Although wetness alone macerates the skin, softening the stratum corneum and greatly increasing susceptibility to friction injury, urine has an additional impact on skin integrity because of its effect on skin pH. While studies show that ammonia alone is only a mild skin irritant, when urea breaks down in the presence of fecal urease it increases skin pH, which in turn promotes the activity of fecal enzymes such as protease and lipase (Atherton, 2004; Wolf, Wolf, Tuzun and Tuzun, 2001). These fecal enzymes increase the skin’s permeability to bile salts and act as irritants in and of themselves.

The interaction between fecal enzyme activity and IDD explains the observation that infant diet and diaper rash are linked, since fecal enzymes are in turn affected by diet. Breast-fed babies, for example, have a lower incidence of diaper rash, possibly because their stools have lower pH and lower enzymatic activity (Hockenberry, 2003). Diaper rash is also most likely to be diagnosed in infants 8–12 months old, perhaps in response to an increase in eating solid foods and dietary changes around that age that affect fecal composition. Any time an infant’s diet undergoes a significant change (i.e. from breast milk to formula or from milk to solids) there appears to be an increased likelihood of diaper rash (Atherton and Mills, 2004).

The link between feces and IDD is also apparent in the observation that infants are more susceptible to developing diaper rash after treatment with antibiotics, which affect the intestinal microflora (Borkowski, 2004; Gupta & Skinner, 2004). Also, there is an increased incidence of diaper rash in infants who have suffered from diarrhea in the previous 48 hours, which may be due to the fact that fecal enzymes such as lipase and protease are more active in feces which have passed rapidly through the gastrointestinal tract (Atherton, 2004).