How is rising air pollution affecting our brains?

Children in New Delhi protesting air pollution in 2017. Image: Getty.

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.”

 
 
 
 

Covid-19 is highlighting cities' unequal access to green space

In the UK, Londoners are most likely to rely on their local park for green space, and have the best access to parks. (Leon Neal/Getty Images)

As coronavirus lockdowns ease, people are flooding back to parks – but not everyone has easy access to green space in their city.

Statistics from Google show that park attendance in countries across the globe has shot up as people have been allowed to move around their cities again.

This is especially true in urban areas, where densely populated neighbourhoods limit the size of private green space – meaning residents have to go to the park to get in touch with nature. Readers from England can use our interactive tool below to find out how much green space people have access to in their area, and how it compares to the rest of the country.

 

Prime Minister Boris Johnson’s announcement Monday that people are allowed to mingle in parks and gardens with groups of up to six people was partially following what people were doing already.

Data from mobile phones show people have been returning to parks across the UK, and also across Europe, as weather improves and lockdown eases.

People have been returning to parks across the world

Stay-at-home requirements were eased in Italy on 4 May, which led to a flood of people returning to parks.

France eased restrictions on 1 May, and the UK eased up slightly on 13 May, allowing people to sit down in public places so long as they remain socially distanced.

Other countries have seen park attendance rise without major easing of lockdown – including Canada, Spain, and the US (although states there have individual rules and some have eased restrictions).

In some countries, people never really stopped going to parks.

Authorities in the Netherlands and Germany were not as strict as other countries about their citizens visiting local parks during lockdown, while Sweden has famously been avoiding placing many restrictions on people’s daily lives.


There is a growing body of evidence to suggest that access to green space has major benefits for public health.

A recent study by researchers at the University of Exeter found that spending time in the garden is linked to similar benefits for health and wellbeing as living in wealthy areas.

People with access to a private garden also had higher psychological wellbeing, and those with an outdoor space such as a yard were more likely to meet physical activity guidelines than those without access to outdoor space. 

Separate UK research has found that living with a regular view of a green space provides health benefits worth £300 per person per year.

Access is not shared equally, however, which has important implications for equality under lockdown, and the spread of disease.

Statistics from the UK show that one in eight households has no garden, making access to parks more important.

There is a geographic inequality here. Londoners, who have the least access to private gardens, are most likely to rely on their local park for green space, and have the best access to parks. 

However the high population in the capital means that on the whole, green space per person is lower – an issue for people living in densely populated cities everywhere.

There is also an occupational inequality.

Those on low pay – including in what are statistically classed as “semi-skilled” and “unskilled” manual occupations, casual workers and those who are unemployed – are almost three times as likely as those in managerial, administrative, professional occupations to be without a garden, meaning they rely more heavily on their local park.

Britain’s parks and fields are also at significant risk of development, according to new research by the Fields in Trust charity, which shows the number of people living further than a 10-minute walk from a public park rising by 5% over the next five years. That loss of green spaces is likely to impact disadvantaged communities the most, the researchers say.

This is borne out by looking at the parts of the country that have private gardens.

The least deprived areas have the largest gardens

Though the relationship is not crystal clear, it shows at the top end: Those living in the least deprived areas have the largest private green space.

Although the risk of catching coronavirus is lower outdoors, spending time in parks among other people is undoubtedly more risky when it comes to transmitting or catching the virus than spending time in your own outdoor space. 

Access to green space is therefore another example – along with the ability to work from home and death rates – of how the burden of the pandemic has not been equally shouldered by all.

Michael Goodier is a data reporter at New Statesman Media Group, and Josh Rayman is a graphics and data visualisation developer at New Statesman Media Group.