Py-GC/MS has proven to be an informative and reliable method to determine plastics concentrations in liquid and solid tissue samples, with ample assurance of accuracy, quality, and rigor2,3,9,10. Decedent liver and kidney MNP concentrations were similar, with means of 465 and 666 µg/g, respectively, from 2024 samples (Figure 1A). These were higher than previously published data for human placentas (126 µg/g)10, but comparable to testes (329 µg/g)11. Liver samples had significantly higher concentrations in 2024 than in 2016 samples (145 µg/g; p<0.001). The brain samples, all derived from the frontal cortex, revealed substantially higher concentrations than liver or kidney, at 3,057 µg/g in 2016 samples and 4,806 µg/g (0.48%, by weight) in 2024 samples, ranging as high as 8,861 µg/g. Five brain samples from 2016 (highlighted in orange, Figure 1A,B) were analyzed independently by colleagues at Oklahoma State University, and those values were consistent with our findings.
A non-parametric analysis of variance (Kruskal-Wallis) confirmed that MNP concentrations in brains were significantly greater than all other tissues (P<0.0001). Furthermore, from 2016 to 2024, there was a significant increase in MNP concentrations in both livers and brains. The predominant polymer found in all tissues was polyethylene, which independently displayed similarly increasing trends from 2016 to 2024 in the liver and brain (Figure 1B). The proportion of polyethylene in the brain (74%) appeared significantly greater relative to other polymers in comparison to the liver and kidney (44-57%), although kidney samples from 2024 also had increased relative PE (71%; Figure 1C,D). This was also confirmed with ATR-FTIR spectroscopic analysis from 5 brain samples (Figure 1D).
Because we suspected that much of the MNPs measured were actually in the nanoscale range, transmission electron microscopy (TEM) was conducted on the dispersed pellets obtained from kidney, liver, and brain (Figure 2; see methods supplement). While TEM does not provide spectroscopic identification to confirm particulate composition, we observed common shapes and sizes among the numerous samples and tissue types. Notably, there were innumerable particulates with shard-like appearance, often less than 200 nm in length. Currently, MNP uptake and distribution pathways are incompletely understood; this new appreciation of the size and shape aids in our appreciation of potential mechanisms. Importantly, these observations bring into question the relevance of the many recent studies utilizing polystyrene microspheres4,12, as polystyrene was infrequently detected in human tissues and MNPs were rarely spherical.
The concentrations in liver and kidney were not as high (relative to brains) as we would have suspected, as these are "front line" organs for xenobiotic uptake and clearance. That said, the lipophilic nature of plastics may make them easily handled by the liver, which has a major role in uptake and repackaging of dietary triglycerides and cholesterol. A recent study found higher MNP numbers in the cirrhotic liver compared to the healthy liver; whether the microplastics promote disease or are simply accumulating along with intracellular fats has not been elucidated13.
Following this logic, the human brain has the second highest lipid content in the body, with only adipose tissue being higher; brain MNP concentrations are comparable to recently published Py-GC/MS data from carotid plaques, which are also a lipid depot3. Furthermore, the brain receives a high blood flow, approximately 25-30% of the cardiac output, and has a tremendous metabolism. The blood-brain barrier poses a notorious challenge. However, modeling of transfer across cellular membranes suggests the uptake is dependent on the association of particulates with cholesterol and, furthermore, that particles <1µm rapidly traversed the blood-brain barrier within 2h of ingestion in mice14. Longer-term gavage studies similarly found that larger (5 µm) polystyrene microspheres could access the brain and promote metabolomic alterations15. Lastly, clearance rates from the brain are unknown for polymer particulates. The lack of correlation with the decedent age suggests that an equilibrium occurs and may depend on genetic, dietary, and lifestyle factors that ultimately contribute to the wide between-subject variability in MNP concentrations. In zebrafish exposed to constant concentrations, nanoplastics uptake increased to a stable plateau and cleared after exposure16; however, the maximal concentrations were increased proportionately with higher exposure concentrations. While the time course for kinetics is assuredly longer in humans, we postulate that the exponentially increasing environmental concentrations of MNPs1,17 will analogously increase internal maximal concentrations, which is corroborated by our finding that total plastics mass concentration in brains increased over 50% in the past 8 years.