The skin of newborns (NBs) has indispensable functions for their development: it acts as a protective barrier,1 helps thermoregulation, exchanges gases, maintains hydration, and contributes to innate immunity.2 Additionally, its mildly acidic pH provides additional protection against pathogens.1 The disruption of this natural barrier allows opportunistic microorganisms to affect infants with bloodstream dissemination, especially in premature infants due to immunological system immaturity.3 Studies indicate that the infant's skin continues to develop for up to 12 months after birth4,5 and that it differs from the adult skin in several aspects, such as composition, structure, function, and its susceptibility to infections.4,6,7

The potential of hydrogen (pH) of the slightly acid skin is an important factor in the protection against microorganisms – it is essential for the epidermal barrier maturation and repair processes.8 In adults and adolescents, skin pH is <5 (pH<5). In the thinner skin, especially in preterm infants, the pH tends toward neutral, resulting in significant loss of defense against microbial proliferation, as well as higher transepidermal water loss.9 At birth, a full-term NB's skin has a pH ranging from 6.3 to 7.5.10,11 Within the first 2 weeks of life, the pH drops to approximately 5.11 Between the second and fourth weeks of life, the pH becomes gradually acid, ranging from 4.2 to 5.9, depending on the area of the body, with higher values found in the axillary, genital, and interdigital areas.12

Bath time is a moment of relaxation and mother–child interaction, in addition to being indispensable for the maintenance of skin hygiene and health.13 The bath keeps the skin free of irritants (saliva, nasal secretions, urine, feces, and fecal enzymes), dust, and microorganisms.1 According to the World Health Organization (WHO), it is recommended that the NB be given the first bath 6h after birth.9 It has been suggested that a bath given with only water is the least harmful method of cleaning the NB, which has been adopted in the national postnatal care protocols in many countries, including the United Kingdom.9 However, the buffering capacity of water has been questioned, as it can increase skin pH from 5.5 to 7.5. Moreover, the use of water alone was identified as an ineffective cleaning agent, as it does not remove oily substances such as feces and sebum.14,15 American guidelines recommend the use of lukewarm drinking water, with the option of associating a mild cleaning product with a physiologically adequate pH (5.5–7.0).14

It is believed that the repeated use of cleaning agents can alter the skin surface pH in the long term.12 Traditional soaps have an alkaline pH, which can destroy the skin lipid layer9 and elevate skin pH above 8.0, leading to skin dryness and irritation.12,14,16–19 A pH of 7.5 is capable of increasing skin protease activity and inhibiting lipid lamellae synthesis, leading to a breakdown of the skin barrier.14

The “syndets”, a term derived from “synthetic detergent”, are formulated from synthetic surfactants that have good detergent effect, with neutral or slightly acid pH, and cause less irritation.9 Studies show that taking a bath using this type of soap is comparable or even superior to a bath with water only.20

A good cleaning product for NBs must have a pH close to 5.5 and some buffer capacity to maintain the pH close to that level.14 The cosmetic industries offer a great variety of such products and classify them as “mild”. However, there is no international criterion to establish the mildness of cleaning agents, and often, various products advertised as adequate for sensitive skins exhibit significant irritative effects.21

Thus, the aim of this study was to evaluate the pH of children's soaps, antibacterial soaps, and syndets commonly used to bathe children and infants, in order to verify whether they are slightly acidic and, therefore, consistent with the maintenance of the water–lipid mantle and barrier function and, secondly, to determine whether the labels of these products have information about their pH.

This was an observational, analytical, cross-sectional, quantitative study. Products from all brands (liquid or bar) of children's soap were obtained, antibacterial and syndets found at points of purchase (supermarkets, drugstores, and cosmetic stores) located within 7km of Hospital de Clínicas, located in downtown Curitiba, state of Parana, Brazil. These samples were paid for using the researchers’ own funds, in order to avoid any conflicts of interest. Ninety types of soaps were analyzed (62 bars and 28 liquid soaps), divided into three categories, including 67 children's soaps, 17 antibacterial soaps, and six syndets. These groups were further divided into five categories according to pH ranges: pH<5; between 5 and 5.9; between 6 and 6.9; between 7 and 7.9; and ≥8.

The pH of all the soap samples was measured using a pHmeter® (Hanna Instruments, model H19321, TX, USA) at a dilution of 1% in distilled water (pH 9.1). A comparison between 1% and 10% dilutions was performed and it was verified that pH values remained the same. Thus, due to the fact that smaller volumes are easier to use, the 1% dilution was chosen.

Data were stored in a Microsoft Excel® spreadsheet and evaluated using R® software, version 3.0.2 (Microsoft®, Washington, USA). Summary measures used in descriptive statistics were mean, standard deviation, median, minimum and maximum values, and frequencies, depending on the type of the studied variable. The Pearson–Yates chi-squared test and Student's t-test were applied, considering the significance level of 5%. The tests used for inferential statistics (data comparison) were the Wilcoxon–Mann–Whitney test, Fisher's exact test, and the Kruskal–Wallis test.

This study did not require the approval by the ethics committee, as it did not involve human subjects, but only the biochemical analysis of soaps; opinion No. 064.2012.

The pH of the soaps ranged from 4.4 to 11.5, with a median of 10.7. Two (2.2%) had pH<5, 8 (8.8%) soaps had pH between 5 and 5.9, 9 (10%) had pH between 6 and 6.9, 10 (11.1%) had pH between 7 and 7.9, and 61 (67.7%) had pH>8.

In the group of children's soaps, 47 were bars and 20 were liquid soaps (Table 1). Only one of bar soaps had pH<8 (2.2%). Among the liquid soaps, 1 (5%) had pH<5.0; 3 (15%) had pH from 5.0 to 5.9; 9 (45%) had pH from 6.0 to 6.9; and 7 had (35%) pH from 7.0 to 7.9.

In the group of antibacterial soaps, 14 were bars and three were liquid soaps (Table 1). All bar soaps (100%) had pH>8.0; among the liquid soaps, 1 (33.3%) had pH<5.0; 1 (33.3%) had pH between 5.0 and 5.9 (33.3%), and 1 had pH>8.0. Only two antibacterial liquid soaps had the pH close to the physiological range.

In group of syndets, two were bar and four were liquid soaps (Table 1). Among the bar soaps, 1 (50%) had pH of 5.0–5.9, and 1 (50%) had pH>8.0; and among the three liquids soaps, 3 (75%) had pH between 5.0 and 5.9, and 1 (25%) had pH between 7.0 and 7.9.

When comparing the pH values between the liquid and bar soaps using Fisher's exact test, there was a statistically significant difference (p<0.0001), with the liquid soaps showing lower pH values (Fig. 1).

Figure 1.

Potential of hydrogen (pH) variation according to the soap presentation n=90.

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When divided by soap category: children's soaps, antibacterial soaps, and syndets, different pH values were also found between them using the Kruskal–Wallis test (p=0.0017).

The multiple comparison test (Fisher's exact test) was used, which showed a statistically significant difference in pH (p=0.0032) between the children's soap and syndet groups, and between the antibacterial soap and syndet groups (p=0.0002). The syndets had significantly lower pH when compared to the others (p<0.05; Fig. 2). Only two of the assessed soaps had information on the label indicating the pH.

Figure 2.

Potential of hydrogen (pH) variation according to the group of soaps (children's, antibacterial, or syndet) n=90.

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Skin pH alterations caused by the use of different types of soaps are well known.1,7,9,12,13 It is also known that skin pH is slightly acidic, and that highly-alkaline pH can damage the acid mantle that acts as an antibacterial barrier, as well as disrupt the lipid lamellae of the epidermis, resulting in skin dryness due to higher transepidermal water loss and allowing the entry of potential irritants and allergens.7,9,12,13 This occurs because the soap, in contact with water, undergoes a hydrolysis reaction, releasing the alkali contained in these products and increasing the skin pH to 10–11.12 Based on the results, it can be observed that the soaps in liquid form are the most appropriate for everyday use, especially in children and infants, whose skin is more delicate and sensitive.

As demonstrated in the present study, Volochtchuk et al.,12 in a study carried out at the same institution, have already shown significant differences in pH between liquid and bar soaps, but the study evaluated both adult and children's soaps. The bar soaps showed pH>6, with most between 9 and 10. As for the liquid soaps, most showed pH between 6 and 6.9.11

Liquid syndets have a pH closer to the physiological range (approximately 5), and are especially recommended to patients with diseases that alter the skin barrier (such as atopic dermatitis and ichthyosis) or children with sensitive skin. In these individuals, the use of soaps with high pH worsens xerosis and can generate solutions of continuity of the skin, which can be a gateway to pathogens. Syndet action occurs in two ways: first, by reducing the interaction between tensioactive agents and skin proteins and lipids; and second, by restoring lipids and moisturizing agents lost during the washing.19 Thus, the skin does not dry, remaining hydrated.

Antibacterial bar soaps showed the highest pH values of all assessed soaps. This demonstrates that these products can be aggressive to the child's skin and should not be routinely used, but only in specific situations and for short periods of time, and on restricted parts and not the entire body, preferably in liquid form.

It was verified that only two of the included soaps had any mention of pH levels on their package. The study by Tarun et al.22 mentioned that the labels of all assessed soaps, except for one, had no information related to the product pH. It was observed that even those products whose packaging contained phrases such as “neutral pH”, “balanced pH”, or “dermatologically tested” had pH above the expected range. Therefore, it was found that many of these products provide information that can confuse the consumer, since the fact that the skin is slightly acidic is not generally known by the public, as well as the importance of maintaining the skin's barrier function. Only one brand (Eucerin bar and liquid soap, Eucerin®, USA) displayed pH specification on the packaging, which makes it difficult to choose or recommend these products, by both the lay population and health professionals.

Most analyzed soaps are manufactured and sold in Brazil. Although some of them represent internationally-renowned brands, it cannot be verified that their chemical composition and hence their pH is the same worldwide. However, as this study selected product brands that are also available in other countries, in easily accessible and popular stores, and also because as many different brands as possible were acquired, it is believed that the results can be extrapolated to other cities in Brazil and other regions of the world, which should be supported by studies in these locations.

This study highlights the pH inadequacy of several children's products freely available in points of purchase. Pediatricians, parents, and caregivers should be aware of the characteristics of products used in pediatric patients, as some may even cause damage to the skin of children and infants.

Based on this fact, it is important that health surveillance institutions that regulate the selling of products for infant use establish stricter criteria for their commercialization, as well as for the information that should be included in their packaging, including the product pH.

The authors declare no conflicts of interest.