Hygrometers and thermohygrometers: environmental monitoring ensures the potency and stability of compounding agents.

: Although the terms humidity and relative humidity are often used interchangeably, they are not synonymous. Humidity is the amount of water in the air, and relative humidity is the ratio of the amount of water vapor in the air at a specific temperature to the maximum possible amount of water vapor in the air at that temperature. Thus humidity and temperature are inextricably bound in their effects on the environment. In a compounding pharmacy, humidity can affect the stability and quality of the compounds prepared, as well as equipment, chemicals, and polymers. Devices that measure relative humidity (hygrometers) or humidity and temperature (thermohygrometers) are essential instruments in a compounding pharmacy. They must be chosen carefully, however, to ensure that the measurements they yield are accurate, that they are reliable over time. Most desirable are devices that alert the pharmacist immediately at any time if levels of humidity or temperature at a designated site differ from a specific norm. In this report, we discuss the effects of humidity on the process of compounding and on the agents used in customized preparations. A Table that lists essential features of a variety of hygrometers and thermohygrometers appropriate for use in a compounding pharmacy is presented for easy reference.