## 4 Uptake and Bioaccumulation of Microplastics and Nanoplastics in the Human Body
### Dermal Exposure  
Another significant source of plastics is the health and beauty industry, particularly the body and face scrubs applied directly to the skin [13], the exposure is through the dermal application of nanocarriers for medication delivery. Small particle size and stressed skin conditions are crucial for skin penetration, despite no definitive data on the effects of nanocarriers [60]. There is currently no research that has examined how well NPs can penetrate the skin's outer layer. Thus far, one study documented nanoparticle penetration in small quantities via the skin [90].  
The stratum corneum, the skin's outermost layer, serves as a barrier to protect the skin from harm, toxins, and microorganisms. Corneocytes make up the stratum corneum, which is bordered by lamellae of hydrophilic lipids such as cholesterol, ceramide, and long-chain free fatty acids [91]. Plastic particles could enter the body through sweat glands, skin wounds, or hair follicles, however, absorption through the stratum corneum by polluted water is deemed as uncommon because MPs and NPs are hydrophobic [60].  
Plastic particles' entry into the body and subsequent distribution throughout the skin tissue was studied by [PERSON] et al., where pig skin tissue and fluorescent PS particles with a diameter of 20-200 nm were used [92]. A confocal laser scanning micrograph of the skin showed that the concentration of 20 nm PS NPs was higher than that of 200 nm NPs in the hair follicles. To embed themselves into the deeper skin tissue, the particles need to pass through the stratum corneum, which neither was able to do. These results were confirmed by [PERSON] et al., who also found that PS particles with a diameter of 20-200 nm can only penetrate the skin's top layers at a depth of 2-3 \(\upmu\)m [93]. Further, [PERSON] et al. found fluorescent PS nanoparticles with a diameter of 40 nm in the perifollicular tissue of skin explants that had undergone cyanoacrylate follicular stripping, and this research established that when particles were given transcutaneously, the langerhans cells then absorbed them [94].  
Moreover, skin damage from UV exposure weakens the skin barrier [95] and causes an increase in the skin's penetration by different particles [96; 97]. Common components in body lotions such as urea, glycerol, and hydroxyl acids improved the ability of nanoparticles to penetrate the epidermal barrier [98].  
[PERSON] et al. highlighted the effects of oleic acid, ethanol, and oleic acid-ethanol enhancers on the transdermal transport of 10 nm zinc oxide nanoparticles by passing through the multilamellar lipid areas between corneocytes [99]. They found that each chemical had the potential to increase the ability of zinc oxide nanoparticles to penetrate the epidermal barrier. [PERSON] et al. established a three-layer \"sandwich model\" through crystalline structure analysis of different lipid lamellae compositions in stratum corneum samples obtained from human and swine sources [95]. This model presumably prevents big nanoparticles from penetrating healthy skin.  
In conclusion, experiments conducted in vitro and in vivo have proven that MPs and NPs can enter the human body through the skin barrier. Nevertheless, the basis for all these investigations is PS particle modeling. It would be beneficial to undertake additional research using environmental sample collection to completely comprehend the penetration properties of MPs and NPs as those samples could contain a range of plastic particles with various properties.