Mildew is a type of mould that can grow on many different surfaces. It usually appears thin and dark, but may also be white. Mildew grows in moist heat. It can develop on seat belts if they get wet from rain or spilt liquids and then are retracted while still damp. Seat belts must be cleaned without removing them to ensure their safety functioning remains sound.
Open the vehicle doors and pull the affected seat belt out as far as it will go. Brush off the surface mildew outside the car, if possible, to prevent scattering it around the car interior. Otherwise, you can brush it into a plastic bag and then throw the bag away.
Use a sponge or rag to hand-wash the seat belt with washing powder; then rinse it with a clean wet rag.
Wipe down the seat belt with a rag moistened with one part rubbing alcohol and one part water if the detergent doesn't do the trick, and rinse again.
Use a commercial disinfectant household cleaner if any mildew still remains, following directions on the product labelling. These cleaners may discolour the seat belt fabric, so you should spot-test the cleaner on a very small area first.
Leave the windows open and air-dry the seat belt (in direct sunlight, if possible). Prevent the belt from retracting by looping it over the steering wheel, the shift lever, or a window crank handle or door handle.
Remove mildew from seat belts (and any other surface) as soon as you notice it, because it can cause permanent damage if left for a prolonged time.