Fig. 1.

Bio-distribution and clearance pathways of ambient UFPs in the body. After inhaling via the respiratory system, UFPs are first deposited in the lung, and then they may penetrate the ABB and be deposited in extra-pulmonary organs such as the heart, liver, kidney, and spleen through blood circulation. In addition, UFPs in the circulation may also penetrate the BBB and PB, leading to deposition in the brain and placenta. It is worth noting that there is also a nasal olfactory pathway regarding the deposition of UFPs in the brain. The main clearance pathway of UFPs in the body is by the kidney, with the vast majority of particles excreted in urine. In addition, clearance by liver and gastrointestine also plays an important role in some cases. The lymphatic circulation also plays an essential role in the clearance process of UFPs in vivo.