A vacuum filter can pick up and retain all of the germs, bacteria, impurities and fungi from your carpets, floors and living environment. If you fail to clean your filter regularly, these nasty particles can linger and cause unpleasant odor. Among other things, you may encounter the musty aroma of mildew.
Mildew requires two things for growth: moisture and nutrients. The nutrients can come from dirt and soil picked up by the vacuum. So if your vacuum filter picks up a combination of dirt, water and mold spores, mildew can begin to feed on the soil and grow within the moisture. This ultimately leads to the presence of the musty aroma, as mildew smell always indicates live mildew.
The odor itself emerges as the mold spores feed on their nutrients. The process of absorbing nutrients involves complex chemical reactions, which result in the release of vapors known as microbial volatile organic compounds, or mVOCs. These vapors cause the characteristic mildew odor. When trapped in a small, contained space such as a vacuum filter, the odor can become especially pungent, especially if the fungus has remained in the filter for a long time.
To remove the mildew odor, you will need to kill the mildew. First (and most importantly), take the filter outdoors. This will prevent you from spreading the spores indoors. Empty the filter and wash it out thoroughly with a hose. Then scrub the entire filter with a mildew-fighting solution with dish detergent (preferably one that contains phosphoric acid), white vinegar or hydrogen peroxide. Rinse the filter again and let it dry completely in the sun before returning it to the vacuum. If you cannot dry it in the sun, use a heat source such as your hair dryer.
Always empty and wash your vacuum filter after vacuuming any part of your home. This will prevent spores from accumulating and growing. Also, do not use your vacuum to pick up spills or moisture unless the vacuum is specifically designed as a wet vac. Mold spores inside of a vacuum indicate mold spores elsewhere in your home, so inspect your home for other visible signs of mildew if you notice the problem inside your vacuum filter.