Replacement Gas & Odor (GO) Smokestop particle and gas filters for the Blueair 400 series air purifiers. Includes 1 polypropylene pleated filter with 3 lbs. of activated coconut shell carbon impregnated with an 70/30 blend of Potassium Iodide (KI), Phosphoric acid (H3PO4), and Potassium Hydroxide (KOH) for gas, smoke and odor control. Filter can be vacuumed to prolong filter life and efficiency. Replace filter every 6 months.