UpStart Battery Replacement Blueair 203 Air Purifier Filter - Compatible Blueair 200 & 300 Air Filter Replacement Blueair 203 Humidifier Filter Replaces Blueair 200 & 300 Air Filter Gradient filter design maximizes airflow while removing harmful particles to 0.1 microns in size For best results, change air filter at least once per season or more often depending on air quality and use. Essential for Allergy Sufferers. On Sale for a Limited Time!