Study objective: To examine the short term effects of raised concentrations of air pollutants on myocardial infarction deaths, the authors measured if incidence rate ratios increased after the concentration of suspended particulate matter (SPM) reached various critical values and were sustained for various periods of time.
Design: Retrospective analysis of a database, which contained hourly SPM concentrations and myocardial infarction deaths.
Methods: The rate of death (events/1000 hours) attributable to myocardial infarction was compared among SPM categories. SPM categories were classified by five different critical values that measured high SPM levels, and by the hours of exposure (exposure window) after reaching the critical SPM level.
Main outcome measures: The adjusted incidence rate ratios in the various SPM categories were compared with the reference category (0-99 microg/m(3)) to determine the incidence rate ratio.
Setting: Tokyo Metropolitan area, Japan. Patients/
Participants: 14,950 people who died of myocardial infarction in the Tokyo Metropolitan area from 1990 to 1994.
Main results: When the exposure window was one hour, the adjusted incidence rate ratio in each category were 1.13 (95% CI: 1.07 to 1.20) in the 100-149 microg/m(3) category, 1.17-1.24 in the intermediate categories, and 1.40 (1.00 to 1.97) in the highest (300 microg/m(3) over) category. When the exposure window was one to six hours, increased rate ratios were seen in the highest category (1.17 (four hours) to 1.40 (one hour)). Gradual increases in the incidence rate ratio were seen as the SPM category increased when the exposure window was less than six hours.
Conclusion: An increased rate ratio of myocardial infarction deaths was seen within a few hours after reaching a high concentration of SPM. When the exposure window was less than six hours, there was a gradual increase in the incidence rate ratio as the SPM concentration increased.