There's no need to use harsh chemicals like ammonia or bleach to remove a mildew smell under a sink and dishwasher. Vinegar, hydrogen peroxide and baking soda deliver a one-two-three knockout punch to both mould and mildew. All three are natural substances that will not harm family pets or small children, and they are safe for the environment. Follow these simple steps to eliminate mould and mildew odours.
Remove everything from under the sink. Discard items that are damp or have mildew on them. If possible, move the dishwasher out from under the countertop.
Fill one spray bottle with distilled vinegar---the white vinegar---and the other with hydrogen peroxide. Use them both at full strength; do not dilute them, and don't mix both in one container. Mixing diminishes their effectiveness.
Spray the affected areas with the vinegar. Allow it to sit for 10 to 15 minutes, but don't allow it to dry. Although the odour of the vinegar may be unpleasant for a short time, it is completely non-toxic, and the smell dissipates in just a few minutes. Wipe the areas clean with a damp sponge or towel.
Spray the same areas immediately with hydrogen peroxide. Allow this to sit for the same amount of time, and then wipe dry. Heavily affected areas may require a second application.
Dissolve 1/4 cup of baking soda in a gallon of water and clean the areas with a sponge. Allow the areas to dry completely. A fan will shorten the drying time. Place an open box of baking soda under the sink to absorb future odours. Replace the baking soda every 30 days.
Mildew forms where there is moisture. To ensure that mildew does not reappear, the source of the moisture must be found and the problem corrected. Check all plumbing connections to ensure that there are no leaks. Check for seepage around the seal under the sink where it contacts the countertop. Check the dishwasher drain and hose fittings. If any of these are questionable, get them repaired.