Most people spend a huge amount of face time with their pillowcases, quite literally for those stomach-sleepers out there. Despite this, the all-important bed linen doesn't get washed nearly as often as it should be to keep it hygienic. A Mattress Advisor survey found that American men went 29 days and American women 20.4 days before changing their pillowcases. That makes an average of 24.6 days between pillowcase changes, accounting for both sexes.
The case for changing pillowcases more frequently is pretty compelling. The average human spends somewhere between 49 and 60 (or more!) hours per week nestled up to their pillowcase. During this time, we transfer a whole bunch of nasty stuff from our face, body and head to the case, including sweat, body oils and dead skin cells. Exposure to these things can cause everything from skin breakouts/acne to exacerbated allergy or asthma symptoms.
Couple that with the fact that microscopic dust mites (not to mention their fecal matter and itty-bitty dead bodies) multiply rapidly in such settings, and it's easy to see why the pillowcase needs more regular changing. Failure to get rid of the little buggers can cause allergic reactions, including skin problems and even rashes. People with asthma can suffer serious attacks due to constant exposure to dust mites, while others with milder allergies could wind up contending with a chronic runny nose, sneezing or watery eyes.