The word humidity gets used all the time in environmental and product control discussions. The problem is that it often means different things to different people. In practice, there are three distinct types of humidity, and confusing them can lead to poor design decisions and inconsistent process results.
Absolute Humidity
Absolute humidity is the total amount of water present in the air. It is expressed as the weight of water per unit weight of air, typically in grams/kg.
The key point is this. Temperature has no effect on absolute humidity. If the moisture content of the air stays the same, the absolute humidity does not change, regardless of whether the air is hot or cold.
Relative Humidity
Relative humidity is what most people are familiar with. It represents the ratio of the actual moisture in the air ( g/Kg) to the maximum amount of moisture the air can hold at that temperature.
Because of this, relative humidity is always tied to temperature. Saying 50 percent RH on its own is incomplete. It must be stated as, for example, 50 percent RH at 21°C.
As temperature changes, relative humidity changes even if the absolute moisture content remains constant.
Equilibrium Relative Humidity (ERH)
Equilibrium Relative Humidity is the most critical concept for product stability and process control.
ERH refers to the relative humidity in the thin layer of air that surrounds a product. This microclimate is controlled by the product’s own moisture content. Equilibrium is reached when the relative humidity of this layer matches the relative humidity of the surrounding air.
Both product temperature and ambient air temperature strongly influence ERH. Like relative humidity, ERH is expressed as a percentage and must always be qualified by temperature.
Which Humidity Really Matters?
In real-world applications, relative humidity and equilibrium relative humidity are the two parameters that actually need to be controlled. When these are managed correctly, absolute humidity naturally falls into place.
Failing to identify which humidity you need to control is one of the most common reasons for unstable processes and inconsistent product quality.
The Effect of Relative Humidity on People
Relative humidity has a direct impact on human comfort and health.
Low humidity increases moisture loss from the skin, making people feel colder. It also dries the skin, increasing susceptibility to irritation and infection. High humidity slows down evaporation from the body, making us feel warmer and more uncomfortable.
Most people can adapt unless conditions become extreme, but temperature and relative humidity together play a major role in overall well-being.
The Effect of Equilibrium Relative Humidity on Products and Processes
Products constantly exchange moisture with the surrounding air.
Sugar, for example, absorbs moisture and becomes sticky above about 70 percent RH. Cooked rice dries out quickly when left uncovered at room conditions. These changes happen naturally as products try to reach moisture equilibrium with their environment.
Moisture transfer is driven by differences in vapor pressure and continues until equilibrium is reached. In unventilated spaces, hygroscopic materials can actually control the surrounding humidity by absorbing or releasing moisture until a stable balance is achieved. This process usually involves only small changes in the product’s total water content, because it depends on both absolute humidity and the material’s inherent moisture properties.
In ventilated environments, the supplied air dictates the product’s moisture content. True equilibrium can only be maintained when both air and product temperatures are matched and relative humidity is controlled within tight limits.
This is why precise control of temperature and humidity is essential. It protects product quality, stabilizes processes, and directly impacts yield and profitability.
