These optional Ultra Fine Filters are designed for use with all Luna G3 & React G3 X Series CPAP & BiPAP machines. Using an Ultra Fine Filter, along with the standard Reusable Foam Filter, with your Luna G3 or React G3 X provides higher air filtration for a cleaner, fresher sleep therapy experience. Ultra Fine Filters are especially useful in households with pets and during high allergy seasons. For optimal performance Ultra Fine Filters should be changed regularly when they appear soiled or discolored.