There is a tendency to equip filtering respiratory protective devices with a blower system to lower the breathing resistance. Because there is a continuous flow of air through the filter, the possibility that adsorbed contaminants are released from the filter is enlarged, which evidently is undesirable. The purpose of this work was to explore whether there is a significant risk that physisorbed or chemisorbed contaminants desorb from the filter. Measurements were performed under various conditions with cyclohexane, a physisorbed vapor, and ammonia, a chemisorbed gas. During a certain time, an activated carbon filter bed was exposed to either cyclohexane or ammonia, followed by a period of clean air flow. The experiments showed that respiratory protective devices with a continuous airflow do have a risk to release previously adsorbed contaminants at too high concentrations. Under humid conditions, the release of physisorbed contaminants occurs even more rapidly than under dry conditions.