Compatible with GermGuardian/Guardian Technologies FLT4100 Filter E. Engineered to fit air purifier models AC4100, AC4150BL, AC4150P, AC4150BLCA, AC4150PCA. Recommended to replace every 6-8 months under normal conditions, more often in harsh environments. Approximately 8 ¼ x 4 ¾ x 1 3/8 inches in size. Filter-Monster branded replacement filters. Package contains two (2) filters and two (2) attached pre-filters. Filter-Monster branded replacement filter designed to be compatible with GermGuardian Air Purifier models AC4100, AC4150BL, AC4150P, AC4150BLCA, AC4150PCA. Replaces Guardian Technologies Filter E, part number FLT4100. Recommended to replace filter every 6-8 months under normal conditions, more often in harsh environments. Each filter is approximately 8 ¼ x 4 ¾ x 1 3/8 inches in size. Package contains two (2) filters with attached pre-filters.