Mold Remediation Certification Practice Exam

How is equilibrium relative humidity (ERH) related to water activity (aw)?

1. ERH is the inverse of water activity

2. ERH is the absolute value of water activity

3. ERH is an alternative name for water activity

4. ERH is the percentage of water activity

The correct answer is: ERH is the percentage of water activity

Equilibrium relative humidity (ERH) is a measurement that indicates the amount of moisture in the air relative to the maximum moisture the air can hold at a given temperature, expressed as a percentage. Water activity (aw), on the other hand, is a measure of the availability of water in a substance for microbial growth and is defined as the ratio of the vapor pressure of water in the substance to the vapor pressure of pure water at the same temperature. The relationship between ERH and water activity can be understood through the principles governing the behavior of water in materials. Specifically, water activity can be expressed in relation to relative humidity. The value of water activity increases as the moisture content of a material increases, which directly correlates with an increase in relative humidity in the atmosphere around that material. When a substrate, such as food or building material, reaches a state of equilibrium with its environment, the water activity of that material can be expressed in terms of the relative humidity of the surrounding air. In this context, as relative humidity rises, the water activity of the material will approach a corresponding value that reflects the available moisture for processes such as microbial growth. Thus, indicating that ERH is presented as a percentage of water activity accurately captures this relationship. In essence