Water Activity: USP 795 Changes
Water Activity: USP <795> Changes Sarah Taylor, PharmD Beyond-use dating in the newly proposed USP General Chapter <795> underwent significant changes, particularly to the recommended maximum beyond-use dates allowed for different types of formulations. One of the biggest changes is a new focus on microbial growth and preservative efficacy. The guidelines define aqueous preparations as those having a water activity (a w ) greater than or equal to 0.6, and only preparations with an a w less than 0.6 are considered anhydrous dosage forms. 1 What is water activity? Water activity is a measurement of the ratio of vapor pressure of the substance in question when at equilibrium with the surrounding air to the vapor pressure of distilled water under identical conditions. 2 So, for example, an a w of 0.6 means that the vapor pressure of the substance in question is 60% that of pure water. Water activity should not be mistaken for water content. Water content ...