Dermal exposure can be evaluated using the conceptual framework elaborated by Schneider et al. [23]. The source of contamination is NPs produced or released from objects. From the source domains, the NPs can deposit on the skin or on other surfaces (direct contact). NPs can be transported to the air zone, and from there they can contaminate the skin and surfaces by deposition. Protective gloves and aprons can be contaminated themselves and resuspension of NPs can happen. Finally NPs can be transferred to other surfaces and to the perioral region causing inadvertent intake and ingestion. More information on risk resulting from skin contact can be found in BauA [24].

Van Duuren-Stuurman et al. in 2008 [25] studied dermal exposure to NPs in ten European enterprises using an observation model (Dermal Exposure Assessment, DREAM). In the majority of tasks observed (39/45), dermal exposure cannot be excluded, and the main routes for exposure were transfer and direct contact.

NPs emitted from processes or released from objects or textiles agglomerate and settle on the skin and on surfaces. Therefore, in general, the skin comes in contact with agglomerated NPs with a diameter bigger than 100 nm, and the presence of acid pH and sweat can increase their tendency to aggregate. Moreover, in contact with sweat, NPs can release metals or impurities in higher amount than bulk material for the high ratio surface/mass. These aspects are more relevant when toxic or sensitized metals can be released (Cd, Ni, Pd, Ag, etc.) [12].

Larese Filon et al. in [12] suggested some critical sizes to evaluate NP skin hazard: for NPs <4 nm, penetration has been demonstrated; for NPs 4–20 nm, skin penetration/permeation is possible, probably through follicles; for NPs 21–45 nm, skin absorption can be possible only on impaired skin; and for NPs >45 nm, skin absorption is unlikely in healthy skin.

Due to their high surface/mass ratio, NPs can release metals in ionic form that can penetrate the skin inducing local or systemic effects. NPs can induce in more efficient way skin sensitization with respect to bulk materials. Ni, Cd, Co, and Pd NPs can release metals that can cause allergic contact dermatitis (Fig. 11.1). Journey and Goldman reported in 2014 [13] a case of skin and respiratory sensitization in women exposed to NiNPs. No other case reports are available in literature.

Quantum dots containing Cd can be absorbed through the skin reaching general blood circulation causing sign of systemic intoxication of Cd in exposed animals [26], while no data are available in exposed workers. Polycyclic aromatic hydrocarbons can be released from NPs [27], and their carcinogen effect could be exerted on the skin. Therefore, no human data are available on that.

Very few data are available on this topic, but coating and pH could influence dissolution, release of sensitized or toxic metals, and also persistence on the skin. While many data are available on different cells in vitro, no data are available on real work scenario. Ovissipour et al. [28] studied decontamination of tomatoes treated with different NPs, and he found that decontamination could be difficult from surfaces in particular conditions of pH of the solution used and isoelectric point of NPs.

Absorption of NPs through the skin is a function of surface area contaminated, applied dose, time of contact, thickness of the membrane, and characteristics of the NPs, On the other site, the use of protective measures to avoid skin contamination is extremely important, due to the difficulties in removal of NPs from the skin.

The amount of NPs that come in contact to the skin is important for decontamination purposes and for the loading on the skin surface. Nevertheless, in the condition of substances with very low permeation rate, such as NPs, the dose could be not relevant because a decontamination is not easy or possible, and NPs can be stored in hair follicles and from there they can release ions. Moreover, NPs can penetrate and permeate the skin if their size is less than 45 nm, as suggested by Larese Filon et al. [12].

Skin effect or absorption can be more effective when skin is exposed for longer period. One of the crucial aspects is that after contact with NPs, it is quite difficult to clean the skin because NPs are very adhesive to the skin and they reach the hair follicles where they cannot be removed. In this condition also, after a short contact with the skin, NPs cannot be removed by the cleaning procedures. There are no, in our knowledge, studies on this topic, but there are experimental data that demonstrated that hair follicles are the “storage” place for NPs that can come in contact to the skin [29]. Moreover, one study evaluated the cleaning effect on tomatoes contaminated by NPs washed using deionized water. The washing was effective to remove alumina NPs but not titania and silica [29]. The NPs persistence on the surface was explained with differences between pH of the washing solution and NPs isoelectric point. The Fourier transform infrared spectroscopy results showed that some NPs can bind to certain biochemical components such as polysaccharides and proteins on the surface of tomato skins.

In human skin, NPs can penetrate between the stratum corneum and inside the hair follicles when NPs are flexible or when rigid NPs are smaller than 40–45 nm, particularly when the skin is impaired [12]. From hair follicles NPs can release toxic or sensitized substances or can cross the epithelia reaching the derma. Rancan et al. [29] demonstrated that silica NPs can reach Langerhans cells after they have been stored in hair follicles.

Occupational skin diseases (OSD) are extremely common in worker populations. The European Agency for Safety and Health at Work, EU-OSHA, EU-25 report 2008 [30], reported that they constitute a top-priority public health problem. The economic burden of OSD in the EU exceeds € 5 billion spent every year on treatment, compensation, and loss of productivity [31]. They represent up to 35 % of all reported occupational diseases [32, 33]. The impairment of the skin is frequent in some professions, particularly in wet workers, construction workers, health-care workers, hairdressers, mechanics, etc. that are doing “wet work” or are in contact with irritants and sensitizing agents.

The impairment of the skin barrier increases the risk of NPs penetration because NPs can reach easily the basal layers of the epidermis and hair follicles [29]. The impairment of the skin increases NPs penetration 4–100 times [34–36], and for some NPs, such as titanium dioxide, skin penetration is possible only in impaired skin [37]. Frequent exposure to water, irritant chemicals, detergents, disinfectants, and powders can lead to fissuring of the skin [38]. Moreover it is possible for the skin barrier to be compromised, although there are no visible signs [39]. In this case, the contamination of the skin with NPs can cause an increased risk of NPs penetration into the skin and possible effects. In vitro data performed using human skin and Franz cells demonstrated that, obviously, the impairment of the skin increases significantly the metal content inside the skin and skin absorption for many metal NPs [18, 34, 36, 40, 41].