Researchers from the National Institute for Environmental Studies have found a direct correlation between the death rates of Tokyo residents and the density of suspended particulate matter emitted in exhaust fumes from diesel cars and factories.

The study, led by Masaji Ono, head of the NIES international public health laboratory, shows there is a marked increase in the death rate across Tokyo's 23 wards on days when the SPM index is high.

Scientists in the United States have observed this correlation in the U.S. since the mid-1990s. The NIES study marks the first time a correlation between death rates and SPM has been demonstrated in Japan.

"The results are about the same as those reported in the United States," said Ono, noting that researchers have not been able to explain the mechanism linking SPM density and mortality.

Ono is expected to present the results of the study at an academic conference on atmospheric environmental studies that opens Saturday in Kitakyushu.

The Ono group analyzed the number of nonaccident-related deaths of people aged under 65 in the Tokyo area between 1990 and 1994. The death rates were then correlated with the density of atmospheric SPM during the period, which varied between 3 micrograms and 225.5 micrograms per cubic meter.

With an increase of 100 micrograms of SPM in the air, the death rate rose 6 percent, according to the study.

After dividing the data between deaths caused by respiratory failure and those caused by circulatory factors, the group found no significant difference in the mortality rates between the two groups. The SPM study was conducted as part of the Environment Ministry's ongoing efforts to disclose the impact of particle matter on public health.