25 September 2006Planetark
Because this risk appears to be independent of 24-hour particulate matter levels, they suggest that air quality standards be based on hourly data, as well as 24-hour levels.
Particulate matter, or PM, is the term used to describe the tiny particles emitted by automobiles, especially diesel vehicles.
Dr. Shin Yamazaki, an epidemiologist at Kyoto University, and associates collected data from the 13 largest cities in Japan regarding concentrations of suspended particulate matter 7 m diameters or higher (PM7), ambient temperature, plus other components of air pollution, from January 1990 to December 1994.
During that period, 17,354 residents age 65 or older died due to hemorrhagic or bleeding stroke, and 46,370 died from ischemic stroke, the type caused by blood clots.
According to their analyses, reported online in the journal Occupational and Environmental Medicine, the odds ratio (OR) of death from ischemic 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 hemorrhagic stroke nearly 2.4-fold, an association independent of 24-hour mean PM7 concentrations.
In contrast, death due to ischemic stroke was not associated with 1-hour PM7 levels. Yamazaki's team suggests this discrepancy may be due to the longer interval from ischemic stroke onset to death, or to the fact that inhaled particles raise blood pressure, a risk factor for bleeding in the brain.
Moreover, they write, "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."
They propose that, if hourly measures of air quality are not available, stricter standards for 24-hour mean levels of suspended particulate matter may be an effective substitute.
SOURCE: Occupational and Environmental Medicine 2006.
