Prevalence of atopic dermatitis in infants by domestic water hardness and season of birth: Cohort study

Atopic dermatitis is a chronic skin disorder in which the skin becomes dry, itchy and thickened.  Even though it is very common in children, its exact causes are not well-known.  Because water is a known skin irritant and the skin of infants are very sensitive, it has been thought that hard water, that is water that contains high calcium carbonate, may be a risk factor.  In this month’s issue of JACI, Engebretsen and colleagues investigate whether early exposure to hard domestic water is associated with the prevalence of atopic dermatitis (J Allergy Clin Immunol 2017; 139(5): 1568-1574).

To do this, they looked at the Danish National Birth Cohort study which collected nearly 100,000 children born between 1996 and 2002.  Out of these, the mothers of 55,092 children completed an interview at 6 and 18 months to get more information on atopic dermatitis.  What the authors found was that hard water is associated with a higher incidence of atopic dermatitis.  This effect was dose-dependent, and they attribute a 2% risk for atopic dermatitis on hard domestic water.  In addition, they found that children born in autumn and winter had a higher incidence as well.  However, combined evaluation of these two effects did not cause an even greater incidence on atopic dermatitis.

The reasons for this association are unclear.  The authors suggest that hard water may change the acidity of skin and thus change the activity of skin enzymes, or maybe that it requires more irritant soap for lather production with hard water.  It may even be that hard water changes the growth of bacteria on the skin that may modulate risks for atopic dermatitis.   It also opens a lot of other questions that have not yet been explored.  Can water softening reduce the risk of developing AD?  What role do skin moisturizers and other emollients have in preventing hard water-induced skin damage? Does this effect extend to infants outside of Denmark and other Nordic countries?   Although these are all unanswered, this study opens a new window for research and helps point the way for further directions.