Pillows accumulate many asthma- and allergy-provoking items such as bacteria, pollen, mold, and dust mites. In general, most experts in this field recommend replacing pillows every 3-5 years at most. It can also be helpful to cover your pillows with allergy-proof protective coverings. And don’t forget to wash your pillowcases at least once a week to keep them free of these allergy-provoking elements.