Among industrialized countries, Japan still maintains an old set of guidelines for particulate matter (PM); therefore, we assessed the public health impacts of PM exposure in various Situations using data from the Tokyo metropolitan area. Subjects were 7.8 million people aged older than 30 years. Based on a linear relationship between exposure and health effects, we estimated attributable cases of mortality caused by PM. Even at the recent exposure level, the number of deaths will occur after both short- and long-term exposure. When the guideline for PM2.5 (particles < 2.5 mu m in diameter) long-term exposure was set at 12 mu g/m3, we could prevent 8% of all causes mortality or 6,700 deaths in the Tokyo metropolitan area per year. This assessment shows that guidelines for long-term exposure, especially for PM2.5, should be recommended in Japan.