Airborne Fine Particulate Matter Linked to Stroke Risk

By Todd Neale
ANN ARBOR, Mich., 31 may 2008Even low levels of airborne fine particulate matter appear to be associated with an increased risk of cerebrovascular events, researchers found.Levels of particulate matter in a southeast Texas county were associated with "borderline significant" increases in the risk of ischemic stroke and transient ischemic attack, Lewis Morgenstern, M.D., of the University of Michigan, and colleagues reported online in the Annals of Neurology.When levels of fine particulate matter rose by 5.1 mg/m3 the risk of suffering one of the events increased by 3% that day (RR 1.03, 95% CI 0.99 to 1.07) and the day after (RR 1.03, 95% CI 1.00 to 1.07).
"Although the magnitude of increased risk for stroke/[transient ischemic attack] because of [particulate matter] exposure was relatively small," the researchers said, "most of the public is exposed to ambient air pollution at the levels observed in this community or greater every day, suggesting a potentially large public health impact."
The median particulate matter level was 7.0 mg/m3, less than half the standard set by the Environmental Protection Agency (15 mg/m3).
The results, therefore, "call into question current standards for fine particulate matter and whether these standards are sufficient to protect the public from stroke," the researchers said.
Past studies have found a link between air pollution and cardiovascular disease in general but data on the association with stroke have been more limited, the researchers said.
Fine particulate matter -- a diameter less than 2.5 mm -- can penetrate airways and alveoli and enter the circulation because of its smaller size and chemical activity, they said.
The particles may raise the risk for cerebrovascular events by inducing arterial vasoconstriction or acute increases in blood pressure and plasma viscosity, they said.
To explore the issue, Dr. Morgenstern and colleagues turned to the Brain Attack Surveillance in Corpus Christi Project, which is designed to capture all strokes in Nueces County, Tex. There are a large number of oil refineries, chemical plants, and other sources of particulate matter pollution in this area.
Nevertheless, the level of particulate matter during the study was low, most likely because of the prevailing winds and coastal location, they said.
Air pollution and meteorological data were obtained from the Texas Commission on Environmental Quality.
From Jan. 1, 2001 through Dec. 31, 2005, the researchers identified 2,350 ischemic strokes and 1,158 transient ischemic attacks in patients ages 45 and older (median age 72). Half of the patients were Mexican-American, 44% were white, and 5% were African American.
The increased risk of suffering a stroke or transient ischemic attack was independent of ozone levels after accounting for temperature, day of the week, and temporal trends in cerebrovascular events.
Season did not modify the association.
"These data contribute to the literature because there is currently a scarcity of data on the effect of environmental exposures on risk for stroke in the United States," the researchers said.
Future studies on the effects of particulate air pollution on the risk for subtypes of ischemic stroke could reveal the biological mechanisms at work, they said.
The authors acknowledged some limitations to the study, including the fact that, as an ecological study, it should be used primarily for generating hypotheses. The results should be replicated, they said, and future studies should take into account any unmeasured confounders.
In addition, measurement of particulate matter is subject to error. The values obtained in this study may not be generalizable over a wider geographic area, they said.
Finally, they said, residual confounding by other pollutants may have occurred.
The study was funded by a grant from the National Institute of Neurological Disorders and Stroke.
The authors made no disclosures.
Primary source: Annals of NeurologySource reference:Lisabeth L, et al "Ambient air pollution and risk for ischemic stroke and transient ischemic attack" Ann Neurol 2008; DOI: 10.1002/ana.21403.