Replacement 4 Stage Filter Set (2 pieces). Includes an H11 HEPA Filter and Nano-Cold Catalyst/Carbon Filter. COMPATIBILITY: Compatible with and designed to fit Hathaspace Smart True HEPA Air Purifier HSP001. MOST EFFICIENT: HEPA removes tiny particles, pure filter prevents growth and spread of harmful agents, catalyst filter removes larger and more dangerous agents, and carbon filter absorbs odors, smoke, and gases (4 stage filtration process). made filter change is recommended every 6 months (depending on air quality and use