An industrial air filter should be evaluated for replacement when its measured differential pressure at normal operating airflow reaches the documented final-pressure-drop limit for the filter or system. Earlier replacement may be necessary if airflow becomes inadequate, the filter is damaged, wet, or contaminated, or process and hygiene requirements are no longer met. A universal pressure-drop threshold is not reliable because filter design, efficiency, fan capacity, and operating conditions vary. Trend readings over time and inspect pre-filters and instruments before deciding.