Because this risk appears to be independent of particulate matter, or PM, levels measured over 24 hours, they suggest air quality standards be based on hourly data as well as 24-hour levels.

PM is the term used to describe the tiny particles emitted by automobiles, especially diesel vehicles.

A team led by Shin Yamazaki, an epidemiologist at Kyoto University, collected data from the 13 largest cities in Japan to check concentrations of suspended PM, ambient temperature, plus other components of air pollution, from January 1990 to December 1994.

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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 in Occupational and Environmental Medicine, the odds ratio of death from ischemic stroke was increased with temperatures above 30 degrees Celsius in the warmer months compared with moderate temperatures of 15 to 22 degrees Celsius. In contrast, the risk of death due to bleeding in the brain was increased in cold weather.

But during warmer months, high one-hour mean concentrations of PM increased the risk of death from hemorrhagic stroke nearly 2.4-fold, an association independent of 24-hour mean PM concentrations.

In contrast, death due to ischemic stroke was not associated with one-hour PM 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.

They propose that, if hourly measures of air quality are not available, stricter standards for 24-hour mean levels of suspended PM may be an effective substitute.

On the same problem, the US government imposed stricter standards on air quality for the first time in nearly a decade, ruling that communities must cut back on the amount of soot on any given day. But the Environmental Protection Agency did not go as far its own scientists had urged in curbing soot, which is linked to heart and lung disease as well as childhood asthma.

The decision sparked complaints on both sides of the pollution debate, with public health experts saying it was inadequate and industry executives calling it too stringent.

REUTERS, THE WASHINGTON POST