What is Relative Humidity?

If you've ever wondered why dew or condensation forms overnight when there has been no precipitation, the answer has to do with relative humidity.

Relative humidity (RH) is a measure of how much water vapor is currently in the air compared to the maximum amount of water vapor the air can hold at that specific temperature. It is expressed as a percentage, where 100% RH means the air is completely saturated and cannot hold any more moisture, leading to condensation, dew, or fog.

Understanding relative humidity (RH) is critical in packaging because it dictates how fast moisture ruins a product and how much desiccant is needed to prevent it. Without tracking RH, packaging will either fail from under-protection or waste money from over-engineering.

The Core Mechanism of Relative Humidity

To understand how desiccants work, it is vital to understand two core properties of relative humidity.

• Temperature Dependence: Warm air has a higher capacity to hold water vapor than cold air. If the absolute amount of water in the air stays the same but the temperature drops, the RH increases. If the temperature rises, the RH decreases.
• Vapor Pressure: Water vapor exerts a specific pressure (partial vapor pressure). Saturated air exerts maximum vapor pressure (saturation vapor pressure). RH is essentially the ratio of current vapor pressure to saturation vapor pressure. Moisture naturally moves from areas of high vapor pressure (high humidity) to areas of low vapor pressure (low humidity) to reach equilibrium.

How Desiccants Interact with Relative Humidity

Desiccants are hygroscopic substances—meaning they have a high affinity for water—used to induce or maintain a state of dryness in their local environment. They control relative humidity by manipulating vapor pressure.

The Driving Force: Vapor Pressure Differential

A desiccant maintains a very low surface vapor pressure because its structure is dry. When placed in an environment with high relative humidity, a steep vapor pressure differential is created between the surrounding air and the desiccant's surface. To equalize this pressure, water molecules migrate out of the air and bind to the desiccant, successfully lowering the RH of the enclosed space.

The Capture Mechanism

Desiccants remove moisture through one of two primary mechanisms:

• Adsorption (Physical trapping): Materials like silica gel, molecular sieves, and activated alumina have vast networks of microscopic pores. Water molecules from the air physically cling to the internal surface area of these pores without altering the desiccant's chemical structure.
• Absorption (Chemical reaction): Materials like calcium chloride chemically react with water vapor to form a new compound or a liquid brine solution, locking the moisture away.

The Equilibrium Point

A desiccant will not completely strip 100% of the moisture from the air forever. It collects moisture until its internal vapor pressure matches the vapor pressure of the surrounding air. Once this Equilibrium Relative Humidity (ERH) is reached, the desiccant stops capturing moisture. If the ambient temperature rises dramatically, the desiccant's vapor pressure may exceed the air's vapor pressure, causing it to release captured moisture back into the environment (desorption).