The present study performs a meta-analysis (an analysis combining data from multiple previous studies) on more than 18 million stroke patients demonstrating an association between recent exposure to environmental pollution and the risk of ischaemic stroke and its mortality. Ischaemic stroke is the most common type of stroke (80-85% of all strokes) and is caused by a lack of cerebral blood flow due to occlusion or thrombosis of a cerebral artery.  

The analysis shows that different environmental particles such as carbon monoxide (CO), sulphur dioxide (SO₂), nitrogen dioxide (NO₂) and ozone (O₃) as well as particles of different sizes increase the risk of ischaemic stroke and the likelihood of death from stroke, even in short-term exposure to this pollution (less than 5 days).  

The data obtained are robust as they evaluate data from different studies with a large number of stroke patients (18 million) in multiple countries in Asia, Europe and America.  

Several studies have previously shown that continuous exposure to air pollution increases the risk of stroke (both ischaemic and haemorrhagic) and heart disease but, until now, there was no conclusive data on short-term exposure and the relationship with stroke risk.  

One limitation of the work is that it is based on observational studies, so there could be other causes for these findings, such as socioeconomic status, access to the health system, diet, exercise, etc. On the other hand, there seems to be a publication bias of positive studies that find an association between pollution and stroke. However, the large number of patients studied from multiple countries and different studies provides robust data for the study and its conclusions.

In this study, the authors conduct a systematic review and meta-analysis of the relationship between exposure to different air pollutants and the occurrence of ischaemic stroke. The study draws on more than 100 published observational studies and a total of more than 18 million cases from large clinical databases such as MEDLINE, Scopus, Cochrane Central and Web of Science, and finds a positive correlation between the incidence of cerebral ischaemia and exposure to numerous compounds from environmental pollution. These include sulphur dioxide (SO₂), nitrogen dioxide (NO₂) and PM2.5 and PM10 particles (derived from the combustion of fossil fuels and classified according to their size), which are associated with an increased risk of mortality due to these neurological accidents.  

The importance and significance of this study lies not only in the enormous size of the population sample analysed and the studies included, but also in the fact that it analyses the effect of individual chemical components. Moreover, it has the novelty of analysing the short-term effect between exposure and the onset of neurological problems, as until now most studies have analysed the long-term effect (chronic exposure).  

The study has certain methodological limitations, such as the inclusion of a population mostly from developed countries; data from developing countries such as those in Africa are missing. Nevertheless, this study warns of the significant risk of stroke following recent (and not just chronic) exposure to air pollution, and should be taken into account when establishing measures to curb pollution, especially in cities and industrial areas.

The Neurology article is of great interest and reinforces previously published results on the association between environmental pollution and incident stroke. It is a meta-analysis that includes more than 110 studies with more than 18 million strokes analysed. In this particular study, this association is analysed with acute exposure to pollutants ('peaks' of pollution), observing an effect on the incidence of stroke after 5 days of exposure, as well as on stroke mortality.   

These results are undoubtedly very relevant for establishing policies aimed at reducing environmental pollution. In the latest update of the Spanish Society of Neurology stroke prevention guidelines (2021*), a specific section on environmental pollution was established, recommending, based on the evidence available at the time, the application of public health policies to reduce air pollution due to its potential effect on stroke prevention.