The Kenmore Replacement Charcoal Pre-Filter is the sure way to ensure your HEPA filter continues to work properly. For maximum effectiveness, filters should be replaced every three months. This will keep odors and contagions out of your lungs. just like you want your water to be filtered, you should keep the air you breathe clean and filtered as well. If you suffer from allergies or a respiratory illness, then frequently replacing your filter is a must. Keep the air clean! •Ensure that the air your family breathes is contagion free with the Kenmore Replacement Charcoal Pre-Filter