In Helsinki, Finland, from 1987 to 1993, the authors studied the associations between daily concentrations of sulphur dioxide, nitrogen dioxide, ozone, total suspended particulates, and particulates with aerodynamic diameters less than 10 microm (PM10), and the daily number of deaths from all causes and from cardiovascular causes. Investigators used Poisson regressions to conduct analyses in two age groups, and they controlled for temperature, relative humidity, day of the week, month, year, long-term trend, holidays, and influenza epidemics. The PM10 levels were associated significantly with all-cause and cardiovascular mortality among persons under the age of 65 y of age. In the less-than-65-y age group, sulfur dioxide and ozone were also associated significantly with cardiovascular mortality. The effect of ozone was independent of the PM10 effect, whereas sulfur dioxide became nonsignificant when modeled with PM10. An increase of 10 microg/m3 in PM10 resulted in increases in total mortality and cardiovascular mortality of 3.5% (95% confidence interval=1.0, 5.8) and 4.1% (95% confidence interval=0.4, 10.3), respectively. A 20 microg/m3 increase in ozone was associated with a 9.9% (95% confidence interval=1.1, 19.5) increase in cardiovascular mortality; however, ozone results were inconsistent. Moreover, in addition to their separate effects, high concentrations of PM10, ozone, and nitrogen dioxide had a further harmful additive effect. Typically, PM10 was a better indicator of particulate pollution than total suspended particulates. The authors' findings suggest that (a) even low levels of particulates are related to an increase in cardiovascular mortality; (b) ozone--even in low concentrations--is associated, independently, with cardiovascular mortality; and (c) PM10, ozone, and nitrogen dioxide--the essential components of summertime pollution--have harmful interactions at high concentrations.