Usually, one filter can be used for 30 days, It is recommended to change at any time during heavy pollution periods Filter Activated Carbon PM 2.5 FM Filter Activated Carbon?PM 2.5 FM. Do you have problem with dust chemicals, smoke and particles. Do you want to enjoy a free outdoor life? Common Filter? It has no effect. Moving purifying respirator? It is large and burden. That’s a good reason to buy this Filter Activated Carbon PM 2.5 FM. Refused to be a human vacuum cleaner. A must-have for outdoor activity suitable for dust, anti-vehicle exhaust, anti-pollen allergy, PM2.5, for cycling, hiking, skiing, and other outdoor activities.