Humidity, packaging and storage

When storage conditions come up, temperature always leads. Humidity tends to be remembered only when something has already gone wrong: a softened carton, a leaning pallet, a label nobody can read.

What does relative humidity mean?

Air always carries some water vapour. Relative humidity is the amount of water vapour in the air as a share of the most the air could hold at that temperature, expressed as a percentage. Warm air can hold more water vapour than cold air. So when the same air cools, its relative humidity rises, and at a certain point, the dew point, the vapour starts to condense.

That relationship explains many humidity problems in a warehouse. The same air can show very different relative humidity on a warm afternoon and on a cool night.

How does humidity affect the product?

Some products are affected directly. Effervescent tablets, some powders and capsules are well-known examples. For these, the protection the packaging gives matters, but the packaging itself is affected by humidity too.

Packaging and labels

Corrugated cartons lose compression strength in humid air. In a stack, that means the bottom cartons get crushed and the pallet leans, or even tips over.

Humidity also weakens label adhesion. A box whose batch number or expiry date can no longer be read is a problem for traceability.

Condensation

When cold goods meet warm, humid air, the water vapour in that air condenses on their surface. The droplets on a pallet that has just left the cold room are exactly that. A wet carton loses strength and puts its label at risk.

The fix is simple but needs patience: let the cold goods reach room temperature inside their closed packaging, and open the packaging only after that.

What does “dry place” mean?

“Store in a dry place” is a common label phrase that seems open to interpretation. USP General Chapter <659> defines a dry place as one that does not exceed 40% average relative humidity at 20 °C, or the equivalent water vapour pressure at other temperatures. That definition turns the label phrase into a condition you can measure.

What to do in practice

A few habits prevent most of the trouble humidity causes:

• Relative humidity is recorded together with temperature. Many data loggers can measure both at once.
• Goods are placed so that they touch neither the floor nor the walls.
• Dock doors are not left open longer than needed, because in humid weather the outside air brings its moisture in with it.
• Dehumidification is used where needed.
• “Dry place” on a label is treated as a condition to monitor, not as a note.

Humidity across an area can also be measured during temperature mapping. How condensation appears in transport for cold goods is covered in Where does the cold chain break?