As the body's first line of defense against external insult, the skin's vast surface area (1.5 to 2 m2) is repeatedly exposed to chemical assaults and other types of injuries. As such, it may serve as a portal of entry for topical contactants. Recognizing the potential hazards of skin exposure, the National Institute of Occupational Safety and Health (NIOSH) characterizes skin disease as one of the most pervasive occupational health problems in the United States. In 2012, NIOSH ranked skin disease as the second leading work related illness based on frequency, and estimated its annual cost at greater than 1 billion dollars (NIOSH, 2017a). Although substantial reduction in the reported incidence has occurred in recent years due to better workplace environments and personal protective equipment, further improvements in prevention and management are clearly needed. Skin conditions resulting from occupational or consumer exposures not resulting in lost work are poorly recorded. Hence, their incidence is likely to be greatly underestimated.
NIOSH recently changed its cutaneous categorization to more accurately reflect the risks to workers after topical exposures (NIOSH, 2017b). Previously, inhalation exposures were thought to produce the most risk to workers, with skin exposure being only a secondary pathway. The new guidelines reflect increased understanding of several mechanisms by which skin exposure can lead to disease: systemic toxicity via skin absorption (SYS), direct effects that damage the skin (DIR), and immune-mediated responses to chemicals that contact the skin (SEN). The new notations utilize these categories to designate hazard specific skin exposures, and have incorporated standardized criteria to allow for consistency in how chemicals are designated. Determining the hazard potential of a particular chemical is based upon physicochemical properties of the substance, toxicokinetic studies, epidemiological data, in vitro or in vivo laboratory testing, and in silico computational predictions. These criteria are specified in the revised chemical designation issued by NIOSH (NIOSH, 2017b).
A large (10% of body mass) and highly accessible organ, the skin protects the body against external insults, thus maintaining homeostasis. Its biochemical properties allow it to perform a myriad of functions beyond that of barrier. Physiologically, the skin participates directly in thermal, electrolyte, hormonal, metabolic, antimicrobial, and immune regulation. Rather than merely repelling noxious agents, the skin may react to them with a variety of defensive mechanisms preventing widespread cutaneous and/or internal injuries. If an insult is severe or sufficiently intense to overwhelm the protective function of the skin, acute or chronic injury manifests in various ways. The specific presentation depends upon a variety of intrinsic and extrinsic factors (Belsito, 1989) including body site, duration of exposure, other environmental conditions, and the physicochemical properties of the insult (Table 19-1).
Table 19-1Factors Influencing Cutaneous Responses