Tiny air pollution particles have been revealed in the brain stems of young people and are intimately associated with molecular damage linked to Alzheimer’s and Parkinson’s disease.
If the groundbreaking discovery is confirmed by future research, it would have worldwide implications because 90% of the global population live with unsafe air. Medical experts are cautious about the findings and said that while the nanoparticles are a likely cause of the damage, whether this leads to disease later in life remains to be seen.
There is already good statistical evidence that higher exposure to air pollution increases rates of neurodegenerative diseases, but the significance of the new study is that it shows a possible physical mechanism by which the damage is done.
The researchers found abundant pollution nanoparticles in the brainstems of 186 young people from Mexico City who had died suddenly between the ages of 11 months and 27 years. They are likely to have reached the brain after being inhaled into the bloodstream, or via the nose or gut.
The nanoparticles were closely associated with abnormal proteins that are hallmarks of Alzheimer’s, Parkinson’s and motor neurone disease. The aberrant proteins were not seen in the brains of age-matched people from less polluted areas, they said.
“It is terrifying because, even in the infants, there is neuropathology in the brain stem,” said Prof Barbara Maher, at Lancaster University, UK, and part of the research team. “We can’t prove causality so far, but how could you expect these nanoparticles containing those metal species to sit inert and harmless inside critical cells of the brain? That’s the smoking gun – it seriously looks as if those nanoparticles are firing the bullets that are causing the observed neurodegenerative damage.”
Maher said the work provides hypotheses that could now be tested. For example, brain stem damage would affect the movement control and gait of young people and this should correlate with pollution exposure if the nanoparticles are the cause.
The causes of neurodegenerative disease are complex and not fully understood. “There’s definitely going to be genetic factors and there’s highly likely to be other neurotoxicants,” said Maher. “But the thing that’s special about air pollution is how pervasively people are exposed to it. I don’t think that human systems have developed any defence mechanisms to protect themselves from nanoparticles.”
She said it was important to study children as they have not experienced other factors associated with dementia such as alcohol consumption: “So they become the canaries in the coalmine.”
The research was led by Lilian Calderon-Garciduena at the University of Montana, US, and is published in the journal Environmental Research. It found the metal-rich nanoparticles matched the shape and chemical composition of those produced by traffic, through combustion and braking friction, and which are abundant in the air of Mexico City and many other cities.
Prof Louise Serpell, at the University of Sussex, UK, said the nanoparticles were a plausible cause of the brain damage, but that there was not enough evidence that nanoparticles could cause the neurodegenerative diseases: “There are many other likely causes for neurodegenerative diseases.” But she said: “Our environmental exposure to pollution and pathogens is probably very important in triggering disease.”
Jordi Sunyer, doctor in medicine and surgery at the University of Barcelona, said animal experiments had shown that inhaled nanoparticles could reach the brain and cause damage, but he said inflammatory chemicals triggered by air pollution in the lung could also reach the brain.
The research found the nanoparticles in the substantia nigra, a key brain area in Parkinson’s disease. David Dexter, associate director of research at Parkinson’s UK, said: “We still don’t fully understand what causes Parkinson’s, but this study builds on research that has linked poor air quality and neurodegeneration, as well as links with metal toxicity. Parkinson’s is the fastest growing neurological condition in the world, so [the role of the environment] is a really important area within global research.”
But he said: “The pathology in this study is quite distinct and not something we have seen in our brain bank from typical Parkinson’s cases.” Maher said this might be because the levels of air pollution varies between cities.
Dr Susan Kohlhaas, director of research at Alzheimer’s Research UK, said: “Air pollution is linked to many adverse health conditions and a growing body of evidence suggests this includes our risk of developing dementia. Proteins do build up in the brain years before we see visible dementia symptoms, but more research is needed before we can suggest air pollution drives brain changes associated with disease in children.”
She said it was critical that action is taken, in particular measuring the number of nanoparticles to which people are exposed. Usually only the overall weight of particles smaller than 2.5 microns is measured.
“If you measure it, and you understand where the problem is greatest, then you can start to do something,” she said. “Policymakers must take account of these findings, and actually begin to work out how we can reduce as much of this exposure to air pollution as possible.”