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Study finds link between air pollution and stroke risk

Wednesday 25 March 2015

"Air pollution is linked to an increased risk of stroke," BBC News reports, prompted by a large global study in The BMJ. Researchers found an association even with brief upsurges in air pollution levels.

Previous research has shown a strong link between air pollution and heart attacks, but until now the research looking at air pollution and stroke has had mixed results.

In this study, the researchers summarised all the relevant research published on the topic worldwide. This showed stroke risk was higher on the day of an increase in air pollution and the days immediately after. They also found the effect of pollution was stronger in lower-income countries such as China.

While this type of study cannot prove air pollution is responsible for some strokes, it shows people are more likely to have strokes in the immediate aftermath of episodes of raised air pollution.

The researchers speculated that the association could be the result of a number of possible factors, such as pollution raising blood pressure or constricting blood vessels.

They concluded that governments around the world need to continue to take efforts to reduce the public health burden caused by air pollution.

Where did the story come from?

The study was carried out by researchers from the University of Edinburgh and was funded by the British Heart Foundation. No conflicts of interest were reported.

It was published in the peer-reviewed BMJ on an open-access basis, so it is free to read online or download as a PDF.

Generally, the study was reported accurately in the media. The Mail Online and BBC News gave more detail about the types of pollution causing the problems, and asked questions about why some regions of the UK breach EU pollution limits.

The study has been published in tandem with a related study looking at the links between air pollution and anxiety, which is also open access.

Some media sources have combined a report on both studies into a single story. We have not analysed this second study, so we cannot comment on whether the coverage is accurate.

What kind of research was this?

This was a systematic review of 103 observational studies that looked at the link between air pollution levels and risk of stroke.

The studies included two types of observational study: case-crossover studies and time series studies. A meta-analysis was performed on 94 of the studies, which pooled the results.

Observational studies cannot show that a risk factor such as pollution directly causes an event such as a stroke, although this type of study can show if there is a likely link between the two. The difficulty is adjusting the figures to take account of anything else that might have affected the chances of having a stroke (confounders).  

What did the research involve?

Researchers trawled the scientific literature for studies that included measures of air pollution, deaths from stroke, or admissions to hospital for stroke. They then pooled the estimates of risk of stroke from the individual studies to come up with an overall risk figure for each type of pollutant studied.

The researchers specified the types of studies they would include at the start of their work, and explained in the paper how they excluded research that did not meet quality requirements or did not give the data in a way that they could use.

They included research published in any language, which increased their chances of including research from low- and middle-income countries.

They assessed 2,748 articles and included 103 in the review. Of these, 94 provided data that they were able to include in their analysis. The papers provided information about 6.2 million stroke hospital admissions or deaths from 28 countries.

The researchers used standard analytical techniques to show the increase in stroke risk for each incremental increase in pollution levels for the gases sulphur dioxide, nitrogen dioxide and ozone (all assessed by additional 10 parts per billion), as well as carbon monoxide (assessed by additional part per million).

They also analysed the increase in stroke risk for each incremental increase in fine particles or coarse particles. In addition, they looked at the time lag between stroke and raised pollution levels, and the nation's income status.  

What were the basic results?

Researchers found a "robust and clear" link between gas and particle air pollution levels and admission to hospital for stroke or death from stroke. The link was weakest for ozone and strongest for sulphur dioxide.

Fine particles were more strongly linked to stroke risk than coarse particles, and the link with higher stroke risk lasted longer for high levels of polluting particles than high levels of polluting gases. The increase in relative risk of stroke for each additional increment of pollutant ranged from around 1% to 2%.

To give one example, average (median) pollution levels measured in high-income countries were around 22.6 parts per billion for nitrogen dioxide (the most commonly measured polluting gas).

The increase in stroke risk for each additional 10 parts per billion for nitrogen dioxide was 1.4% (relative risk [RR] 1.014, 95% confidence interval [CI] 1.009 to 1.019).  

How did the researchers interpret the results?

The researchers say they have demonstrated a "marked and close" association between air pollution exposure and risk of stroke. They point out that the study shows low- and middle-income countries have the highest levels of air pollution, and also a "disproportionate burden" of the numbers of strokes worldwide.

They concluded that their study provides sufficient evidence to think that environmental policies intended to reduce air pollution "might reduce the burden of stroke", considering some potential ways that pollution might affect the risk of stroke.

They say air pollution can affect the linings of the blood vessels and the nervous system. This can lead the blood vessels to constrict, blood pressure to rise and blood clots to form – all of these things might increase the chances of having a stroke.  


This study showed a clear link between rises in gas and particle pollution and the chances of being admitted to hospital or dying because of a stroke. The researchers showed the link was strongest on the day of exposure to raised pollution levels.

But this study has some limitations. While systematic reviews are a good way to summarise all the research that has been published on a topic, they are only as good as the individual studies they include.

About two-thirds of the studies used a time series design, which the researchers say is less effective in taking account of trends such as the season of year, rather than the more reliable case-crossover design.

It's also possible that stroke was not diagnosed correctly in some studies. The air pollution data in some studies came from monitoring sites away from city centres, where most people live. This would be likely to underestimate the effect of pollution, as pollution levels are higher in the city centre.

The increase in the chances of having a stroke for any one individual, as demonstrated in this study, is small. However, people cannot usually choose to avoid exposure to air pollution, and many thousands of people are affected when pollution levels rise. According to the Stroke Association, there are around 152,000 strokes a year in the UK.

While there is little people can do to avoid air pollution on an individual level, the study provides new information that governments need to consider when setting policies likely to affect pollution.

Observational studies cannot prove beyond doubt that factors such as pollution directly cause events such as stroke. But this was a comprehensive and careful analysis where the evidence pointed in one direction.

We already know that pollution is likely to increase the risk of heart attacks, and a similar increase would seem to now exist with strokes.

It seems implausible that air pollution alone would trigger a stroke in a healthy individual. But a particularly heavy upsurge in pollution could be the tipping point in people with pre-existing risk factors for stroke, such as obesity and atherosclerosis (hardening of the arteries).

While much has been done to reduce levels of air pollution, it would appear there is much more we could be doing.

Analysis by Bazian
Edited by NHS Website