Water is necessary and fundamental to life. Water is a molecular substance that has unique chemical and physical properties related to functions in the human body [1]. However, water itself can cause skin irritation as demonstrated by occlusion experiments. Functional damage of the skin is shown by increased transepidermal water loss (TEWL). Factors that might account for the irritancy of water include pH, hardness, osmolarity, temperature and extraction of natural moisturizing factors in the stratum corneum [2]. The chemical content of water may also be another factor. Water rich in calcium salts is likely to irritate skin more easily [3].

A key feature of patients with atopic dermatitis (AD) is dryness of the skin caused by dysfunctions of the skin barrier with increase of TEWL [4]. Patients with AD have increased susceptibility to irritants [5]. Subramanyan [6] suggested that soaps and hot water temperature during showering or bathing are irritating factors. Thus, contacts with water should be minimized, moderately heated water should be used, and mild syndets with an adjusted hydrogen ion concentration (pH) (acidified to pH 5.5-6 in order to protect the acid mantle of the skin) should be used for cleansing [6, 7]. The value of diluted sodium hypochlorite baths and intranasal mupirocin for moderate to severe AD in infection-prone patients are suggested by some studies [8, 9].

Some patients with AD have various concepts or questions of using various type of water for bathing or skin cleansing. In underdeveloped or developing countries, some patients are afraid of contamination in tap water or underground water. They sometimes use bottled water, normal saline, or even distilled water for their facial cleansing. Thus, our aim is to study the pH of water from various sources to give an overview for physicians to recommend to patients with AD or even patients with dry sensitive skin.

Available water from various sources was collected for measurement of the pH. These included bottled drinking water (still water, sparkling water, still mineral water, and sparkling mineral water), water derived from water filter, tap water, boiled tap water, swimming pool water, rainwater, waterfall water, river water, seawater, distilled water, normal saline 0.9%, facial mineral water and make up remover water.

pH measurement was performed using a pH meter (Thermo Scientific Orion 2 Star, Beverly, MA, USA) and pH-indicator strips (pH 0-14 Universal indicator strips, Merck, Darmstadt, Germany). Each sample was measured twice. Then average pH values were calculated.

The average pH value of water from various sources measured by pH meter and pH indicator strips were shown in Table 1. Bottled drinking water using reverse osmosis plus ultraviolet (UV) and/or ozone treatment to kill organisms had pH between 6.9 and 7.5. Bottled drinking water of sparkling type which carbon dioxide gas is filled during bottling process had an approximate pH between 4.9 and 5.5.

Bottled mineral drinking still water had pH between 7.1 and 7.5 while the sparkling ones had pH between 5.3 and 6. Water derived from home water filters had an approximate pH of 7.5 as same as tap water, whereas boiled tap water had a slightly higher alkaline pH.

Swimming pool water had had pH between 7.2 and 7.5 while seawater from the Gulf of Thailand had a pH of approximately 8. Rain water in Bangkok had a pH of 6 while the water from the Chao Phraya River in Bangkok had pH of more than 7. Normal saline and distilled water had pH of 5.4 and 5.7, respectively. Facial mineral water had pH between 7.5 and 8, while facial makeup removing water had an acidic pH.

Conflicting views about cleansing the skin in patients with AD are proposed. Some authors suggested that the dry skin of patients with AD could not tolerate soaps and frequent baths [10, 11]. In contrast, even normal of skin of AD is colonized by Staphylococcus aureus, so frequent baths, regular use of soap or antiseptic soap to remove crusts, scales, dirt, and organisms on the skin are suggested by some authors [12].

However, some patients with AD have exacerbated skin lesions or dry skin when frequent swimming or bathing. Seki et al. [13] reported that the water-holding capacity of the stratum corneum in patients with AD is sensitive to free residual chlorine exposure. Most of water supply systems in many countries use chlorine as a disinfectant. Others are chloramines, chlorine dioxide, ozone and UV [14]. Public swimming pools are usually disinfected by gaseous chlorine or sodium hypochlorite and cartridge filters. Some of disinfection by products (DBPs) such as halogenated acetic acid and haloketones can irritate eyes, skin and mucous membrane [15]. According to the Centers for Disease Control and Prevention (CDC), pH of swimming pool should kept between 7.2 and 7.8 in order to keep the germ-killing power of chlorine and cause less irritation of the eyes and skin of swimmers [16]. In our study, swimming pool water had pH between 7.2 and 7.5.

Bottled water is an alternative to municipal water because of its taste and smell. Drinking bottled water implies a hygienic lifestyle and good looks. This may be the reason why some people use bottled water to wash their faces or even to bath. For sparkling water, the carbon dioxide gas is added in the bottling process which affects the pH of the water. Bottled natural mineral waters have various mineral contents. There are many regulations and standards for water intended for human consumption which limit the levels of disinfectants, DBPs (bromate, chlorine, chlorite, haloacetic acids, total trihalogenometals), inorganic chemicals (aluminum, amonium, antimony, arsenic, barium, beryllium, boron, cadmium, chloride, chrome, copper, cyanide, fluoride, iron, lead, manganese, mercury, molybdenum, nickel, nitrate, nitrite, selenium, silver, sodium, sulfate, thallium, uranium, zinc), organic chemicals (acrylamide, benzene, total pesticides, polycyclic aromatic hydrocarbons, phenolics) [17]. In our study, bottled drinking still water had slightly alkaline pH, while the sparkling ones had lower pH.

Boiling or filtration of tap water through home purification systems is widely used to abolish the contamination in the water to improve water quality by treating it in the home. There are many types of home water filters, for example, faucet mounted activated carbon/hollow fiber membrane filter, ceramic and biosand filters [18]. These water treatment technologies empower people to believe that the water they used is safe. Boiled water and filtered tap water in our study gave an alkaline pH.

The river water used in our study derived from the Chao Phraya River in Bangkok which contains clay and humus substances from the northern part of Thailand down to Bangkok. The water supply in Bangkok Metropolitan area is prepared from the Chao Phraya River water. The Chao Phraya River water showed pH of approximately seven in this study. However, water content of the rivers depends on geographic locations which affect the water pH.

Panyakapo and Onchang [19] reported 4-year investigation of rainwater at a rural area of Nakorn Pathom Province which is located near Bangkok. The pH value ranged from 4.0 to 7.8 with an average of 6.1. The ions in rainwater in their study included Ca2+, NH4+, SO4(2-), HCO3-, NO3-, Na+, Cl-, CH3COO-, Mg2+, K+, H+, HCOO-, and PO4(3-). The influence of various local emission sources such as soil, biomass burning and agricultural emissions might affect the chemical composition of rainwater. In our study, fresh rainwater in Bangkok in March had a pH of 6.

Ionized waterfall aerosol was shown to improve lung function parameters of some pediatric patients with allergic asthma [20]. However, to our knowledge, there is no study concerning waterfall water and AD. In our study, water derived from the waterfall in Trat Province, which is located in the Eastern part of Thailand, had a pH of 7.

Seawater derived from the Gulf of Thailand had an alkaline pH. Water treatment in forms of balneotherapy and spa therapy has become popular as an adjunctive treatment for some dermatologic conditions [21]. Balneotherapy involves the immersion of patients in mineral waters baths or pools. Mineral waters contain different natural solutions according to different geologic conditions. Inoue et al. [22] reported that balneotherapy using Kusatsu hot-spring water in Japan can control the skin symptoms of the acute flares of patients with refractory AD. The proposed mechanism is due to the bacteriocidal activity against Staphylococcus aureus by the coexistence of manganese and iodine ions in water under acidic condition (pH 2-3).

Climatological and balneotherapies are recognized as natural spa treatments. The Dead Sea is one of the most famous for balneologic properties and for providing climatotherapy. Chemical composition of the Dead Sea water includes magnesium, calcium, potassium, bromide, sodium, sulfate, bicarbonate, chloride, sodium chloride, potassium chloride [23]. Bathing in a magnesium-rich Dead Sea salt solution was reported to improve skin barrier function, enhance skin hydration, and reduce inflammation of atopic skin [24]. Moreover, there is more UVA radiation at the Dead Sea than elsewhere [23]. Balneophototherapy using salt water baths combined with artificial UV was reported useful in patients with AD [25].

Distilled water or deionized water is commonly used for lavage to check homeostasis at the end of various operations [26]. The efficacy of pleural lavage with distilled water during surgery for esophageal squamous cell carcinoma was proposed from the cytocidal effects of hypotonic shock on tumor cells [27]. It is also used as an alternative agent for wound cleansing. Distilled water in this study had pH similar to pH of normal skin of normal people (pH 5.4-5.9) [28]. Normal saline or 0.9% sodium chloride solution is a sterile, nonpyrogenic solution for fluid and electrolyte replenishment. It is composed of 154 milliequivalents (mEq/L) of both sodium and chloride with the pH between 5.0 to 6.0 and an osmolarity of 308 mOsm/L [29]. Normal saline is widely used to clean wounds because of its isotonic nature and it does not interfere with the healing process [30]. Some AD patients prefer to use normal saline to wash their face as it is an isotonic solution and therefore it should not damage the skin.

Pigatto [31] reported the ef fectiveness of mineral water h ydrotherapy in some AD patients. Castex-Rizzi et al. [32] showed that mineral thermal spring water could inhibit tumor necrosis factor-α-induced ICAM-1and E-selectin expression in human endothelial cells in vitro. A sampling of facial mineral water spray in this study had slightly alkaline pH.

Makeup remover water is designed as a gentle formula that will remove makeup without rubbing, while simultaneously hydrating skin. Some are labeled as soap-free, oil-free and alcohol-free so that they should not cause stinging, burning, or irritating against the skin. A sampling of makeup removing water in this study had pH near that of normal skin of normal people.

In conclusion, our study showed that normal saline, distilled water, bottled sparkling water and facial makeup removing water had an approximated pH as that of healthy normal skin of normal people. However, other factors, for example benefits of mineral substances in the water in terms of bacteriostatic and anti-inflammation should be considered in the selection of cleansing water. Our study shows an overview of knowledge about water for physicians for recommendation to patients with AD, or even patients with dry sensitive skin.