## 4 Human Exposure through Different Routes  
Human exposure to plastic particles is based on generalized estimations and pooled data derived from literature mainly on MPs, which unavoidably suffers from crude assumptions and simplifications, since exposure is highly subject- and lifestyle-dependent or based on human standard food intake. Nevertheless, estimations on exposure to MPs are useful to represent indirectly the exposure to NPs, which most likely could be even higher and undetermined because of the biased methods of analyses [65]. [PERSON] and [PERSON] [68] demonstrated that ingestion is the most studied pathway, but other means of exposure to MPs and NPs, such as inhalation and dermal contact, should not be neglected. Fruits, vegetables, meat, fish, grains, legumes, and water constitute the fundamental human diet. It is estimated that the intake of particles through the consumption of vegetables and fruits amounts to an average of 132,740 particles per gram (p/g) of MPs and NPs in five frequently consumed fruits and vegetables (apples, pears, broccoli, lettuce, and carrots). Considering these food categories and complying with the WHO recommendation of a daily intake of at least 400 g of fruit and vegetables, an average of around 53 million particles/day (p/d) are ingested [68; 69]. Evaluating approximately 15% of Americans' caloric intake, [PERSON] et al. [20] estimate that MP consumption ranges from 39,000 to 52,000 p/y depending on age and sex, increasing to 74,000 and 121,000 p/y when inhalation is considered. Additionally, individuals who follow recommended water intake through only bottled sources may be ingesting an additional 90,000 p/y instead of 4000 p/y in case of only tap water consumption. Other authors report high variability in the numbers of MPs and NPs in both bottled and tap water. Regarding the extent of exposure to plastic particles, the following data on mean concentrations obtained by various methodologies are reported, clearly showing different orders of magnitude. An average of 325 p/L was detected in 259 samples from 11 different brands of bottled water purchased globally [26], while [PERSON] et al. [70] and [PERSON] et al. [71] detected 2649 \(\pm\) 2857 p/L and 5.42 \(\times\) 10\({}^{7}\) \(\pm\) 1.95 \(\times\) 10\({}^{7}\) p/L in bottled water, respectively. [PERSON] et al. [72] detected 3.57 \(\pm\) 1.79 p/L in tap water. Considering these estimates, humans ingest around 13.5 million p/L in single used bottled water and a total of around 27 million p/d, assuming a daily intake of 2 L of bottled water [73]. The lack of standardized methods for identifying and quantifying MPs and NPs clearly is the cause of this significant variability in exposure data [68]. The actual numbers of MPs and NPs in drinking water have yet to be fully evaluated, but according to [PERSON] et al. [74], a human intake of almost 5 thousand p/y per person could be estimated for direct consumption. Additionally, food processing should be considered, as drinking water could contribute to the MPs and NPs content in processed food items. A range from 9.77 p/kg to 506 p/kg of MPs and NPs in table salt is reported [75; 76], and humans would ingest 0.714 p/d with a consumption of 5 g per day (g/d) for healthy adults. In sugar, [PERSON] and [PERSON] [77] measured 217 fibers/kg and 32 fragments/kg, suggesting that humans could ingest around 2150 fibers/year and around 320 fragments/year with a consumption of 27 g/d per capita [78]. A general exposure of over 50 to billions of fibers/day, depending on dietary habits, is estimated [79]. Considering an airborne concentration of MPs and NPs of about 0.7 particles per cubic meter (p/m\({}^{3}\)), a respiratory frequency of 12 breaths per minute, and a tidal volume of 0.5 L, the inhalation rate is roughly 8 cubic meters per day, leading humans to inhale approximately 5900 p/d.