Stroke of inspiration: researchers implicate air pollution. (Science Selections).

Strokes, one of the world's leading killers, are caused by more than just sedentary lifestyles and poor diets, says an international team of researchers [EHP 110:187-191]. Air pollutants also play a part. In one of the few detailed studies of stroke death and air pollutants conducted to date, a team from four Korean institutions and the Harvard School of Public Health led by Yun-Chul Hong found that deaths from stroke increased consistently with rising concentrations of either particulates 10 microns in diameter and smaller (P[M.sub.10]), carbon monoxide (CO), sulfur dioxide (S[O.sub.2]), nitrogen dioxide (N[O.sub.2]), or ozone. The two most susceptible groups were found to be women and people 65 and over.

The team based its findings on data from Korea's largest metropolitan area, Seoul, a city of both relatively high particulate concentrations and high numbers of fatal strokes. Hourly air pollution readings from 20 stations over the years 1995-1998 revealed a wide range of pollutant concentrations during the study period. Government records documented a high rate of stroke deaths during the same period, totaling 22,341. As the team analyzed the data, it factored in potential confounders such as temperature, relative humidity, barometric pressure, and day of the week. Team members did not evaluate several other potential influences, such as lifestyle, income level, living conditions, or health care history, because these weren't expected to vary daily as air pollutant concentrations would.

The team found that rises in P[M.sub.10] and ozone concentrations had a strong, nearly immediate link with stroke deaths: for every interquartile range increase of these two pollutants, death rates rose 1.5% and 2.9% respectively on the same day. Analysis of N[O.sub.2], S[O.sub.2], and CO showed increases of a similar scale, but with a two-day lag. When researchers performed a two-pollutant analysis with P[M.sub.10] and each of the other pollutants, they found a correlation in each case, but sometimes in opposite directions. The risk from P[M.sub.10] was significantly elevated with higher concentrations of ozone, but it was reduced with higher concentrations of N[O.sub.2], S[O.sub.2], and CO. The team didn't evaluate the effects of the four gaseous pollutants interacting with each other.

Some of the varying findings might be explained by the shortage of data for all scenarios; the number of days when both P[M.sub.10] and each other pollutant were near their peaks were rare. The team also acknowledges that it is difficult to separate out the effects of individual pollutants because various pollutant concentrations tend to be interrelative. In addition, the data from the 20 stations don't necessarily reflect the actual exposures of the people who suffered strokes.

Nonetheless, the generally consistent link between stroke deaths and single pollutants at concentrations that often were well below many countries' regulatory standards suggests that stroke incidence might be reduced through air quality improvement. However, additional studies will be needed to determine whether the results of this study apply in different cultures and settings.

It also would be useful to understand the mechanism by which air pollutants boost deaths from stroke. The research team speculates that free radicals may increase after pollutant exposures, causing inflammation, increased plasma viscosity, and subsequent stroke in susceptible people.