It is important to consider that most HEPA filters are protected by a number of preliminary fine particulate filters put in place to prevent the main filter from clogging. These filters must be replaced more often.
There are no GMP guidelines regarding HEPA filter use time. As a result of this, organisations typically establish internal guidelines on the frequency at which HEPA filters should be changed, created in line with their usage levels, and air hygiene requirements (with many being changed annually). In order for HEPA filters to operate properly, they should be free from leaks (and regularly tested for them), as per ISO 14644-3. When operating in high-risk areas where more sterile conditions are required, the filter should be checked every six months. In situations where maintaining aseptic conditions is less critical, the filters should be checked every 12 months. If leaks are found, they can be repaired given that the leaks take up only 0.5% of the filter surface per filter. Otherwise, they should be changed.
In addition to this, in ventilation systems in which the pressure drop is too high, HEPA filters may have to be changed.