Air pollution raises stroke risk

Exposure to high concentrations of air pollution raises the risk of death due to stroke. The risk appears to be independent of 24-hour particulate matter levels. Therefore the air quality standards should be based on hourly data, as well as 24-hour levels. Particulate matter (PM) is the term used to describe the tiny particles emitted by automobiles, especially diesel vehicles. Researchers from the Kyoto University collected data from the 13 largest cities in Japan regarding concentrations of suspended particulate matter 7 µm diameters or higher (PM7), ambient temperature, plus other components of air pollution, from January 1990 to December 1994. During that period, 17,354 residents of age 65 years or older died due to haemorrhagic or bleeding stroke, and 46,370 died from ischaemic stroke, the type caused by blood clots. According to their analyses, the odds ratio (OR) of death from ischaemic stroke was increased with temperatures above 30 degrees centigrade in the warmer months compared with moderate temperatures of 15 to 22 degrees (OR 1.333). In contrast, the risk of death due to bleeding in the brain was increased in cold weather (0 to 8 degrees, OR 1.225). However, during warmer months, high 1-hour mean concentrations of PM7 increased the risk of death from haemorrhagic stroke nearly 2.4-fold, an association independent of 24-hour mean PM7 concentrations. In contrast, death due to ischaemic stroke was not associated with 1-hour PM7 levels. Researchers suggest that this discrepancy may be due to the longer interval from ischaemic stroke onset to death, or to the fact that inhaled particles raise blood pressure, a risk factor for bleeding in the brain. Moreover, during the 4 years covered by this study, there were 443 hours in which the concentration of PM7 was over the 1-hour air quality standard (in Tokyo), and that 49 of those hours (11 percent) occurred on days when the 24-hour mean concentration of PM7 was within the air quality standard for 24-hour periods. If hourly measures of air pollutants are not available, stricter standards for 24-hour mean levels of suspended particulate matter may be an effective substitute for monitoring the air quality.