Nextron Products Alen 350 Generic Air Purifier Filter - Two Extra 2-Stage prefilters Included!! Fits Alen A250, A350, A375 Life expectancy 2-3 years for non-smokers by using two filters systems in one. Two stage (M-class) pre-filter, Greatly extends life of main. Three stage main filter (M-class) including  ultra-high performance activated carbon. Fresher air through our exclusive biological pollutant removal system.