Before you apply bleach to kill mildew, brush away as much of the powdery substance as you can. Use a vacuum to suck up the spores. Remove the source of any dampness that may be feeding the mildew, such as a leaky roof or standing water. Cover surrounding surfaces to protect them from the bleach solution.
Bleach is one of the most effective treatment for mildews. It kills the organism responsible for mildew. For fabric with mildew spots, wash in hot water with a combination of regular laundry detergent and bleach, and dry in sunlight. Wash down hard surfaces such as walls with a solution of 1 cup of bleach to 1 gallon of water. Apply the bleach by wiping, brushing or spraying the solution.
Bleach is caustic and poisonous, so use care when treating it. Work in a well-ventilated area and wear rubber gloves to protect your hands. Direct contact with bleach can burn the skin. Avoid breathing in bleach fumes. Never mix bleach and ammonia, as this creates a poisonous gas. Bleach also removes dyes, so be careful using it on colored fabrics or leather. Store bleach in a labeled bottle out of reach of children and pets.
Humidity contributes to mildew growth, so lowering the humidity of the area and keeping surfaces dry can prevent mildew. Use a dehumidifier for the whole house, or dessicants in small areas. Run the ventilation fan in bathrooms and exhaust fans in the kitchen to remove excess moisture. Fix leaks in roofs and walls and divert water so it doesn't stand around or under a house's foundation.