Over time and many washings, the white fabric dye in pillowcases and sheets can fade and become yellow. There could be a build-up of detergent and bleach residue, or maybe you've washed them with coloured items in the past. Long-term storage could be to blame. Still, it's possible to restore your white pillowcases and sheets to their original brightness.
Fill a clean bucket or bathtub with water and a half to 1 cup of oxygen bleach and mix around.
Add your unfolded, yellowed pillowcases and sheets one at a time.
Let your old yellow pillowcases and sheets soak overnight.
Treat any specific stains on the wet fabric by rubbing in thoroughly a stain remover.
Wash your pillowcases and sheets alone and using a hot water cycle, unless otherwise specified on the care instructions. Set your washing machine for a delayed final rinse cycle.
Add a half cup of white vinegar to the wash just before the final rinse cycle.
Hang your sheets and pillow cases to dry outside and in the direct sunlight all day.
Wash your sheets with a whitening agent or a white fabric dye if they are still not white enough. Follow the instructions on the package.