For most households, washing bed sheets about once a week is a sensible routine. Sheets collect sweat, body oils, dead skin cells, dust, pet dander and microorganisms while we sleep. Leaving them unchanged for three weeks is not automatically dangerous, but buildup may cause odors, skin irritation or worsening allergy symptoms.
Some people may need to wash bedding more frequently. Consider changing sheets every few days if someone sweats heavily, sleeps with pets, has allergies or asthma, goes to bed without showering, or is recovering from an illness. Pillowcases may also benefit from more frequent washing because they come into direct contact with facial oils, hair products and cosmetics.
Always check the care label before choosing the temperature. Use detergent, avoid overloading the machine and dry the bedding completely. Hotter settings may help reduce dust-mite allergens when the fabric permits them, but regular washing and thorough drying are more important than using extreme heat every time.
So, washing sheets once every three weeks is less frequent than commonly recommended. However, it is your daughter-in-law’s home and personal routine. Rather than repeatedly confronting her, it may be better to avoid commenting unless the bedding directly affects you or there is a genuine health concern.