Humidity can be expressed as an absolute, specific or relative value. In the pharmaceutical community, the latter is generally used when controlling humidity; for example, either in stress stability studies performed to ICH conditions (e.g. 25°C/75% RH accelerated storage or 25°C/60% RH real time condition); or a manufacturing process step such as dry blending.
RH can be expressed as the ratio of the vapour partial pressure of the air to the saturation vapour partial pressure of the air at the actual dry bulb temperature. Hence, there is a different amount of water vapour per unit volume at, for example, 50% RH at 15°C compared with 25°C. Even over such a small temperature difference, the amount of water the air can hold increases by a factor of 1.8. Thus, even small differences in temperature can dramatically affect humidity sensitive processes.
In pharmaceutical manufacture, the need to control humidity may not be just to ensure stability of the API and prevent any degradation due to hydrolysis, for example, but also to ensure that quality drug products with respect to other critical quality attributes1 are produced.
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