Skin represents both an important physical barrier to absorption of toxic substances and simultaneously a significant portal of entry of such substances.

The absorption of compounds through the child’s skin may be more problematic than that of adults because metabolic differences make such substances more toxic to children.

Skin Function and Development

Skin represents both an important physical barrier to absorption of toxic substances and simultaneously a significant portal of entry of such substances. It is also a locus for biotransformation or photochemical transformation processes that reduce the toxicity of some xenobiotics. Skin and its associated structures - hair, nails, sweat or sloughed cells - are also a route of excretion of toxicants or their by-products.1 Skin is also important because of its function within the body’s immune system.

The skin of young children is thinner and less impervious to the passage of foreign substances. It is more susceptible to damage from environmental agents. Dermal absorption can be a significant exposure route for some toxicants, and given that children’s body systems are underdeveloped they may suffer greater adverse effects from such exposure. Childhood exposure to UV radiation is a known risk factor associated with the etiology of skin cancers.

Functions of Skin

The stratum corneum of the epidermis is the most significant in providing some degree of physical protection from percutaneous absorption of chemicals. The rather complex structure of the stratum corneum and the composition of the cornified cells therein provide a “highly chemically resistant envelope.”1 Skin’s impermeability varies with age and physical condition of the skin.2

The barrier feature of the epidermis also lies in its function as a natural defense mechanism against invasion by microorganisms. The immune system properties of skin, however, reside largely with the functioning of Langerhans’ cells. Together with neighbouring keratinocytes, Langerhans’ cells comprise the skin immune system or SALT (skin associated lymphoid tissue). These epidermal cells fulfill an immunosurveillance role by recognizing and presenting antigens. They also induce delayed hypersensitivity reactions in the skin.3

Melanin, the brown pigment of skin manufactured in specialized cells at the base of the epidermis, contributes to protecting skin from sun-induced damage, although the exact mechanism by which it does so has not been fully determined. Melanin is an oxygen scavenger and may reduce genetic damage to the nuclear material of skin cells by removing mutagenic oxygen radicals before genotoxic effects occur.4

Skin is also among those tissues that have rapid turnover and constant renewal throughout life, representing a vulnerable target from environmental exposures at any stage in pre-adult development and over the lifetime of the individual.

Skin Development

A child’s skin is different from that of an adult’s. Developing skin is extremely permeable and a less effective barrier to entry of toxic substances. The skin of the newborn, but particularly that of the premature neonate, is highly permeable and therefore will more readily absorb substances. By the end of the first year of life, skin permeability is closer to that of an adult.5

The skin of children is also thinner and therefore more vulnerable to direct damage from exposures such as ultraviolet radiation. Being of smaller overall body size, a child’s skin surface to volume ratio is greater than an adult’s. so they potentially absorb more chemicals through skin on a per weight basis when compared to adults.

It is also important to acknowledge that the absorption of compounds through the child’s skin may be more problematic because metabolic differences make such substances more toxic to children. Case reports have shown that infants can develop specific health effects entirely from transdermal exposure alone. For example, when Argentinian infants developed acrodynia, a rare syndrome that may result from mercury toxicity, it was discovered that they were exposed from diapers rinsed in phenylmercury used as a fungicide by a commercial laundry.6 Use of hexachlorophene as an antibacterial skin cleanser resulted in neurotoxic effects in infants, especially premature babies.7

There may also be greater risk of health effects from dermal absorption due to the windows of vulnerability of developing systems such as reproductive tissues and the brain. For example, several case reports in the medical literature show evidence of children suffering from excessively early or abnormal sexual development after exposure to androgens found in various products, such as topical testosterone-containing ointments or estrogen- or placenta-containing hair treatments.8,9,10,11,12,13 Some ethnic folk remedies that are considered toxic to children may also be administered topically and therefore represent a direct dermal exposure.14

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