Whether you detect it in your basement, your closet or your bedroom, a musty smell means mold, and where there's mold, there's moisture. If the smell is coming from your mattress, the most important question to ask yourself is: "How did my mattress get damp?" The answer could involve your pets or your children (hopefully not), but in many cases, it's simply a lack of ventilation in your bedroom. It's difficult to eradicate mold once it gets a foothold in something as porous as a mattress, but if you can eliminate the moisture problem, the mold should go dormant. You can then neutralize the odor with vinegar, baking soda, sunlight or all three.
Dry It Out
You mattress needs to breathe. Expose it to a stream of fresh air, and the moisture should evaporate. Remove all the covers, pillows and sheets, and then open the door and a window to create a cross draft. Once one side is dry, flip the mattress to dry the other side.
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It may take a day or two for the mattress to dry out completely, especially if your bedroom is dark, so you might want to plan alternative sleeping arrangements. Don't have any? Turn up the heat in the bedroom and run a fan to increase circulation. You can also enlist a helper to assist you in carrying the mattress outside to dry in the sun. Letting the sun dry your mattress is the best option, because ultraviolet sunlight kills active mold and is a natural deodorizer.
Vacuum and Neutralize
Although you can't see it, your musty mattress probably has a thin layer of mildew on the surface, and it will still be there, ready to spring back to life, after drying. This is why it's important to vacuum both sides thoroughly. After you're done, empty the vacuum bag outside to prevent spores from flying right back onto the mattress where they came from. This may be all you need to do to eliminate the musty odor, but if not, it's time to call on two of a homeowner's best friends -- vinegar and baking soda -- to balance the pH of the mattress fibers.
Vinegar is acidic, and baking soda is alkaline. Using both at the same time is an exercise in futility, because they cancel each other out. However, if you use one, wait a bit, and then use the other, you can create a neutral environment in which odor-creating microorganisms can't survive. Here's how to do it:
Step 1: Apply the Vinegar First
Fill a spray bottle with distilled white vinegar and spray the entire mattress until the surface feels damp. When you're done, flip the mattress and spray the other side. Let the mattress air dry.
Step 2: Apply Baking Soda
Dust baking soda on one side of the mattress; let it sit overnight, then vacuum it off. Turn the mattress over and repeat on the other side.
Step 3: Let the Sun Help
Leave the mattress outside in the sun for the better part of a day. Stand it up on one edge and support it so the air and sunlight can reach both sides. By the end of the day, the mattress should be odor-free and ready to put back on your bed.
Can't Take the Mattress Outside?
You may have to leave the mattress on your bed while deodorizing it. If so, you can do the procedure in stages. Spray vinegar on one side; let it dry, then apply the baking soda. Flip the mattress after vacuuming the baking soda, and do the other side.