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空污有毒微粒入侵人腦 恐與阿茲海默症有關
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本報2016年9月9日綜合外電報導，姜唯編譯；蔡麗伶審校
英國新研究指出，科學家在人的腦組織中發現大量有毒空污微粒，引發關注。而另一份近期研究才顯示，阿茲海默症與腦組織中磁鐵物質有所關連；同時，空污顯著提高了罹患阿茲海默症的風險。不過，要推論空污導致或惡化阿茲海默症仍言之過早。
新研究指出，科學家在人腦組織中發現大量有毒空污微粒，並間接推論空氣污染有可能為阿茲海默症的環境因子。圖片來源：Ben Amstutz（CC BY-NC 2.0）
研究主持人、蘭開斯特大學教授梅爾（Barbara Maher）說，研究結果顯示空氣污染很可能是阿茲海默症的環境因子，接下來應該進行流行病學和毒性檢測，因為這些粒子充斥空氣中，人們每天都會接觸到。
這份發表於國家科學院院刊的新研究，檢視英國曼徹斯特和墨西哥37位年齡介於3歲至92歲的人類腦組織，發現其中含有大量磁鐵與氧化鐵粒子，每克冷凍乾燥腦組織中含有高達數百萬個磁鐵顆粒。
梅爾說，磁鐵對腦組織來說是劇毒，會產生自由基，「氧化細胞損傷是阿茲海默症的代表性徵狀之一，因此大腦中有磁鐵粒子存在是非常驚人的事，它的生物活性太高了。」
阿茲海默症的特色之一是大腦中金屬物質異常累積，而近期研究顯示，磁鐵與阿茲海默症患者大腦的損傷直接相關。人腦會自行合成磁鐵，但是量很少而且呈晶體狀，並非新研究觀察到的較大的圓形顆粒。
梅爾說明，「我們也在腦組織中觀察到其他金屬粒子，包括鉑、鈷和鎳。鉑之類的物質不太可能是腦部自行合成的，比較可能來自（汽車）觸媒轉化器。」
其他科學家表示，雖然新研究的腦組織樣本中大部分的磁鐵粒子來自空氣污染，但是與阿茲海默症之間的連結仍有待更深入的研究。
未參與研究的佛羅里達大學教授多布森（Jon Dobson）說：「我不懂的是，這份研究發現的磁鐵粒子含量比過去在人腦中發現的要多，需要進一步研究確認是因為腦組織的位置不同、樣本來源居住在工業區，還是實驗室污染所致。」研究人員則表示已盡全力避免實驗室內污染。
一份2015年的研究顯示空氣污染和阿茲海默症的風險之間呈正相關，其他研究顯示接觸空氣污染的兒童和青少年腦部出現了與阿茲海默症相關的損傷。而空氣污染也被證實和老年痴呆有關。
Toxic air pollution particles found in human brains
Detection of ‘abundant’ magnetite particles raises concerns because of suggested links to Alzheimer’s disease
Toxic nanoparticles from air pollution have been discovered in human brains in “abundant” quantities, a newly published study reveals.
The detection of the particles, in brain tissue from 37 people, raises concerns because recent research has suggested links between these magnetite particles and Alzheimer’s disease, while air pollution has been shown to significantly increase the risk of the disease. However, the new work is still a long way from proving that the air pollution particles cause or exacerbate Alzheimer’s.
“This is a discovery finding, and now what should start is a whole new examination of this as a potentially very important environmental risk factor for Alzheimer’s disease,” said Prof Barbara Maher, at Lancaster University, who led the new research. “Now there is a reason to go on and do the epidemiology and the toxicity testing, because these particles are so prolific and people are exposed to them.”
Air pollution is a global health crisis that kills more people than malaria and HIV/Aids combined and it has long been linked to lung and heart disease and strokes. But research is uncovering new impacts on health, including degenerative brain diseases such as Alzheimer’s, mental illness and reduced intelligence.
The new work, published in the Proceedings of the National Academy of Sciences, examined brain tissue from 37 people in Manchester, in the UK, and Mexico, aged between three and 92.
It found abundant particles of magnetite, an iron oxide. “You are talking about millions of magnetite particles per gram of freeze-dried brain tissue - it is extraordinary,” said Maher.
“Magnetite in the brain is not something you want to have because it is particularly toxic there,” she said, explaining that the substance can create reactive oxygen species called free radicals. “Oxidative cell damage is one of the hallmark features of Alzheimer’s disease, and this is why the presence of magnetite is so potentially significant, because it is so bioreactive.”
Abnormal accumulation of brain metals is a key feature of Alzheimer’s disease and a recent study showed that magnetite was directly associated with the damage seen in Alzheimer’s brains. Magnetite particles are known to form biologically in human brains, but these are small and crystal-shaped, unlike the larger, spherical particles that dominated the samples in the new study.
“Many of the magnetite particles we have found in the brain are very distinctive,” said Maher. “They are very rounded nanospheres, because they were formed as molten droplets of material from combustion sources, such as car exhausts, industrial processes and power stations, anywhere you are burning fuel.”
“They are abundant,” she said. “For every one of [the crystal shaped particles] we saw about 100 of the pollution particles. The thing about magnetite is it is everywhere.” An analysis of roadside air in Lancaster found 200m magnetite particles per cubic metre.
Furthermore, said Maher: “We also observed other metal-bearing particles in the brain, such as platinum, cobalt and nickel. Things like platinum are very unlikely to come from a source within the brain. It is a bit of an indicator of a [vehicle] catalytic converter source.”
Other scientists told the Guardian the new work provided strong evidence that most of the magnetite in the brain samples come from air pollution but that the link to Alzheimer’s disease remained speculative.
“This is a very intriguing finding and it raises a lot of important questions,” saidProf Jon Dobson, at the University of Florida and not part of the research team. But he said further investigation was needed: “One thing that puzzles me is that the [particle] concentrations are somewhat higher than those previously reported for the human brain. Further studies [are needed] to determine whether this due to regional variations within the brain, the fact that these samples are from subjects who lived in industrial areas, or whether it is possibly due to [lab] contamination.” The researchers said they had gone to great lengths to avoid contamination.
Air pollution was linked to a significant increase in the risk of Alzheimer’s disease by a major study published in 2015, while other research showedbrain damage related to Alzheimer’s disease in children and young adults exposed to air pollution. Air pollution has also been linked to dementia in older men and women.
“We have not demonstrated a causal link between these particles and Alzheimer’s disease but when you consider that magnetite has been found in higher concentrations in Alzheimer’s brains and you know that magnetite is pernicious in its effect on the brain, then having a direct [air pollution] source of magnetite right up your olfactory bulb and into your frontal cortex is not a great idea,” said Maher.
Prof David Allsop, an Alzheimer’s disease expert at Lancaster University and part of the research team, said: “There is no blood-brain barrier with nasal delivery. Once nanoparticles directly enter olfactory areas of the brain through the nose, they can spread to other areas of the brain, including hippocampus and cerebral cortex – regions affected in Alzheimer’s disease.” He said it was worth noting that an impaired sense of smell is an early indicator of Alzheimer’s disease.
“Knowledge is power,” Maher said. “So if there’s at least a possibility that exposure to traffic pollution is having even worse health impacts than were previously known, then take the steps you can to reduce your dose as far as you can.”
“What this is pointing towards perhaps is there needs to be a major shift in policy and an attempt to reduce the particulate matter burden on human health.” Maher said. “The more you realise the impact this is having, the more urgent and important it is to reduce the concentrations in the atmosphere.”
Dr Clare Walton, research communications manager at the Alzheimer’s Society, said: “This study offers convincing evidence that magnetite from air pollution can get into the brain, but it doesn’t tell us what effect this has on brain health or conditions such as Alzheimer’s disease. Further work in this area is important, but until we have more information people should not be unduly worried. There are more practical ways to lower your chances of developing dementia such as regular exercise, eating a healthy diet and avoiding smoking.”
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