There are several factors that contribute to the development of diaper rash.  They include wetness, friction/chafing, presence of urine and stool and sometimes, infection from organisms.

Wetness and chafing/friction:  The persistence of wetness from urine and stool causes skin over-hydration which can result in damage (maceration).  This is made worse due to the mechanical friction between the skin and diaper or baby wipe, as well as chafing from skin-on-skin rubbing.  The over-hydration occurs also due to the covered (occlusive) environment of the diaper which traps moisture in place.  Repeated wiping of the diaper area can also contribute to tissue irritation and damage from friction.

The moist environment also favors the growth of certain organisms, especially the fungus Candida, which can sometimes result in an infection of the diapered area skin.

Digestive enzymes: Besides wetness, prolonged contact of stool and urine with the skin results in irritation in other ways as well.  Digestive enzymes that help break down food for digestion get passed through the stool.  When these enzymes come in contact with the skin, they can also break down the skin resulting in irritation and damage.  Examples of these enzymes include proteases (help digest proteins) and lipases (break down fat).  Urine can also interact with another enzyme in the stool called urease to produce ammonia – ammonia can cause an increase in the pH which can also compromise the skin.  The increased pH also further activates the enzymes in stool.  One of the key functions of skin is to act as a barrier.  If the enzymes, wetness or chafing compromise the skin barrier, these enzymes and irritants can penetrate the skin further causing greater irritation.  Protection from these digestive enzymes is important to both the prevention of diaper rash as well as the healing of an existing rash.