Air Pollution Linked to Higher Stroke Mortality

November 17, 2016

More evidence showing that living in areas with high levels of air pollution is linked to a higher stroke risk has come from two new studies.

Both studies are published online in the journal Stroke. The first, from the United Kingdom, shows a higher risk for death after a stroke in patients who live in areas of high air pollution, and the other, a Japanese study, suggests a higher risk for a new stroke the same day as exposure to high levels of air pollution.

"We have shown a significantly increased risk of death after stroke in patients who had long-term exposure to high levels air pollution before their stroke occurred," senior author of the UK study, Charles Wolfe, MD, professor of health & social care at King's College London, commented to Medscape Medical News. "This was particularly pronounced for high exposure to smaller particulate matter — particles below 2.5 μm in diameter (PM2.5) — which are found in high quantities in exhaust fumes."

For the study, Professor Wolfe and colleagues analyzed data from the South London Stroke Register, a population-based register covering an urban, multiethnic population.

They linked mortality data from this registry with data on air pollution represented by averaged annual concentrations related to residential postcode before stroke.

In total, 1800 strokes were recorded between 2005 and 2012 (74.3% were ischemic, 14.5% were hemorrhagic, and 11.2% were of unknown type).

Results showed an increased risk for death up to 5 years after stroke in patients living in areas of high air pollution.

Hazard ratios (HRs) for death for a 1 interquartile range increase in PM2.5 were 1.28 (95% confidence interval [CI], 1.08 - 1.53) for all strokes and 1.32 (95% CI, 1.08 - 1.62) for ischemic strokes.

Within ischemic subtypes, PM2.5 pollution was associated with a two-fold increase in mortality risk for total anterior circulation infarcts and a 78% increase for lacunar infarcts.

PM10 pollution was associated with 45% increased mortality risk for lacunar infarct strokes.

Separating PM2.5 and PM10 into exhaust and nonexhaust components did not show increased mortality.

"While this study adds to the evidence linking air pollution to cardiovascular disease, it cannot prove causality as it has an observational design," Professor Wolfe commented. "And while we did control for socioeconomic status, other illnesses and age and sex, there will of course be other confounders that we won't have accounted for — people who live in an area with high air pollution probably have other factors that may affect cardiovascular and cerebrovascular disease as well.

"So it is difficult to say for certain that it is the air pollution that is responsible but there are many studies now that have shown similar associations," he said.

He noted that the smaller particles (PM2.5) were associated with a worse effect on mortality and this correlated with biological studies that have shown a greater inflammatory effect of small particulate matter vs larger particulate matter on the vessel wall.

"Our study suggests that people who have previously had an ischemic stroke, but not a hemorrhagic stroke, may be more vulnerable and at a higher risk of death to chronic, long-term exposure of PM," they conclude.

"By identifying the groups that are particularly vulnerable to air pollution–associated mortality (eg, survivors of ischemic stroke and, in particular, [total anterior circulation infarcts] and [lacunar infarcts]), our study may have implications for wider health policies."

Acute Risk

The Japanese study, by lead author Ryu Matsuo, MD, PhD, Department of Health Care Administration and Management Graduate School of Medical Sciences, Kyushu University, Fukuoka, Japan, and colleagues, looked at acute exposure to air pollution and shows a small increase in the risk of having a stroke within a day of high exposure to pollution.

For the study, the researchers analyzed data on 6885 ischemic stroke patients from a multicenter hospital-based stroke registry in Japan who were previously independent and hospitalized within 24 hours of stroke onset. Time of symptom onset was confirmed. They also obtained hourly data on air pollutants from the National Institute for Environmental Studies.

A case-crossover analysis was conducted to evaluate the effects of short-term exposure to particulate matter on ischemic stroke events, with 3 or 4 control periods from the same day of the week, month, and year as the case period.

Results showed that increased ambient PM2.5 concentrations within 1 day before stroke onset were associated with the occurrence of ischemic stroke: hazard ratio, 1.03 (95% CI, 1.00 - 1.06) per 10-μg/m3 increase in PM2.5. This association was maintained after adjustment for other air pollutants (nitrogen dioxide, photochemical oxidants, or sulfur dioxide) or influenza epidemics, and was evident in the cold season.

No significant association could be seen for days 2 to 6 before the stroke.

The researchers conclude: "Subjects with any risk factor should, thus, take care during exposure to high PM2.5 concentrations, particularly in winter. However, it remains unclear whether avoiding exposure to ambient PM2.5 or paying particular attention to health after exposure is actually beneficial."

They also point out that this study was performed in a limited area of western Japan that is susceptible to transboundary air pollution, and further studies are needed to investigate the generalizability of the findings.

Professor Wolfe said his group have conducted a similar study looking at exposure of air pollution in the year before stroke, which showed a 23% increase in stroke risk in those exposed to higher levels of PM2.5.

The research by Dr Wolfe and colleagues was funded/supported by the National Institute for Health Research (NIHR) Biomedical Research Centre based at Guy's and St Thomas' National Health Service (NHS) Foundation Trust and King's College London, and the NIHR Collaboration for Leadership in Applied Health Research and Care South London at King's College Hospital NHS Foundation Trust. The study by Dr Matsuo and colleagues was supported by Japan Society for the Promotion of Science KAKENHI Grants from the Japanese Ministry of Education, Culture, Sports, Science and Technology. None of the authors have disclosed any relevant financial relationships.

Stroke. Published online November 3, 2016. UK study, Japanese study

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