UpStart Battery 4-Pack Replacement Vornado Model 40 Humidifier Filter - Compatible Vornado MD1-0002, MD1-0001 Air Filter Replacement Vornado Model 40 Humidifier Filter - Quantity, 4 Replaces Vornado MD1-0002, MD1-0001 Air Filter Traps Water Impurities & Dissolved Solids Before They Become Airborne 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!