Short-term exposure to air pollution may increase risk of stroke or mini-stroke

Short-term exposure to low levels of particulate air pollution may increase the risk of stroke or mini-stroke, according to findings that suggest current exposure standards could be insufficient to protect the public.

"The vast majority 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," said Lynda Lisabeth, lead author and assistant professor in the University of Michigan School of Public Health.

However, Lisabeth stressed that the association requires further study in other areas with varying climates and alternative study designs. Stroke is the third leading cause of death in the United States.

The study examined particulate air pollution in a southeast Texas community where there is a large petroleum and petrochemical industry presence. Particulate matter is one type of air pollution, defined as tiny particles of solid or liquid that can cause numerous health problems when inhaled. These particles can be man-made or from natural sources.

In the study, researchers identified ischemic strokes and transient ischemic attacks (TIA), sometimes called mini strokes but that often lead to a stroke later. Ischemic attacks are caused by a blockage of blood flow to the brain by a blood clot.

The results showed borderline significant associations between same day and previous day fine particulate matter exposures and ischemic stroke/TIA risk. Similar associations were also seen with ozone, another type of pollution. Despite the fossil fuel industry in the area, fine particulate matter exposures were low relative to other regions of the country, probably because of the proximity to the coast and prevailing wind patterns.

Findings suggest that recent exposure to fine particulate matter may increase the risk of ischemic cerebrovascular events specifically. Some research has shown that particulate air pollution is associated with acute artery vasoconstriction and with increased thickening of the blood, which may enhance the potential for blood clots. However, this requires further study.

Researchers looked at data from the Brain Attack Surveillance in Corpus Christi Project, a population-based stroke surveillance project designed to capture all strokes in Nueces County, Texas. Ischemic stroke and TIA cases between 2001 and 2005 were identified using trained staff and later verified by neurologists. Daily historical air pollutant and meteorological data were obtained for the same time period from the Texas Commission on Environmental Quality's Monitoring Operations database. Data on fine particulate matter and ozone were available from a centrally located monitor in Corpus Christi, Tex., located upwind of the local industrial facilities. The majority of stroke/TIA cases were also located upwind of local chemical plants and refineries.