脳には6〜7gのマイクロプラスチックが蓄積している。

高濃度に蓄積した人ほど認知症リスクが高い。

しかも8年間で約50％増加、認知症との因果関係は不明

　マイクロプラスチック（直径5ミリメートル以下のプラ粒子）は、世界的にプラスチックの使用量が増えるのに伴い、驚異的な速さで環境に浸透している。現在のプラスチック生産量が年間3億トンを超えるなか、世界の海には、2023年時点で推定250万トンのプラスチックが浮遊している。これは2005年の水準の10倍以上にあたる量だ。

　2月3日付けで医学誌「Nature Medicine」に掲載された研究により、マイクロプラスチックとナノプラスチック（さらに小さい直径1～1000ナノメートル）は、人間の肝臓や腎臓よりも高い濃度で脳に蓄積されることが判明した。また、2024年のサンプルは、2016年のサンプルと比べてマイクロおよびナノプラスチックの濃度が大幅に高くなっており、認知症と診断された人の脳内ではさらに高濃度だったという。

　脳内のプラスチック粒子と認知症の因果関係が証明されたわけではないものの、こうした研究結果は、プラスチックが健康に影響を及ぼす可能性について懸念を生じさせるものだ。

「体内にプラスチックがあるという状況は、環境中での蓄積と、人々がそれにさらされる状況がそのまま反映されたものだと考えられます」と、論文の著者である米ニューメキシコ大学の薬学教授マシュー・カンペン氏は言う。「人々がさらされるマイクロプラスチックとナノプラスチックの濃度が、どんどん高まっているのです」

プラスチック汚染が急増

　マイクロプラスチックやナノプラスチック（両方をあわせて「MNP」と呼ぶ）は、ペットボトル、買い物袋、発泡スチロール容器などのプラスチック製品が環境中で分解されて生じる。肉眼では見えないほど小さいものもある。

　科学者らは1970年代から海中に存在するMNPを研究してきた。海洋哺乳類の体内からは、海水から吸収されたり汚染された魚を食べたりして取り込まれたマイクロプラスチックが見つかっている。また、人間が食べるブタ、ウシ、ニワトリなどの動物の組織にも蓄積される。

　MNPは空気中にも存在する。特に屋内の空気は、衣類、家具、家庭用品に含まれるプラスチックから落ちる粒子のせいで、屋外の空気よりもMNPの濃度が高くなる傾向にある。

　人間に吸い込まれた粒子は体内を移動し、やがてさまざまな臓器にたどり着く。研究ではこれまでに、人間の肺、胎盤、血管、骨髄からMNPが見つかっている。

　2024年9月に医学誌「JAMA Network Open」に発表された研究では、MNPが血液脳関門を通過できることを示す証拠が発見された。血液脳関門とは、いわば血流から脳内に入るものを制限するフィルターだ。

　かつては、この関門を通過できるのはナノプラスチックの中でも特に小さな粒子だけだと考えられていた。だが、同研究によって、より大きなマイクロプラスチックも脳内に侵入できることがわかった。

なぜ脳により多いのか

　今回「Nature Medicine」に発表された研究により、脳内にはMNPが存在することが確かめられた。また、その量は驚くほど多かった。

　同研究は、2016年および2024年に亡くなった52人の脳のサンプルを分析している。すべてのサンプルは、判断、意思決定、筋肉の動きを司る脳の領域である前頭葉から取られた。

　研究者らはまた、同じ遺体から採取した肝臓と腎臓のサンプルも調べ、顕微鏡イメージングと分子解析によってすべての組織を分析し、その化学組成を特定している。

　2024年の脳と肝臓のサンプルは、2016年のサンプルと比べて、MNPの濃度が著しく高かった。脳内のプラスチックの濃度は、2016年から2024年の間に約50％増えており、研究者らは、家庭内、大気中、水中におけるMNP濃度の大幅な増加がその原因ではないかと指摘している。

「彼らの研究で見つかったマイクロプラスチックの量は衝撃的です」と、オランダ、ユトレヒト大学の神経毒性学者エマ・カステール氏は言う。「私の予想をはるかに上回る量でした」。氏は今回の研究に関わっていない。

　氏によると、この結果には環境中のマイクロプラスチック濃度の増加が反映されており、プラスチックにさらされる量が増えたことで、臓器内により多くのプラスチック粒子が蓄積された可能性が高いという。

　脳のサンプルには、肝臓や腎臓のサンプルと比べて7～30倍の濃度のMNPが含まれていた。脳内で発見された粒子の大半は、梱包材を中心に世界で最も広く使用されているプラスチックのひとつであるポリエチレンだった。

　MNPがほかの臓器よりも脳に多く蓄積されるというのは理にかなっていると、カステール氏は言う。鼻から吸い込まれたMNPは、嗅覚情報を処理する脳内の部位である嗅球（きゅうきゅう）に直接到達できるからだ。

　サンプルが採取された本人の年齢と、臓器に蓄積したプラスチックの量には関連が見られなかったと、カンペン氏は指摘する。これは、体が何らかの方法でプラスチックを排出していることを示唆している。もし排出されていなければ、年齢が高い人の臓器には、年を追うごとにより多くのプラスチックが蓄積されていくはずだ。

　もう一つ注目すべき発見としては、認知症と診断された12人の脳内では、その他の人々の脳と比べてMNP濃度が約3～5倍高かった点が挙げられる。これは、必ずしもMNPが認知症を引き起こしたという意味ではなく、その関連性についてさらなる研究が必要であることが示されただけだと、研究者らは説明している。

　カステール氏はこの関連性について、血液脳関門のフィルター機能が低下する認知症患者での傾向と関係しているのではないかと述べている。つまり、脳内のMNP濃度が高いのは、認知症の原因ではなく、結果である可能性も考えられるというわけだ。

健康への影響と予防策は

　脳内のMNPが健康にどう影響するのかについてはまだ完全にはわかっておらず、有害かどうかをより詳しく理解するにはさらなる研究が必要だと、科学者らは述べている。

　研究ではこれまでに、動脈内のMNPが心血管疾患のリスク要因になり得ること、また、消化管のがん細胞がMNPと接触するとより速く広がる可能性があることが示されている。

「健康への影響についてはまだ十分にわかっていませんが、MNPが脳内に存在していることと、本来はそこに存在すべきでないことは確かです。それだけでも十分に懸念すべきかもしれません」とカステール氏は述べている。

　カンペン氏のチームは、今後は脳全体を調べて、プラスチックがより多く蓄積されている特定の部位があるかどうか、またそれが特定の健康状態と関連しているかどうかを調べたいとしている。

　プラスチックを完全に避ける方法はない。だが、個人の小さな選択がさらされる機会を減らすことにつながると、カステール氏は言う。たとえば、使い捨てのビニール袋をできるだけ使わない、家の換気を心がける、定期的に掃除機をかけてホコリやプラスチック片を取り除く、プラスチックビーズのスクラブ剤のようなMNPを意図的に添加した化粧品を避ける、などが挙げられる。

　研究者らは、環境中のマイクロプラスチックを減らすための解決策を模索している。これまでに、ポリスチレンを食べる昆虫や、環境中のプラスチックを分解する菌類や微生物が見つかっている。また、飲料水からMNPを除去する新しいタイプのフィルターの開発も進められている。

「プラスチックは至るところにあります。大半の人は、プラスチックのない生活を想像することすらできないでしょう。たとえ今すぐプラスチックの生産をやめたとしても、世界にはすでにマイクロプラスチックがあふれています」とカステール氏は言う。

「だからこそ、予防策を講じ、さらされる機会を最小限に抑えて潜在的な健康リスクを防ぐために何ができるかを考えることが重要なのです」

プラスチックの消費が増え続ける中、微小なプラスチック粒子であるマイクロプラスチックが地球上の至るところに集積している。米ニューメキシコ大学健康科学センターで毒物学を専門とするマシュー・カンペン教授らは、深海や空に浮かぶ雲、そして人間の体内にもマイクロプラスチックが存在するとの研究結果を英医学誌ネイチャー・メディシンに発表した。

この研究では、体内に蓄積したマイクロプラスチックは、臓器の中でも特に脳に多く集積していることがわかった。カンペン教授らは、脳のサンプルには腎臓や肝臓の7～30倍ものマイクロプラスチックが含まれていたと明らかにした。

脳に蓄積するマイクロプラスチックの量も急速に増えているようだ。2024年に採取した人間の脳サンプルからは、わずか8年前の16年に採取したサンプルより50％多くのマイクロプラスチックが検出されたのだ。

研究者らは、正常な成人の脳の前頭皮質（眼球の後ろ側にある脳の部分）から採取した組織の中に、計12種類のポリマーを発見した。カンペン教授は米CNNの取材に対し、結果に基づくと、平均的な人間の脳には1グラム当たり4800マイクログラムのマイクロプラスチックが含まれている可能性があり、これはプラスチック製のスプーン1本分に相当すると説明した。

研究に携わった科学者らは、生物組織中のマイクロプラスチックを測定する独自の方法を考案した。この方法は、これまで人間の胎盤や精巣中の物質を検出するために使用されてきた。

研究者らは最初に各組織のサンプルを溶解し、これを遠心分離機にかけ、小さな濃縮ペレットを作成。これを600度まで加熱し、放出されるガスを調べた。さらにプラスチックを多く含むサンプルを顕微鏡で観察し、200ナノメートル未満のマイクロプラスチックの堆積を発見した。それは極めて微小な粒子で、血液脳関門を通過できるほど小さい。しかし、たとえそうだとしても、マイクロプラスチックがどのようにして脳に入り込み、なぜ脳内にこれほど大量に集積しているのかを説明することはできない。これについては、研究に携わった専門家にもわからないという。

マイクロプラスチックは人間にとって有害なのか？

だが、研究者らは、マイクロプラスチックは主に食物、特に肉類を通じて体内に取り込まれると考えており、実際、肉類には分解されたポリマーが高濃度で含まれていることを発見している。カンペン教授は、畑にプラスチックで汚染された水をやることで、土壌にプラスチックが蓄積されるとみている。そこで育てた作物を家畜に与え、その排泄物を肥料として畑に戻しているため、マイクロプラスチックの「生物濃縮（訳注：食物連鎖の過程で、体外に排出されにくい化学物質が体内に蓄積していくこと）」のような状態に陥っているのかもしれないとの見方を示した。

■マイクロプラスチックは人間にとって有害なのか？

体内に蓄積されたマイクロプラスチックは有害なのか、そうだとすれば、どのような害を及ぼすのかはまだわかっていない。だが、研究に参加した専門家らは、マイクロプラスチックが脳の中に蓄積していることに強い危機感を抱いている。

今回の研究では、認知症の脳には正常な脳より多くのマイクロプラスチックが含まれていることも明らかになった。しかし、その脳に蓄積したマイクロプラスチックが認知症を引き起こしたのか、それとも認知症による脳の血流の変化を反映しているのかはわかっていない。

プラスチック自体は不活性物質だと考えられており、人工関節のような医療用途に幅広く利用されている。このため、研究者らは、マイクロプラスチックに含まれる化学物質ではなく、むしろマイクロプラスチックの物理的な存在に危険性があるのではないかと疑っている。

カンペン教授は、マイクロプラスチックが毛細血管の血流を阻害しているのではないかと考え始めていると言う。同教授は「これらのナノ物質が脳の軸索間の結合を阻害する可能性がある。あるいは認知症に影響するタンパク質凝集の種となる可能性もある」としながらも、現時点では解明できていないと述べた。

マイクロプラスチックが人間に害を及ぼすのか、そうであればどのように害を及ぼすのかを特定するためには、さらなる研究が求められる。現段階では、カンペン教授は今回の研究結果が、人間の脳内にかなりの量のプラスチックが蓄積しているという事実の警鐘となることを望んでいる。同教授は「『私の脳には大量のプラスチックが含まれているが、まったく気にならない』と言う人間にはまだ1人も出会っていない」からだと結んだ。

Bioaccumulation of microplastics in decedent human brains

https://www.nature.com/articles/s41591-024-03453-1

Abstract

Rising global concentrations of environmental microplastics and nanoplastics (MNPs) drive concerns for human exposure and health outcomes. Complementary methods for the robust detection of tissue MNPs, including pyrolysis gas chromatography–mass spectrometry, attenuated total reflectance–Fourier transform infrared spectroscopy and electron microscopy with energy-dispersive spectroscopy, confirm the presence of MNPs in human kidney, liver and brain. MNPs in these organs primarily consist of polyethylene, with lesser but significant concentrations of other polymers. Brain tissues harbor higher proportions of polyethylene compared to the composition of the plastics in liver or kidney, and electron microscopy verified the nature of the isolated brain MNPs, which present largely as nanoscale shard-like fragments. Plastic concentrations in these decedent tissues were not influenced by age, sex, race/ethnicity or cause of death; the time of death (2016 versus 2024) was a significant factor, with increasing MNP concentrations over time in both liver and brain samples (P = 0.01). Finally, even greater accumulation of MNPs was observed in a cohort of decedent brains with documented dementia diagnosis, with notable deposition in cerebrovascular walls and immune cells. These results highlight a critical need to better understand the routes of exposure, uptake and clearance pathways and potential health consequences of plastics in human tissues, particularly in the brain.

Main

Environmental concentrations of anthropogenic microplastic and nanoplastic (MNP), polymer-based particulates ranging from 500 µm in diameter down to 1 nm, have increased exponentially over the past half century1,2. The extent to which MNPs cause human harm or toxicity is unclear, although recent studies associated MNP presence in carotid atheromas with increased inflammation and risk of future adverse cardiovascular events3,4. In controlled cell culture and animal exposure studies, MNPs exacerbate disease or drive toxic outcomes, but at concentrations with unclear relevance to human exposures and body burdens5,6. The mantra of the field of toxicology—‘dose makes the poison’ (Paracelsus)—renders such discoveries as easily anticipated; what is not clearly understood is the tissue distribution and internal dose of MNPs in humans, which confounds our ability to interpret the controlled exposure study results.

So far, visual microscopic spectroscopy methods have identified particulates in organs, such as the lungs, intestine7 and placenta8. These methods are often limited to larger (>5 µm) particulates; thus, smaller nanoplastics are unintentionally excluded. As a new approach, pyrolysis gas chromatography–mass spectrometry (Py-GC/MS) has been applied to blood9, placentas10 and recently major blood vessels3,4 in a manner that appears more cumulative, quantitative and less biased when coupled with orthogonal methods. Py-GC/MS data between labs has been comparable, providing confidence in this method for human tissue analysis3,9,10. Here we applied Py-GC/MS in concert with visualization methods to assess the relative distribution of MNPs in major organ systems from human decedent livers, kidneys and brains.

Results and discussion

We obtained de-identified, postmortem human liver (right central parenchyma), kidney (wedge piece containing cortex and medulla) and brain (frontal cortex) samples, retrospectively from 2016 and 2024 autopsy specimens (Supplementary Table 1), in cooperation with and approval from the University of New Mexico (UNM) Office of the Medical Investigator (OMI) in Albuquerque, New Mexico (NM), under the guidance of a trained forensic pathologist (D.F.G.) who selected consistent regions from all organs. Py-GC/MS measurements of MNP concentrations in decedent liver and kidney specimens were similar, with the median value of total plastics at 433 and 404 µg g−1, respectively, from 2024 samples (Fig. 1a and Supplementary Table 1). These were higher than previously published data for human placentas (median = 63.4 µg g−1)10 and testes (median = 299 µg g−1)11. Brain samples, all derived from the frontal cortex, exhibited substantially higher concentrations of MNPs than liver or kidney (two-way analysis of variance (ANOVA), P < 0.0001), but comparable to recently published Py-GC/MS data from carotid plaques4, with a median of 3345 µg g−1 (25–75%: 1,267–5,213 µg g−1) in 2016 samples and 4917 µg g−1 (25–75%: 4,026–5,608 µg g−1) in 2024 samples (Fig. 1a and Supplementary Table 1).

Liver and brain samples from 2024 had significantly higher concentrations of MNPs than 2016 samples on both post hoc multiple comparisons of the two-way ANOVA (Supplementary Tables 4–7 and Supplementary Fig. 6), consistent with results from a multiple regression analysis of brain concentrations considering the potential influence of other demographic variables (Supplementary Tables 8–10). Five brain samples from 2016 (highlighted in orange in Fig. 1a) were analyzed independently by colleagues at Oklahoma State University using Py-GC/MS, and those values were consistent with our findings (P = 0.49 for a Student’s t test comparing UNM and OSU data). The proportion of polyethylene (PE) in the brain (75% on average) was greater relative to other polymers and compared to PE in the liver and kidney (P < 0.0001; Fig. 1b and Extended Data Fig. 1). PE, polypropylene (PP), polyvinyl chloride (PVC) and styrene-butadiene rubber (SBR) concentrations specifically increased from 2016 to 2024 in liver and brain samples (Fig. 1c and Extended Data Fig. 2). PE predominance was confirmed with attenuated total reflectance–Fourier transform infrared spectroscopic analysis from five brain samples, although other polymers were not as consistent in prevalence, possibly due to differences in prevalence across size distributions and limited sampling (Supplementary Tables 9–13 and Supplementary Figs. 17–25).

To expand these findings, we obtained brain tissue from earlier time frames (1997–2013) with a mean age of death comparable to the NM cohorts (52.8 ± 34.3 years) from locations in the eastern United States, along with samples from a repository of dementia cases at UNM. Py-GC/MS analysis revealed lower overall MNP concentrations in East Coast samples (median = 1,254 µg g−1; Supplementary Table 1 and Fig. 1d). While geographical differences cannot be ruled out, we applied a simple linear regression including all normal brain biospecimen data, which revealed significantly increasing trends for total plastics, PE, PP, PVC and SBR (Extended Data Fig. 2). To extend findings to a specific neurological condition, Py-GC/MS was conducted on 12 dementia cases collected in the NM OMI. These cases included Alzheimer’s disease (n = 6), vascular dementia (n = 3) and other dementia (n = 3) specimens from 2019 to 2024. Py-GC/MS analysis revealed total plastics concentrations in dementia samples (median = 26,076 µg g−1; Fig. 1d and Supplementary Table 1) that were higher than in any normal frontal cortex cohort (P < 0.0001 by two-sided t test). Atrophy of brain tissue, impaired blood–brain barrier integrity and poor clearance mechanisms are hallmarks of dementia and would be anticipated to increase MNP concentrations; thus, no causality is assumed from these findings.

Using scanning electron microscopy (SEM) and polarization wave microscopy, refractory inclusions were identified in all organs histologically (Fig. 2, Extended Data Fig. 3 and Supplementary Figs. 7–16). Within the liver, these inclusions were widely dispersed but also notably aggregated within acellular regions consistent with the expected frequency and morphology of lipid droplets, with rod-shaped particles in the 1–5 µm size range (Extended Data Fig. 3a). In the kidney, an elevated presence of refractile inclusions of similar sizes was noted in glomeruli and along tubules (Extended Data Fig. 3a–d). Based on elevated concentrations of polymers identified by Py-GC/MS in these tissues, we suspected that much of the MNPs may be present in the nanoscale range, too small for visualization by light microscopy. Transmission electron microscopy (TEM) was therefore conducted on the dispersed KOH-insoluble pellets obtained from the liver and kidney (Extended Data Fig. 3e,f and Supplementary Fig. 9). While this visualization method cannot provide spectroscopic confirmation of polymer composition, we observed common shapes and sizes across samples and tissue types. Particulates isolated from the pellets and well-dispersed appeared shard-like and were typically less than 0.4 µm in length, consistent with recent findings of nanoplastics in farmed mussels12. SEM with energy-dispersive spectroscopy confirmed that particles observed in livers, kidneys and brains were principally composed of carbon (Extended Data Figs. 4–7). Based on the larger morphology of particulates observed in situ versus those isolated and dispersed from the pellets of digested tissue, we postulate that aggregation of nanoplastics may occur in the liver and kidney.

In brain tissues, larger (1–5 µm) refractile inclusions were not seen, but smaller particulates (<1 µm) were noted in the brain parenchyma (Fig. 2a–c and Supplementary Figs. 10–15). Given the resolution limitations of light microscopy, we examined resuspended brain pellets by TEM, which revealed largely 100–200 nm long shards or flakes (Fig. 2d and Supplementary Figs. 9 and 16). In situ, we confirmed that particles found in the brain were carbon-based by SEM with energy-dispersive X-ray spectrometry (EDS; Extended Data Figs. 6 and 7). In dementia samples, many refractile inclusions were prominent in regions with inflammatory cells and along the vascular wall (Fig. 2e,f). MNP uptake and distribution pathways are poorly understood, and the mechanism of how nanoplastics are delivered to and taken up into the brain is unknown. Insights from Daphnia magna suggest clathrin-dependent endocytosis and macropinocytosis may underlie nanoplastic translocation within the intestine13; we posit a similar uptake may occur in human ingestion of lipids that would also facilitate selective transfer into the brain. While blood was not cleared from the decedent’s organs during autopsy, it is unlikely that the nanoplastics in the brain are selectively contained in the vascular compartment, as the kidneys and livers would also have comparable blood volumes.

While we suspected that MNPs might accumulate in the body over a lifespan, the lack of correlation between total plastics and decedent age (P = 0.87 for brain data) does not support this (Supplementary Fig. 1). However, total mass concentration of plastics in the brains analyzed in this study increased by approximately 50% in the past 8 years. Thus, we postulate that the exponentially increasing environmental concentrations of MNPs2,14 may analogously increase internal maximal concentrations. Although there are few studies to draw on yet performed in mammals, in zebrafish exposed to constant concentrations, nanoplastic uptake increased to a stable plateau and cleared after exposure15; however, the maximal internal concentrations were increased proportionately with higher nanoplastic exposure concentrations. While clearance rates and elimination routes of MNPs from the brain remain uncharacterized, it is possible that an equilibrium—albeit variable between people—might occur between exposure, uptake and clearance, with environmental exposure concentrations ultimately determining the internal body burden.

Although the current data derive from multiple tissue banks and two analytic sites replicating key results, the new analytical Py-GC/MS methods applied here are yet to be widely adopted and refined into standardized tests for clinical specimens. Both analytical laboratories (UNM and OSU) observed a ~25% within-sample coefficient of variation, which does not alter the conclusions regarding temporal trends or accumulation in brains relative to other tissues, given the magnitude of those effects. Numerous quality control steps ensure that external contaminants are not impacting the results, including Py-GC/MS assessment of KOH and formalin storage control sample ‘blanks’ and measurements of the polymer composition of all plastic tubes and pipette tips that are essential in the digestion and measurement process (Supplementary Figs. 2–4). Decedent specimen collections over the past 30 years were not focused on minimizing external plastic contamination. However, given the consistent nature of handling and processing across all organ samples within objectively clean clinical and forensic settings, the significant accumulation of MNPs in the brain cannot be dismissed as an artifact of contamination. Furthermore, the 2016 samples were stored for 84–96 months compared to only 2–4 months for the 2024 samples, which exhibited greater concentrations of polymer. Thus, contamination from plastic storage vessels should not influence the conclusions. For the brain, especially, greater attention to anatomical features, such as white versus gray matter, vascularization and glia content, should be carefully evaluated in future studies to reduce variation. Finally, by obtaining only a single sample from each organ for each participant, distribution heterogeneity within tissues remains uncharacterized.

Our estimates of polymer mass concentration could be impacted by several factors that may lead to overestimation or underestimation. The KOH digestion extensively eliminated biological material from the pellets through saponification of triglycerides and denaturing of proteins (Supplementary Fig. 5). However, the final pellets still contained unknown residual biomatrix, which could present challenges for mass spectral interference. KOH reduced the liver and kidney mass by 99.4%, while the brain samples were reduced by 91.8%, that is, the resultant average pellet mass derived from 500 mg of starting material was approximately 3 mg and 41 mg, respectively. This discrepancy is proportional to, and consistent with, the mass of the polymer measured. However, unknown organic molecules likely remain and influence the resultant Py-GC/MS spectra. Lipids have been noted as a potential source of interference in Py-GC/MS analysis of PE16. Our method of KOH digestion and physical separation of solids was designed to reduce this concern, rather than augment it with a liquid–liquid extraction in organic solvents that would selectively drive lipid partitioning. Furthermore, the spectra suggest a reduction of longer carbon chains in the pyrolysis chromatogram, which is potentially due to advanced oxidative degradation of the MNPs and excess carbonyl formation that may lead to an underestimation of the concentration, as our standards are created with pristine polymers17,18. Finally, given the observed small size of nanoscale particles isolated from the human specimens (typically <200 nm in length), it is likely that ultracentrifugation incompletely collected nanoplastics in the analytical samples, also contributing to potential underestimation. The shape and size of observed nanoparticles in the isolated material from human specimens taxes the limits of modern analytical instrumentation but may reflect an end-stage product of plastic degradation that is uniquely suited for human uptake and accumulation.

Conclusions

The present data suggest a trend of increasing MNP concentrations in the brain and liver. The majority of MNPs found in tissues consist of PE and appear to be nanoplastic shards or flakes. MNP concentrations in normal decedent brain samples were 7–30 times greater than the concentrations seen in livers or kidneys, and brain samples from dementia cases exhibited even greater MNP presence. These data are associative and do not establish a causal role for such particles affecting health. For this, refinements to the analytical techniques, more complex study designs and much larger cohorts are needed. Given the exponentially rising environmental presence of MNPs19,20,21, these data compel a much larger effort to understand whether MNPs have a role in neurological disorders or other human health effects.

JAMA network

September 16, 2024

Microplastics in the Olfactory Bulb of the Human Brain

Luís Fernando Amato-Lourenço, PhD1,2; Katia Cristina Dantas, PhD2; Gabriel Ribeiro Júnior, PhD2; et al

https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2823787

JAMA Netw Open. 2024;7(9):e2440018. doi:10.1001/jamanetworkopen.2024.40018

Question  

Can microplastics reach the olfactory bulb in the human brain?

Findings  

This case series analyzed the olfactory bulbs of 15 deceased individuals via micro-Fourier transform infrared spectroscopy and detected the presence of microplastics in the olfactory bulbs of 8 individuals. The predominant shapes were particles and fibers, with polypropylene being the most common polymer.

Meaning  

The presence of microplastics in the human olfactory bulb suggests the olfactory pathway as a potential entry route for microplastics into the brain, highlighting the need for further research on their neurotoxic effects and implications for human health.

Abstract

Importance  

Microplastic (MP) pollution is an emerging environmental and health concern. While MPs have been detected in various human tissues, their presence in the human brain has not been documented, raising important questions about potential neurotoxic effects and the mechanisms by which MPs might reach brain tissues.

Objective  

To determine the presence of MPs in the human olfactory bulb and to analyze their characteristics such as size, morphology, color, and polymeric composition.

Design, Setting, and Participants  

This case series study used a cross-sectional design involving the analysis of olfactory bulb tissues obtained from deceased individuals during routine coroner autopsies. The sampling procedures were conducted at São Paulo City Death Verification Service, with laboratory analysis carried out at the Brazilian Synchrotron Light Laboratory (LNLS). Participants included 15 adult individuals who had been residents of São Paulo for more than 5 years and underwent coroner autopsies. Exclusion criteria included previous neurosurgical interventions. Data analysis was performed in April 2024.

Exposure  

The primary exposure assessed was the presence of MPs in the olfactory bulb, analyzed through direct tissue examination and digested tissue filtration followed by micro-Fourier transform infrared spectroscopy.

Main Outcomes and Measures  

The main outcomes were the identification and characterization of MPs within the olfactory bulb, including their size, morphology, color, and polymeric composition.

Results  

The median age of the 15 deceased individuals was 69.5 years, ranging from 33 to 100 years, with 12 males and 3 females. MPs were detected in the olfactory bulbs of 8 out of 15 individuals. A total of 16 synthetic polymer particles and fibers were identified, with 75% being particles and 25% being fibers. The most common polymer detected was polypropylene (43.8%). Sizes of MPs ranged from 5.5 μm to 26.4 μm for particles, and the mean fiber length was 21.4 μm. Polymeric materials were absent in procedural blank and negative control filters, indicating minimal contamination risk.

Conclusions and Relevance  

This case series provides evidence of MPs found in the human olfactory bulb, suggesting a potential pathway for the translocation of MPs to the brain. The findings underscore the need for further research on the health implications of MP exposure, particularly concerning neurotoxicity and the potential for MPs to bypass the blood-brain barrier.

Introduction

The ubiquity of microplastic (MP) pollution has become a pervasive environmental concern,1 raising questions about its occurrence within the human body and its harmful effects.2 While MPs have been detected in various organs of the human body, such as the lungs,3,4 large and small intestines,5 liver,6 placenta,7,8 semen,9 and bloodstream,10 to our knowledge, there have been no published studies to date reporting their presence in the human brain.

The presence of the blood-brain barrier (BBB) is likely an important limiting factor for the access of MPs to the human brain via hematogenous translocation. Despite this, some animal studies have shown that MPs can impair the BBB and reach the brain via oral ingestion, leading to neurotoxic effects.11-13 Another potential entry site for micro- and nanoplastics (MNPs) in the human brain is the olfactory pathway.14 This pathway involves olfactory neurons in the nasal that transmit information about odors to the central olfactory system of the brain. Olfactory axons pass through the cribriform plate (CP) of the ethmoid bone and reach the olfactory bulbs (OB), which are connected to the limbic system of the brain.

There are different levels of evidence suggesting that the olfactory pathway might allow the translocation of exogenous particles to the brain. Environmental black carbon particles have been detected in various human brain regions, with one of the highest concentrations found in the OB, measuring 420.8 particles/mm3.15 Rarely, the 15- to 30-μm–sized ameboid form of Naegleria fowleri penetrates the brain via the nose, causing amebic meningoencephalitis.16 Affected individuals typically present with the disease after contact with contaminated freshwater bodies or after rinsing the nose with nonsterile tap water.17 Furthermore, the permeability of this barrier has been evoked as a possible quicker and safer drug delivery route to the brain,18,19 as well as access to cerebrospinal fluid through nasal lymphatic vessels.20

In this study, given the ubiquitous presence of MPs in the air21 and their previous identification in the human nasal cavity,22,23 we hypothesized that the smallest-size fraction of MPs could reach the OB. Therefore, we conducted an investigation into the presence of MPs within human OB obtained from 15 deceased individuals during coroner autopsies. We identified and analyzed various characteristics of the MPs, including their size, morphology, color, and polymeric composition.

Methods

This case series study was approved by the ethical board of the São Paulo University Medical School, in compliance with the Helsinki Declaration. Written informed consent was provided by the deceased individuals’ next of kin. The study was conducted from February 2023 to May 2024 and followed the Reporting Guideline for Case Series.46

Study Population

We obtained the bilateral OBs from 15 adult individuals who underwent routine coroner autopsies at the São Paulo City Death Verification Service of University of São Paulo to determine the cause of death. All individuals had been residents of São Paulo for more than 5 years. Cases in which the deceased had previously undergone neurosurgical interventions were not selected for the study. Information regarding previous occupations and underlying diseases was obtained through questionnaires administered to the next of kin. Additionally, autopsy reports were reviewed. We also collected samples from the OB of 2 stillbirths at 7 months gestation, as a negative control for the study. The collection of OBs took place between February 2023 and February 2024.

Quality Control and Quality Assurance and Evaluation of Sample Processing

We implemented a plastic-free approach to safeguard the integrity of our results. This strategy facilitated a thorough assessment of potential sources of variability and error, thereby enhancing the reliability of our collected data. All procedures, from the OB sampling to the micro-Fourier transform infrared (μFTIR) spectroscopy analysis, followed the protocols recommended by several studies.24-26 Briefly, all solutions were prefiltered through a Whatman cellulose filters with a mesh size of 0.45 μm. Stainless steel materials, glassware, and samples were covered with aluminum foil (before and after processing) to avoid airborne sample contamination. Ultrapure water with a resistivity of 18.2 mΩ was obtained from a Milli-Q purification device (Millipore Corp). Glass and stainless-steel materials were washed thoroughly using the purified water 3 times and then using acetone P.A. to remove any particles or fibers that have adhered to the glass. The scientific staff responsible for handling samples wore exclusively 100% cotton laboratory coats and were required to remove any plastic or textile bracelets, rings, and watches to minimize the risk of sample contamination. Clean latex gloves were used for all procedures. The samples were processed in a clean laminar flow cabinet (ISO class 5, SKU330313, Hipperquímica, SP, Brazil). Blank filters (47 mm) were used from the OB collection to the sample filtering to assess possible airborne contamination. A clean filter was also used as a negative control. Access to the μFTIR spectroscopy and the digestion/filtration room was restricted to the operators only, to avoid air flow in the room and the suspension/resuspension of possible atmospheric contaminants.

Sample Processing

The presence of MPs in the OB was assessed in 2 ways: directly on the tissue and a digested assessment. The cryo-cuts method preserves the spatial context of MPs within the tissue, allowing their proximity to anatomical structures such as blood vessels to be observed. This is crucial for understanding potential pathways of MPs translocation and accumulation within the OB. The digestion method ensures that MPs that are deeply embedded in the tissue are not overlooked. Postdigestion, MPs are concentrated on filters, which can then be analyzed for a more accurate quantification and identification without interference from the tissue matrix. By combining these 2 methods, the study maximized the probability of detecting and characterizing MPs within the OB.

OB Cryo-Cuts

The left OB of each case was horizontally cryo-sectioned using a Leica CM1860 UV cryostat (Leica Biosystems) at 10 μm thickness and thaw-mounted onto 5 mm × 5 mm gold/chromium-coated silicon dioxide/silicon substrates. No fixatives were used for the tissue sections. The samples were then freeze-dried for 48 hours (Freezone 6 [Labconco Corp]) and examined by optical microscopy (Eclipse LV100ND [Nikon Instruments Inc]). The freeze-drying process maintains the integrity of biological tissues by extracting water without substantially compromising their structure. Futhermore, the presence of water molecules, characterized by strong hydrogen bonding, poses a considerable challenge in FTIR measurements, as they mask specific signals indicative of chemical compositions.27

The procedures took place in a biosafety level 2 room in the Cryogenic Preparations Laboratory (LCRIO) at the Brazilian Synchrotron Light Laboratory (LNLS), National Center for Energy and Materials Research (CNPEM).

Sample Digestion and Filtering

Immediately after sampling, the right OBs from 10 selected cases were individually frozen at −20 °C in glass vials, covered with aluminum foil, and sealed with a glass lid until the digestion. For 5 patients, there was no available tissue for digestion. The tissues were then incubated for 12 hours at 40 °C using the enzyme mixture Corolase 7089 (20 UHb/mL)4 inside the laminar flux hood.

The solution was then filtered using a glass vacuum filtration system (Sigma-Aldrich) and silver membrane filters (25 mm in diameter and 0.45 microns pore size [Millipore]). Subsequently, the filters were kept individually in closed Petri dishes inside a glass dissector until the spectroscopy analysis. Due to the material characteristics, a recovery test was not feasible.

Micro-Fourier Transform Infrared Spectroscopy

We performed single-point μFTIR microspectroscopy measurements in reflection mode using a diffraction-limited IR microscope (Cary 620 [Agilent Technologies]). The IR microscope is coupled to a Michelson interferometer responsible for the frequency demultiplexing of the mid-IR broadband response. We used a 1000 K Globar source and illumination and interferograms detection was done by using a high-sensitivity cryo-cooled Mercury–Cadmium-Telluride (MCT [Infrared Associates Inc]). After the interferometer, the IR beam was directed to a 25 × objective that produced an illumination spot of 420 μm × 420 μm on the sample’s surface. This field of view was further reduced to 50 to 100 μm by slits to concentrate the analysis around specific particles. The reflected light was collected through a confocal arrangement by the same objective lens and then directed to the MCT detector. FTIR spectra were generated by calculating the Fourier transforms of the recorded interferograms. The spectral resolution was configured at 16 cm−1, encompassing the range from 4000 to 700 cm−1. Each μFTIR spectrum was normalized to the spectrum of a clean gold surface, which served as a reference background. The cryo-cuts and digested filters were fully analyzed. The μFTIR analyses took place in the IMBUIA beamline at the Brazilian Synchrotron Light Laboratory (LNLS), National Center for Research in Energy and Materials (CNPEM).

The acquired spectra were processed manually using the KnowItAll Informatics System 2024 (John Wiley and Sons Inc). The comparative analysis was performed with the help of FTIR spectra libraries developed for MPs research, including the FTIR Library of Plastic Particles (FLOPP),28 FTIR Library of Plastic Particles Sourced from the Environment (FLOPP-e),28 siMPLe database,29 and KnowItAll IR Spectral Library. We adopted a Hit Quality Index greater than 75% of agreement between characteristic bands of polymers observed in reference materials with bands observed in unknown particles or fibers.30,31

Microphotograph Analysis

We determined particle sizes by analyzing microphotographs obtained through μFTIR spectroscopy. ImageJ 1.54g software (US National Institutes of Health) was used for accurate measurements.

Statistical Analysis

Descriptive analyses were performed using SPSS Statistics 26.0 software (IBM Inc). These analyses were performed in April 2024.

Results

The median (range) age of the 15 deceased individuals was 69.5 (33-100) years. They included 12 males and 3 females. Demographic information is detailed in Table 1. Apart from the 2 cases with histological evidence of previous ischemic cerebral infarction and 1 case with a subarachnoid hematoma due to a ruptured aneurysm of the middle cerebral artery, there were no cerebral histological abnormalities in the remaining cases. The mean (SD) mass of the OB (left or right) was 0.187 (0.050) g, ranging from 0.100 to 0.273 g.

A total of 16 synthetic polymer particles and fibers were identified in 8 out of the 15 deceased individuals, with a range from 1 to 4 MPs per OB. Of these, 75% were particles, of which 83.4% were fragments and 16.6% were spheres, while 25% were fibers with a length-to-width ratio exceeding 3. The particles had a mean (SD) length of 12.1 (7.2) μm, ranging from 5.5 to 26.4 μm, and a mean (SD) width of 8.9 (6.4) μm, ranging from 3.0 to 25.4 μm. The fibers exhibited a mean (SD) length of 21.4 (2.6) μm, ranging from 19.0 to 24.5 μm, and a mean (SD) width of 3.8 (1.8) μm, ranging from 3.0 to 6.0 μm.

In the procedural blank filters, we detected 2 cotton fibers, 2 silica beads, and 1 silicate fragment. Polymeric materials were absent in both the procedural blank and negative control filters. From the 2 collected samples in stillborn, we were able to analyze 1 case, which did not show the presence of MPs. The other case had insufficient material for analysis.

Polypropylene was the most prevalent polymer (43.8%), followed by polyamide, nylon, and polyethylene vinyl acetate (12.5%). This was followed by polyethylene, perlon polyamide, and wool-polypropylene, which accounted for 6.3%). Upon comparison with the reference spectral library of plastic materials, the identified MP particles and fibers exhibited indications of weathering. The μFTIR spectra of the weathered MPs differed substantially from those of pristine standard samples; multiple peaks in the spectra of weathered MPs were attenuated or entirely absent.

Microphotographs and μFTIR point-spectra showing the main types of MP detected in the OB are shown in Figure 1 and Figure 2. The complete μFTIR point-spectra results of the digested OB are presented in the eFigure in Supplement 1. Table 2 provides details regarding the morphology, color, and chemical characterization of the particles and fibers.

Discussion

To our knowledge, this is the first study in which the presence of MPs in the human brain was identified and characterized using μFTIR, allowing quantification and characterization of the morphology and polymeric matrix. Specifically, we detected particles as the predominant shape in the OB in 8 out of 15 individuals who underwent autopsy in Sao Paulo. Our data extend the notion that not only black carbon15 but also MP accumulate in the OB in humans.

We believe that the anatomy of the cribriform plate of the ethmoid bone may serve as a gateway in the nasal passages from within the skull. This plate, situated between the frontal and sphenoid bones, lies horizontally and contains multiple foramina, each less than 1 mm in diameter.32 The OB lies directly above it, and the olfactory neurons of the nasal mucosa reach the OB via the foramina of the cribriform plate. Recent studies have shown that part of the cerebrospinal fluid outflow occurs via lymphatic vessels that surround the olfactory axons, reaching the nasal mucosa and extending toward the nasal lymphoid tissue.33 Ossification of the CP occurs by 1 year of age,34 and the total area of the perforations is age-dependent; it is 3.79 to 3.99 mm2 in those over 50 years of age and 5.61 to 7.91 mm2 in those under 50 years of age. This decrease in the area over time, causing compression and dysfunction of the olfactory nerves, is thought to explain the decreased olfactory sensation in older individuals.35 Furthermore, in mice, paracellular spaces in the olfactory epithelium can reach 5 to 20 μm in the medial-lateral dimension of the transport and a 10- to 100-μm range observed in the rostral-caudal dimension.36 If a similar situation is observed in humans, this could represent another factor facilitating entry of larger particles in the brain via the cribriform plate.

Given the widespread presence of MPs in the air, some of which are associated with PM2.5,37 the identification of MPs in the nose45 and now in the OB, along with the vulnerable anatomical pathways, reinforces the notion that the olfactory pathway is an important entry site for exogenous particles to the brain. In previous epidemiological studies, exposure to PM2.5 has been associated with neurological and psychiatric adverse outcomes, such as dementia.38,39 Some neurodegenerative diseases, such as Parkinson disease, seem to have a connection with nasal abnormalities as initial symptoms.40 In experimental studies, both exposures to PM2.5 and MPs have shown to cause several neurotoxic effects, including disturbances on the brain development.41,42 The cribriform plate reaches maturation at 1 to 2 years of age, which is a critical time window during which MP penetration into the brain could have negative effects on the organ maturation.

In this study, the MP polymeric matrix found in the OB corresponds to the most produced and manufactured plastics, such as polypropylene, nylon/polyamide, polyethylene and polyethylene vinyl acetate, present in packaging, clothes and home accessories, suggesting indoor environments as a major source of inhaled MPs.21,43

Limitations

This study has certain limitations. Although the olfactory pathway seems a likely exposure route, we cannot dismiss the possibility of multiple entry routes. MPs might have reached the OB either through systemic circulation, crossing the BBB, or via the respiratory pathway through the trigeminal nerve.44 The biologic matrix of the OB tissues can be a confounding factor when analyzing MP spectra due to its similarity to some polymeric materials. Therefore, we were cautious to consider suspect particles as polymeric material only when spectral bands highly matched with weathered bands from MP libraries (HQI >75%). In the filtered samples, the biological matrix was previously digested, not being an issue. Given the maximum spatial resolution (3 μm) of μFTIR spectroscopy setup and the limited capacity of analysis for other techniques, we were unable to detect nanoplastics. It is likely that the number of plastics in the submicron range with the potential to cause substantial biological damage would be far more numerous.

Avoiding contamination is one of the biggest challenges when analyzing MP. Due to the presence of MP fibers and particles in the air, we have used blank samples in all methodological procedures to detect contamination of the air. We found no MP in our procedural blanks, which supports the validity of our results. Furthermore, we had the opportunity to analyze the brains of 2 stillbirths. However, the status of brain tissue maceration made the analysis challenging due to difficulties in sampling and processing.

Conclusions

This case series describes the presence of MPs in the OB, mainly particles of the most commonly produced/processed polymers for clothing and packaging such as polypropylene and nylon. Our data support the idea that the olfactory pathway is an important entry site for environmental air pollutants. Considering the potential neurotoxic effects caused by MPs in the brain, and the widespread environmental contamination with plastics, our results should raise concern in the context of increasing prevalence of neurodegenerative diseases. Noninvasive imaging technologies, such as magnetic resonance imaging, are needed to overcome the current limitations in tissue analysis of different human organs and to improve the understanding of the health hazards of MPs.