If you suspect that an old pillow on your bed is getting heavier with age, you're absolutely correct. Pillows get heavier over the years because of dust mites and dead skin that accumulate in them, which makes them less comfortable and also less healthy. Because old pillows can cause problems for your health, it's important to replace your pillow before it gets too old.
Dust mites exist throughout your home, though they're too small to see with the naked eye. The Ohio State University Extension says that one-third of the dust mites in your home live in your bed, and this habitat includes your pillows. The dust mites that live in your pillow will leave droppings throughout their life and also die in your pillow. The droppings and the dead dust mites account for the increased weight of the pillow. According to the Ohio State University Extension, roughly 10 percent of an old pillow's weight is due to dust mites.
The Ohio State University Extension says that humans shed roughly one-fifth of an ounce of dead skin over the course of any given week. Much of this skin is shed during sleep, meaning a significant portion of it will end up in your pillow. Over time, the buildup of dead skin in your pillow will add to its total weight.
While people who are allergic to dust mites will have more severe symptoms after using a heavy pillow laden with dust mites and their droppings, no one's health benefits from breathing this material. Change and wash your pillow cases at least once a week to remove as many dust mites from them as possible. When you're vacuuming your bedroom, vacuum your pillows after you've removed the covers.
Even with regular washing and vacuuming, dust mites will continue to inhabit your pillows and make them heavier over time. It's ideal to replace your pillow every one to two years to give yourself a fresh place to lay your head. Doing so is also helpful because over time a pillow can break down and become less comfortable to use.