Three-Stage Filtration Process: - Stage 1: Pre-filter - an inexpensive pre-filter to remove larger particulates from the air. This is important to prolong the life of the HEPA filter. Rated MERV 6. Estimate life - 12 months. - Stage 2: HEPA - removes 99.97% of particulates down to 0.3 micron. Estimate life 2-5 years. - Stage 3: Carbon - 1/2 in. (13 mm) thick carbon filter for removing chemicals and odors from the air. Replace filter when it no longer seems to capture odors, or once every 3 months.