Breathing in dirty air has been conclusively linked to both heart and lung problems, and the consensus is that pollution is, to say the least, bad for us. Around 7m people die from exposure to fine particles in polluted air every years. These particles enter the lungs and cardiovascular system, causing heart disease, lung cancer, respiratory infections and strokes.
But new research has suggested dirty air may also be affecting our brains – impacting our cognitive function and intelligence.
Last month, a study carried out by researchers at the Yale School of Health and Beijing University suggested that chronic exposure to air pollution could be linked to cognitive performance. The results of the study, which involved testing the maths and verbal skills of around 20,000 people in China, showed that high levels of air pollution lead to significantly lower test scores. For those exposed the longest, the drop was equivalent to a loss of a whole year of schooling.
A separate study, published in September, suggested air pollution may increase the chance of developing dementia. The data, collected in London, suggested people aged over 50 in areas with the highest levels of nitrogen oxide in the air had a 40 per cent greater risk of developing dementia than those with the least pollution.
And last year, a UNICEF study warned that 17m babies across the world are breathing in toxic air, which was putting their brain development at risk. It suggested that ultrafine pollution particles are small enough to enter the bloodstream, travel to the brain, and damage the blood-brain barrier.
Children are more vulnerable because they breathe more rapidly, the study suggested, but also because their physical defences and immunities are not fully developed.
The gaps in the data
It’s not all that surprising that air pollution may affect our health in multiple ways. The problem is that researchers still don’t know exactly how – or why – It impacts our brains.
Professor Pamela J Lein, of the department of molecular biosciences at the University of California, Davis, suggests the brain may become damaged due to inflammation.
“The experimental evidence indicates that air pollution can impact the structure of at least the developing brain. And in both the developing and aged brain, air pollution can trigger a neuroinflammatory response,” she says.
“Neuroinflammation is important to protect the brain – but if it is sustained too long, it can become injurious and interfere with communication between neurons in a neural circuit, and eventually may become neurotoxic and cause neuronal cell death.”
Neuronal cell death is thought to be the cause of cognitive decline in many neurodegenerative diseases such as Alzheimer’s disease, Lein explains. “But even persistent interference with how neurons communicate with each other can be sufficient to manifest as cognitive decline,” she adds.
Although we’re beginning to hypothesise about how air pollution might impact the brain, we still don’t know which air pollutants are to blame.
There are lots of pollutants which can cause health risks, including particulate matter (or PM – a mix of solid and liquid particles suspended in the air), ozone, nitrogen dioxide and sulphur dioxide. The smallest particles are the most damaging because they can get farthest into the lungs.
The research for the Chinese study was based on measurements of sulphur dioxide, nitrogen dioxide and particulates smaller than 10 micrometres in diameter, but not carbon monoxide, ozone and larger particulates. Yet it’s not clear which of these pollutants is responsible for the study participants’ cognitive decline.
More research is need to understand exactly how pollutants enter the brain, too.
“I could spin a good narrative yarn, says Dr Ian Mudway, a lecturer in respiratory toxicology at King’s College London. “I could tell you that we breathe particles, and particles travel up the olfactory nerves to the brain, and those particles are rich in metals, and we know that metals seem to be involved in the acceleration of the injury patterns associated with dementia
“I could say all of those things – but if Iwas being completely honest and transparent, I'd have to, at each of those stages, say, ‘But we don't really know how valid those statements are’.
“As with all fields, very often there's a tendency to suggest that everything is much clearer than it really is.”
It’s also not yet possible to pinpoint whether air pollution directly impacts brain function, because there are so many different factors involved.
Living near roads with heavy traffic has been associated with an increased incidence of alzheimer’s disease, Lein highlighted in an op-ed published earlier this year with her colleague Anthony Wexler – but this association poses a number of questions.
For example, is the effect because of the higher concentration of air pollutants? Or is it because housing is less expensive near busy roadways, so people with lower incomes and possibly poorer diets live near these roads?
Epidemiologic studies may find a correlation between air pollution and neurodegenerative disease – but this doesn’t mean pollution is the cause.
Much more research is needed to show how strong the link is between air pollution and brain function, and exactly what is causing this link.
What we do know, though, is that many of the risk factors for cardiovascular disease are the same as the risk factors for dementia – and we have already established that air pollution has a detrimental impact on the heart. “If you look at the risk factors for stroke, for example, hypertension, these are risk factors for dementia, so maybe it's not a tremendous surprise,” Mudway says.
“We are at this very early point in this field – a bit like air pollution was with cardiovascular disease back in the mid-90s,” he explains. “People had found associations, but there were still considerable uncertainties about how we understood this.
“All the pieces are scattered on the table, and you can see what the picture is – but the pieces don't quite fit yet.”