Is Air Pollution Linked to Ischemic Stroke?

Even though some research has suggested a link between air pollution and ischemic stroke, drawing definitive conclusions has proven elusive. Proposed mechanisms include transiently increased blood coagulability and plaque rupture in response to air pollution. But studies have varied by the concentration and types of pollutants studied, which vary geographically.

New research from London has now added to the controversy, showing no consistent link between air pollutants and stroke subtypes or severity. The study was published online in PLOS One.

“Overall… there was no consistent pattern between short term exposure and increased risk of ischemic stroke subtypes or severity. The possible exception was the suggestion that NO₂ [nitrogen dioxide] exposure might be associated with stroke caused by cerebrovascular small vessel disease,” wrote Ravi Maheswaran, MD, of the University of Sheffield (UK), and colleagues.

In the study, researchers analyzed stroke cases from the South London Stroke Register to identify patients with first-time strokes in a geographically-defined area. They used Oxford Clinical and Trial of Org 10172 in Acute Stroke Treatment (TOAST) criteria to classify stroke subtype. To classify stroke severity, they used a clinical classification system, rather than the National Institutes of Health Stroke Scale (not fully implemented until 2001). Data on air pollution concentrations and exposures came from air pollution monitoring stations linked to postal codes. Researchers also evaluated lags in air pollutant effects ranging from zero to six days. They studied the following pollutants: particulate matter less than 10 microns in diameter (PM10), NO₂, ozone (O₃), carbon monoxide (CO), and sulfur dioxide (SO₂). 

Key results:

• Between 1995-2006, 2590 cases of ischemic stroke occurred (mean age 71.7 years, 49.7% female, 75.6% white)

♦ 62.5% had pre-existing hypertension

♦ 59.9% had a past transient ischemic attack

♦ Nearly one-fifth had preexisting coronary heart disease, atrial fibrillation, or diabetes

♦ No clear link between ischemic stroke and pollutants, in relation to pre-existing risk factors, including lifestyle, pre-existing medical conditions, and sociodemographic factors

• Some positive links between several pollutants at various lag times, but no overall consistent pattern between pollutants and risk of ischemic stroke subtypes or severity

• Possible exception: the risk of stroke from small vessel disease increased with increasing exposure to NO₂:

♦ 3-day lag: OR 1.21 (1.00–1.46)

♦ 5-day lag: 1.21 (1.01–1.45)

♦ 6-day lag: 1.26 (1.05–1.52)

♦ Average over 6 days: OR 1.51 (1.12–2.02)

The authors noted that the study may not have been large enough to detect associations. They also pointed out the influence of publication bias. Negative studies may not be published as often as positive studies, which may hinder comparisons of these results with other studies.

“Further studies are needed to examine these associations. Such studies will enhance pathophysiological understanding of the mechanisms by which air pollution could cause ischemic stroke,” they concluded.

Take-home Points

• Some research has suggested a link between air pollution and ischemic stroke, possibly related to increased blood coagulability and plaque rupture, but research is inconsistent.

• Eleven-year study in South London found no overall consistent pattern between pollutants and risk of ischemic stroke subtypes or severity.

• There was no clear link between ischemic stroke and pollutants, in relation to pre-existing risk factors, including lifestyle, pre-existing medical conditions, and sociodemographic factors.

• The possible exception was the risk of stroke from small vessel disease, which increased with increasing exposure to NO₂.

Reference: Maheswaran R, et al. Air pollution and subtypes, severity and vulnerability to ischemic stroke-A population based case-crossover study. PLoS One. 2016 Jun 30;11(6):e0158556.