Sheets allowed to remain damp will eventually mildew, a process that accelerates during warm weather. Mildew causes a foul odor on sheets and may also leave stains. Mildewed sheets usually require special laundering, as a simple wash with detergent is usually not enough to get rid of mildew odors or stains. The sooner you notice and treat mildewed sheets, the easier it will be to remove the mildew. Even if you don't notice the mildew right away, however, there is still hope.
Place mildewed sheets in a sink or washing machine. Fill with just enough water to cover the sheets. Add chlorine laundry bleach at a ratio of 2 tbsp. per quart of water, as advised by the Michigan State University Extension.
Allow the sheets to soak in the bleach and water for 15 minutes. Wash the sheets in hot water and your regular laundry detergent.
Dry the sheets outdoors in direct sunlight. If any mildew stains remain after drying, simply repeat the soaking and washing procedure.
Place mildewed sheets in a sink or on a table. Mix 2 tbsp. of an oxygen bleaching agent or color-safe bleach containing sodium perborate in a bucket with 1 pint of hot water.
Put on rubber gloves. Use a sponge dipped in the solution to wipe it over mildew stains on the sheets. Allow it to sit for at least 30 minutes.
Rinse the sheets in warm water and wash with your regular detergent. Hang to dry or dry in your clothes dryer.