What is relative humidity and how does it affect your boxes?

First things first. Relative humidity is a term used to describe how much water the air can hold at any given time. Imagine a meadow in the morning, with dew collecting on blades of grass. That is relative humidity at its maximum, 100% RH. The humidity in the air is affected by rises or drops in temperature, which ultimately transform the water from one state to another, for example from gas to liquid in this case.

Moisture content in paper strongly depends on relative humidity, and there is an equilibrium relationship. If the relative humidity of the surrounding air changes, the moisture content in the paper will also change until equilibrium is reached. As a consequence of increased moisture content, the paper will soften. The end result is lower strength properties.

If we look at a world map, tracking the changes in relative humidity over a period of a day, we see that 50% RH is seldom the case. It becomes obvious that different locations have large regional and daily variations.

RH variations in cold chain transport

Relative humidity can also vary greatly in cold chain logistics, i.e. transport. In a scientific study we measured the changes in temperature and relative humidity when oranges were transported from Valencia in Spain to Helsingborg in Sweden. The relative humidity in the temperature-controlled truck fluctuated between 70% and 99%, affecting the moisture content of the corrugated boxes.

So, why is packaging paper only measured in 50% RH? The simple answer is that this is what the testing standards say. But if you consider the relative humidity in your value chain, you will probably start testing in other climates.