Use baking soda. Sprinkle the mattress with baking soda....both sides and also the sides of the mattress. Let that sit for at least an hour. Then vacuum it up.
If the baking soda alone doesn't freshen the mattress enough, use powdered Carpet Deodorizer. Sprinkle just like the baking soda. Let sit for at least an hour. Then vacuum.
You can buy Carpet Deodorizer at most grocery stores, Walmart, Target, CVS, Home Depot, and even most Dollar Stores.
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