This pack includes both filters needed in the IQAir HealthPro Air Purifier. The IQAir PreMax Replacement Filter is found in all HealthPro models and is located in the F1 location shown on the display. Replacement is typical every 6-18 months depending on use. The function of this filter is to reduce the coarse and fine dust particulate before it reached the other filters, this esures the other filters are being optimized for proper filtration. Found in the HealthPro, HeathPro Plus, and HealthPro Compact, this filter is cleanroom-grade with a high efficiency of particulate contol. On the machine display this will be the F3 filter with typical replacement occuring every 2-4 years depending on use. This filter controls very fine particles such as allergens, bacteria, and viruses at the HEPA standard of 99.95% effective.