Atopic dermatitis, commonly known as eczema, is a chronic inflammatory skin condition resulting in dry, intensely itchy, and red patches on the skin. The condition is complex, driven by genetics, immune system dysfunction, and environmental triggers. A common concern is whether the water used for bathing acts as a trigger or direct cause of these persistent flare-ups. Investigating the composition of tap water and its effects on the skin barrier helps understand the scientific evidence behind this concern.
How Water Affects the Skin Barrier
The outermost layer of the skin, the stratum corneum, functions as a protective shield. This skin barrier keeps irritants and allergens out while sealing moisture inside the body. In individuals with eczema, this barrier function is compromised, making the skin more susceptible to environmental factors.
Contact with water, especially prolonged or frequent exposure, influences the skin’s moisture balance through transepidermal water loss (TEWL). TEWL is the passive evaporation of water vapor from the skin surface. When the skin barrier is damaged, TEWL increases significantly, leading to rapid dehydration, dryness, and irritation.
Hardness, Calcium, and Magnesium
The primary concern about tap water relates to its hardness, which refers to a high concentration of dissolved divalent minerals, predominantly calcium and magnesium ions. These mineral ions interfere with the cleansing process by reacting with surfactants found in soaps and body washes. This reaction forms an insoluble precipitate, leaving a thin, irritating residue on the skin surface.
This soap residue compromises the skin barrier and increases sensitivity to irritants. Calcium ions can also directly damage the skin barrier and increase TEWL, particularly in individuals with a genetic mutation in the filaggrin (FLG) protein, a factor common in eczema sufferers. Epidemiological studies have found an association between living in hard water areas and the prevalence of atopic eczema in children and infants.
While hard water is linked to increased eczema prevalence, especially in genetically susceptible infants, the evidence suggests it acts as a risk factor and exacerbating agent rather than a sole cause. In adults, increasing hard water levels are associated with a higher prevalence of eczema, but not a higher rate of new incidence. The minerals in hard water also contribute to the overall alkalinity of the water supply, complicating the skin’s defense mechanisms.
Chlorine and Water pH Levels
Tap water is typically treated with chemical disinfectants, with chlorine being the most common. Chlorine is a strong oxidizing agent that strips the skin of its natural surface oils, or lipids, which are a component of the skin barrier. This lipid removal leads to increased dryness and can trigger or worsen existing eczema symptoms.
The pH of the water also plays a role in skin health, as healthy skin maintains a slightly acidic pH, often called the acid mantle (typically 4.1 to 5.8). This acidity is necessary for barrier function, lipid production, and defense against pathogens. Tap water, especially hard water, is often mildly alkaline, with a typical pH value around 7.2 to 7.4.
Repeated exposure to alkaline water disrupts the naturally acidic environment of the skin. This shift impairs the skin’s ability to repair itself and makes it more vulnerable to inflammation and colonization by harmful bacteria. The combined effects of chlorine stripping lipids and alkaline pH disrupting the acid mantle create a challenging environment for eczema-prone skin.
Current Scientific Consensus on Mitigation
The scientific consensus holds that while tap water may not be the underlying cause of eczema, its components serve as significant environmental exacerbating factors. The primary goal of mitigation strategies is to reduce the irritating effects of the water on an already compromised skin barrier. Many people consider installing water softeners to remove calcium and magnesium ions.
Randomized controlled trials, such as the Softened-Water Eczema Trial (SWET), examined the clinical benefit of water softeners in children with established eczema. These studies generally found that installing a water softener did not lead to a statistically significant improvement in eczema severity compared to standard care. However, a newer pilot study, SOFTER, suggested that installing a water softener from birth might reduce the occurrence of eczema in high-risk infants, indicating a potential preventative role.
For chlorine, carbon filtration systems effectively remove the chemical from the water supply. While large-scale clinical trials on the efficacy of chlorine filters for eczema are less common, removing this known lipid-stripping agent is a logical strategy to reduce skin dryness and irritation. Combining filtration with a gentle skincare routine, including immediate application of a moisturizer after bathing, remains the most evidence-based approach to managing tap water effects on eczema.