Middle-aged and older adults who live in towns and cities suffer ageing of the brain and increased risk of dementia and strokes because of air pollution, new research suggests.
Those in the study were also almost 50% more likely to suffer a type of silent stroke which also increases the chance of the degenerative disease. [A “silent stroke” (technically known as a covert brain infarct) are small areas of damage caused by lack of oxygen to the brain tissue, but are not severe enough to cause obvious symptoms. They may be a sign of blood vessel disease, which increases the risk of one type of dementia (vascular dementia).]
Normal pollution levels from traffic fumes, factory and power station emissions and wood fire smoke were sufficient to age the brain by about one year, the research on US adults found.
Previous studies have already linked air pollution and the noise from living near major roads to heart attacks and other types of strokes.
The new research suggests long term exposure to air-borne pollutants can cause damage to brain structures and impair thinking and memory in middle-aged and older adults.
Those living near main roads had smaller brains, and “covert brain infarcts”, a type of “silent” stroke, which results from a blockage in the blood vessels supplying the brain.
Researchers looked at how far patients aged over 60 who did not have a stroke or dementia lived from major roads. They then used satellite images to assess prolonged exposure to air pollution.
Dr Elissa Wilker, a researcher in the Cardiovascular Epidemiology Research Unit at Beth Israel Deaconess Medical Centre, said: Our findings suggest that air pollution is associated with insidious effects on structural brain ageing, even in dementia and stroke-free individuals.”
The evaluation included total cerebral brain volume, which is a marker of atrophy of the brain, hippocampal volume, which reflect changes in the area of the brain that controls memory; white matter hyperintensity volume, which can be used as a measure of pathology and ageing; and covert brain infarcts.
The study, published in the journal Stroke, found that small increases in air pollution – an increase of only two microgram per cubic meter of air, to levels commonly found in cities, was sufficient to increase the risks.
Dr Wilker said the mechanisms were unclear, but that it might be that the body suffered inflammation as a result of the deposit of fine particles in the lungs.
Professor of Neurology Dr Sudha Seshadri at Boston University School of Medicine said: “On average participants who lived in more polluted areas had the brain volume of someone a year older than participants who lived in less polluted areas.
“They also had a 46 per cent higher risk of silent strokes. This is concerning since we know that silent strokes increase the risk of overt strokes and of developing dementia, walking problems and depression.”
Last year researchers suggested that commuters could cut their air pollution intake in half simply by using the side streets in major cities rather than main roads.
Dr Rossa Brugha, a paediatrician and pollution researcher at Queen Mary, University of London, said walkers can make small adjustments to their route which could have major benefits on their health.
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Noise from road traffic ‘increases stroke risk’
Exposure to noise from road traffic can increase the risk of stroke in the over 65s, a study has found.
For every 10 decibel increase in noise, the risk of stroke among that age group increased by more than a quarter (27 per cent).
Authors of the Danish study, published today (WED) in the European Heart Journal (EHJ), said that they had accounted for air pollution and other factors like differences in lifestyle, meaning they believed there was a genuine association between noise and stroke risk.
Dr Mette Sørensen, senior researcher at the Institute of Cancer Epidemiology in Copenhagen, Denmark, who led the research, said: “Our study shows that exposure to road traffic noise seems to increase the risk of stroke.
“Previous studies have linked traffic noise with raised blood pressure and heart attacks, and our study adds to the accumulating evidence that traffic noise may cause a range of cardiovascular diseases.”
About 150,000 people have a stroke in Britain every year. More than nine out of 10 are over the age of 65.
The study looked at 57,000 people in Copenhagen and Aarhus who were aged between 50 and 64 between 1993 and 1997, and followed them for an average of 10 years. A total of 1,881 suffered a stroke during the study period.
The participants lived in homes with estimated noise levels ranging from 40dB – the sound of a quiet conversation to 82dB – that of a busy street.
For older people, there appeared to be a step-change in their risk of stroke at about 60dB, the researchers found.
Dr Sørensen said about one in five strokes in urban areas could be due to living in noisy homes.
He said: “If we assume that our findings represent the true risk, and the association between traffic noise and stroke is causal, then an estimated eight percent of all stroke cases, and 19 per cent of cases in those aged over 65, could be attributed to road traffic noise.”
He emphasised that the study only looked at urban homes, which were likely to be noisier than rural ones, and that it could not prove that noise itself was a causal factor in stroke risk.
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Air pollution linked to silent strokes
Behind the Headlines – NHGS Choices
Friday April 24 2015
The smaller a particle of pollution is, the more harmful it tends to be.
Air pollution could affect the flow of blood through the brain.
“Adults who live in towns and cities suffer ageing of the brain and increased risk of dementia and [silent] strokes because of air pollution,” The Daily Telegraph reports.
A “silent stroke” (technically known as a covert brain infarct) are small areas of damage caused by lack of oxygen to the brain tissue, but are not severe enough to cause obvious symptoms. They may be a sign of blood vessel disease, which increases the risk of one type of dementia (vascular dementia).
This headline is based on a study which took brain scans of more than 900 older adults and assessed their exposure to air pollution. It found that higher levels of small particles in the air around where an individual lived were associated with a greater likelihood of them having signs of a “silent stroke” on a brain scan.
There was some evidence of association between the particles and slightly smaller brain volume, but this link did not remain once people’s health conditions were taken into account.
Limitations of the study include that the researchers could only estimate people’s air pollution exposure based on average air quality of where they lived in one year, rather than lifetime exposure. It should also be noted that the news has suggested a link to dementia, but the study did not actually assess this.
The findings need to be investigated in future studies before firm conclusions can be drawn.
Where did the story come from?
The study was carried out by researchers from Beth Israel Deaconess Medical Center and other centres in the US. It was funded by the US National Institutes of Health and the United States Environmental Protection Agency.
The study was published in the peer-reviewed medical journal Stroke.
The Daily Telegraph headline suggests that air pollution could increase a person’s risk of dementia, but this is not what the study assessed, and none of the participants had dementia, a stroke or mini-stroke (also known as a transient ischaemic attack).
They also suggest that it is living in towns and cities that increases risk, but this was not what the study assessed. It compared people with different levels of particulate matter in the air where they lived, not whether they lived in towns and cities, and in their main analyses they did not include people living in rural areas far from major roads.
The Mail Online similarly overstates findings, by stating that “living near congested roads with high levels of air pollution can cause ‘silent strokes’”. While an association was found, a direct cause and effect relationship remains unproven.
What kind of research was this?
This was a cross-sectional analysis assessing whether there was a link between air pollutant exposure and changes in the brain linked to ageing.
The authors report that long-term exposure to air pollution is associated with, for example, increased risk of stroke and cognitive impairment. However, its effects on the structure of the brain are not known. If air pollution is linked to structural brain changes, these could, in turn, contribute to the risk of stroke and cognitive problems.
This type of study can show links between two factors, but cannot prove that one caused the other. As the study was cross-sectional, it cannot establish the sequence of events and whether exposure to air pollution came before any differences or changes in brain structure. As an observational study, there may also be factors other than air pollution exposure that could be causing the differences seen. The researchers did take steps to try to reduce the impact of other factors, but they may still be having an effect.
What did the research involve?
The researchers took brain scans of 943 adults aged 60 and over. They also estimated their exposure to air pollution, based on where they lived. They then analysed whether those with more exposure to air pollution were more likely to have smaller brain volume or signs of damage.
Participants in this study were taking part in an ongoing longitudinal study in the US state of New England. Only those who had not had a stroke or mini-stroke and did not have dementia were selected to take part.
The type of effects on the brain that the researchers were looking for were referred to as “subclinical”. This means that they did not cause the people to have symptoms and therefore would not normally be detected.
They looked at total volume of the brain and also the volume of the specific parts of the brain using a magnetic resonance imaging (MRI) brain scan. The brain shrinks gradually with age, so the researchers were interested in whether pollution might have a similar effect. The MRI also identified whether the brain showed signs of a “silent stroke” – that is, parts of the brain tissue that had been damaged by having the blood supply interrupted.
These “covert brain infarcts” were not severe enough to cause symptoms, in the form of a stroke or mini-stroke. However, this damage suggests that the person may have some degree of blood vessel (vascular) disease. They are often seen in the brain scans of people who have vascular dementia.
The researchers used satellite data measuring the level of small particles (PM2.5) on the air in New England to assess average daily air pollution exposure at each participant’s current home address in 2001. They also assessed how close each home was to roads of different sizes. The researchers only looked at those living in urban and suburban areas in their main analyses.
They then looked at whether there were any links between estimated particulate matter exposure and distance from roads and brain findings.
They first took into account confounding factors that could affect results, including:
age
gender
smoking
alcohol intake
education
They then carried out a second analysis, taking into account a number of additional factors, such as:
diabetes
obesity
high blood pressure
What were the basic results?
Average (median) daily exposure to small particles in the air was about 11 microgrammes per cubed metre of air, and participants lived an average of 173 metres from a major road. The participants were, on average, 68 years old when they had their brain scan, and 14% showed signs of a “silent stroke” on the scans.
The researchers found that greater estimated exposure to air pollution was associated with a slightly smaller total brain volume. Each two microgramme per cubed metre increase in particulate matter was associated with a 0.32% lower brain volume. However, once this analysis was adjusted for conditions such as diabetes, this difference was no longer statistically significant.
Greater estimated exposure to air pollution was also associated with a higher likelihood of having signs of “silent stroke” damage to the brain tissue. Each two microgramme per cubed metre increase in particulate matter was associated with a 37% higher odds of this silent damage (odds ratio (OR) 1.37, 95% confidence interval (CI) 1.02 to 1.85).
They did not find differences in association across areas with different average income brackets. Distance from a major road was not linked to total brain volume or a “silent stroke” after adjustment for confounders.
How did the researchers interpret the results?
The researchers concluded that their findings “suggest that air pollution is associated with insidious effects on structural brain aging, even in dementia and stroke-free persons”.
Conclusion
This cross-sectional study has suggested a link between exposure to small particles in the air (one form of pollution) and the presence of “silent stroke” in older adults – small areas of damage to the brain tissue that are not severe enough to cause obvious symptoms.
There are a number of limitations to be aware of when assessing the results of this study:
While there was an association between particulate matter in the air and total brain volume, this was no longer statistically significant after taking into account whether people have conditions such as high blood pressure, which can also affect their risk of stroke.
While the researchers did try to take into account factors such as smoking, alcohol intake and diabetes, which could be having an effect on risk, this may not remove their effect totally. There may also be various other unmeasured factors that could account for the association seen. This makes it difficult to be sure whether any link seen is directly due to the pollution itself.
The researchers could only estimate people’s air pollution exposure based on average air quality of where they lived in one year. This may not provide a good estimate of a person’s lifetime exposure.
While the news extrapolated these findings to suggest a link between air pollution and people’s risk of dementia, this is not what the study assessed. While areas of “silent stroke” can often be seen in people who have vascular dementia, none of the study participants had dementia, or a stroke or mini-stroke.
Overall, this study finds some evidence of a link between one measure of air pollution and “silent stroke”, but the limitations mean that this finding needs to be confirmed in other studies.
It is also not possible to say whether the link exists because air pollution is directly affecting the brain.
Analysis by Bazian.
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Environmental NGOs in the UK have called for the government to protect public health by upholding and toughening air pollution laws around airports
They asked the government to:
Ensure full parliamentary scrutiny and debate and adhere to principles of the EU Directive that air quality
(i) can not be allowed to breach legal limits
(ii) must not be worsened if there is a breach and
(iii) should not be allowed to deteriorate in areas that are within legal thresholds
Government should revise the air quality legal limits in the longer term to reflect WHO guideline levels and reduce air pollution around airports to these levels
Government must:
(a) Uphold and strengthen air pollution laws Air pollution is, according to Defra, responsible for 29,000 premature deaths per year in the UK (even if only considering fine particles), and airports can impact air quality both as a result of emissions from aircraft and from associated road traffic accessing the airport, whether freight or passengers. The UK is subject to EU-wide legal limits on pollutants. All principles of the EU Air Quality Directive should be upheld in the context of airport development:
– No breaches of the limits can be permitted
– In areas where legal limits are exceeded, air pollution must not be worsened, but be brought below these limits in the shortest time possible.
– Finally, where pollution levels are below the legal threshold, in line with the non-deterioration principle they should not be allowed to worsen.
To inform air quality assessments and decisions, regular, rigorous independent monitoring is required, with the results reported publicly.
(b) Work to WHO recommendations The UK government should, in the longer term, revise legal limits to reflect levels recommended by WHO for the protection of human health , allowing for any updates to these guidelines.
Air quality around airports should be improved where necessary to ensure compliance with these limits.
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