UpStart Battery 3-Pack Replacement Robitussin DH835 Humidifier Filter - Compatible Robitussin WF2 Air Filter Replacement Robitussin DH835 Humidifier Filter - Quantity, 3 Replaces Robitussin WF2 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!