The effect of water hardness on atopic eczema, skin barrier function: A systematic review, meta‐analysis

BACKGROUND Hard domestic water has been reported to worsen atopic eczema (AE) and may contribute to its development in early life. OBJECTIVE To review the literature on the relationship between the effect of water hardness (high calcium carbonate; CaCO3 ) on a) the risk of developing AE, b) the treatment of existing AE, and c) skin barrier function in human and animal studies. DESIGN, DATA SOURCES AND ELIGIBILITY CRITERIA We systematically searched databases (MEDLINE, Embase, Cochrane CENTRAL, GREAT and Web of Science) from inception until 30/6/2020. Human and animal observational and experimental studies were included. The primary outcomes were risk of AE and skin barrier function. Studies were meta-analysed using a random-effects model. Evidence certainty was evaluated using the Grading of Recommendations, Assessment, Development and Evaluations (GRADE) approach. RESULTS Sixteen studies were included. Pooled observational data from 7 studies on 385,901 participants identified increased odds of AE in children exposed to harder versus softer water (odds ratio 1.28, 95% CI 1.09, 1.50; GRADE certainty: very low). Two mechanistic studies in humans reported higher deposition of the detergent sodium lauryl sulphate in those exposed to harder versus softer water. Two randomized controlled trials comparing water softeners with standard care did not show a significant difference in objective AE severity with softened water (standardised mean difference 0.06 standard deviations higher, 95% CI 0.16 lower to 0.27 higher; GRADE certainty: moderate). CONCLUSIONS & CLINICAL RELEVANCE There was a positive association between living in a hard-water (range: 76 to >350 mg/L CaCO3 ) area and AE in children. There is no evidence that domestic water softeners improve objective disease severity in established AE. There may be a role of water hardness in the initiation of skin inflammation in early life, but there is a need for further longitudinal and interventional studies.