is the most common non-temperature failure in 2 to 8 degree shipping, and it is almost entirely preventable at packing. What follows is where the water comes from, why minus 21 packs make it worse, and the three changes that keep a chilled shipper visually clean.

Where the Water Actually Comes From

Air at 30 degrees and 70 percent relative humidity holds about 21 grams of water per cubic metre. Cool that air to 5 degrees and it can hold only about 6.8 grams, so roughly 14 grams per cubic metre has to go somewhere. A 20-litre shipper contains 0.02 cubic metres of air, which sounds negligible at a quarter of a gram, and it would be if that were the only source.

The larger source is the corrugated carton and the paper inserts, which equilibrate at 8 to 10 percent moisture in a humid packing room. A 600 g carton can carry 50 g of water, and when the inside of the box drops to 5 degrees that water migrates to the coldest surface it can find, which is the pack film and the inner liner. That is the wet patch your customer photographs.

The Minus 21 Pack Mistake

Packing a 2 to 8 degree box with packs straight from a minus 21 freezer creates a surface 25 degrees below the payload target. Every water molecule in the box heads for that surface, condenses, and then freezes into a layer of frost that melts during handling and drips onto the carton. The payload may still be in spec, but the box looks like it has been through a flood.

The fix is conditioning, not more insulation. Hold packs at 0 to 4 degrees for at least 12 hours before packing, so their surface stays above the dew point of the air you are trapping. In a validated 2 to 8 degree shipper, packs conditioned to 2 degrees hold the payload longer than packs at minus 21 in most ambient profiles, because the colder pack spends its first six hours pulling heat out of the payload too aggressively and then plateaus at the same place.

Three Changes at the Packing Bench

First, bag the packs. A 30 micron polyethylene bag around each pack contains the meltwater and stops direct contact between a cold surface and the carton. Cost is around 3 cents per pack and it removes most visible wetness. Second, drop a 10 g silica desiccant sachet into the void space and keep it away from the packs. Third, pack in a room below 60 percent relative humidity if you can, and if you cannot, pre-condition the cartons in that room for 24 hours so they start drier.

A fourth change for high-value lanes: use an absorbent liner under the payload rather than around it. A 2 mm SAP nonwoven pad rated at 400 ml per square metre will take up any meltwater that forms during a 72-hour journey without adding meaningful weight or volume.

What to Show an Auditor

Condensation control needs evidence, not assurances. Keep three records: the conditioning log for packs, with the room temperature and the hours held; the packing room humidity reading at the time of pack-out; and a photograph of a completed box interior before closure. On receipt, ask the consignee to photograph the carton before opening it, because once the box is open the evidence of condensation disappears within minutes.

If your packs are weeping free water rather than condensing it, the problem is upstream of the box. A gel that releases liquid under stack load creates the same wet carton and a much worse temperature profile, and it is worth checking absorbency under load before you add more desiccant. Our pharma-grade SAP is supplied with retention data after freeze-thaw so the conditioning step behaves the same on the tenth cycle as on the first.