In the early 1990s, biomedical researchers began to notice traces of magnetite — a naturally occurring ferromagnetic mineral and oxide of iron — inside people’s brains. That might sound strange, but there has long been evidence that some amount of magnetite might form naturally in brain tissue. Still, not all magnetite is created the same, it would seem, and a study published last week in the Proceedings of the National Academy of Sciences suggests another possible origin: air pollution.
Barbara Maher, co-director of the Centre for Environmental Magnetism and Paleomagnetism at Britain’s Lancaster University and the lead author on the study, noted that like many kinds of air pollution, magnetite is everywhere. But, she added, it’s particularly concentrated in the emissions of car engines and power plants. “Humans are particularly able to make magnetite through combustion processes,” said Maher, whose research suggested that the roadside air in Lancaster had over 201 million particles of magnetite per cubic meter.
In many cases, Maher said, airborne particulate matter like this can be large enough to get trapped in the body’s olfactory bulb — which transmits information about scents to the brain. But according to the new paper, some smaller forms of pollution can get through — including some magnetite particles. The median diameter of the particles examined was 18 nanometers. In comparison, a human hair is around 60,000 nanometers thick. When particles that small are inhaled, they slip right through the olfactory bulb like a minnow through a salmon net, and the magnetite makes it to the brain — or at least that’s what Maher and her team theorize.
In the study, the researchers examined magnetite particles in the 37 brains of deceased people from Mexico City and Manchester. In the samples, certain magnetite particles were particularly abundant — and very different in size and shape from what was presumed to have occurred biogenically. Indeed, the spherical, regularly dimpled shape of the suspect particles, Maher noted, were very much what one would expect from molten magnetite after it cools — and quite like particles collected from the air along a Lancaster road, and from Didcot Power Station in southern England.
Just what the presence of pollution-based magnetite particles in people’s brains might mean is difficult to say, but there are some points of concern. Principal among these: Magnetite particles have been found in especially high concentrations in and around the sticky, beta-amyloid plaques that are often found in the brains of Alzheimer’s sufferers (some of Maher’s samples showed signs of the disease). Other recent research has also suggested a link between Alzheimer’s and abnormal accumulations of brain metals.
“I think it is a very, very interesting paper,” said Jon Dobson, a professor at the University of Florida who researches biomedical applications of magnetic particles and iron’s role in neurodegenerative diseases. “I think it is intriguing, but I think it probably opens up a lot of questions. If it does turn out that there is an external, environmental source of metal and metal oxide particulates in the brain, then this could be huge in terms of health implications.”
Unlike most forms of iron, which our brains can store safely, magnetite forms free radicals, Dobson noted. These unstable chemicals can rip the electrons off of the atoms around them and damage surrounding tissue.
One key question is whether that reaction leads to the formation of beta-amyloid plaques. Another important question — and one that underscores the persistent mysteries of Alzheimer’s — is the precise relationship between those plaques and the disease itself. Many researchers believe that the plaques are a cause, while others view them as symptoms.
“The problem is that while beta-amyloid is associated with Alzheimer’s, there are also older people who have a lot of beta-amyloid who don’t have Alzheimer’s,” Dobson noted. “In addition to that, previous generations of beta-amyloid-clearing drugs were not particularly effective, obviously, or we would have a treatment for Alzheimer’s.”
Still, Maher says she’s begun paying closer attention to the air she breathes. The concentration of particles in the atmosphere, she noted, can drop precipitously as one moves even just a few meters away from a pollution source. So Maher says that when driving, she now tries to leave space between her and other cars. And when she walks up or down city hills, she says she sticks to the side of the road with downhill traffic, because those car engines aren’t working as hard — meaning they are emitting fewer particulates into the air.
“It’s like know your enemy,” said Maher. “Once you understand the nature of the particles you are trying to reduce exposure to, there are things that you can do.”