A mildew odor emanating from a foam cushion indicates that the cushion needs immediate attention. Moisture or humid conditions in a dark environment lead to mold or mildew growth, which creates those musty, mildewy odors. A cushion that smells like mildew most likely contains it, and being near it may aggravate symptoms for those allergic or sensitive to this allergen. A thorough cleaning, as well as airing it out, renders that cushion odor-free and safe once again.
Vacuum the foam cushions thoroughly over all surfaces to remove mold and mildew spores. Replace the vacuum cleaner bag afterwards, if it is a bag-style vacuum, or wash out the cup-style chamber that holds debris using a sponge and mildly soapy water. Replace the vacuum cleaner's air filter as well. Replacing these items helps prevent the vacuum cleaner from releasing the spores back into the air.
Remove the covers from the foam cushions if they are washable. Wash them in the hottest water recommended on the care tag using your usual laundry detergent. Skip this step if the cushion covers are not designed to be removed.
Place the cushions outdoors on a warm, non-humid day. Air them out by clipping them to a clothesline with sturdy clothespins if the cushions are light enough to hang this way. Otherwise, prop them up against a structure such as a chair or table leg so air hits as many sides of the cushion as possible. Allow the cushions to air out for the entire day or as long as possible, rotating them as needed to expose all cushion sides to fresh air.
Wipe visible mildew or mold specks on the cushions with a mixture of equal portions rubbing alcohol and water, using a sponge or soft cloth. Test one area of the cushion fabric with the alcohol first to ensure the fabric is colorfast.
Spritz the cushions lightly with a spray bottle filled with vinegar to remove lingering odors, or dab some vinegar onto a cloth to wipe down the cushions, using as little liquid as possible. Air-dry the cushions outdoors or in a well-ventilated room full of daylight.