BACKGROUND It is known that particulate air pollution affects cardiorespiratory health; however, it is unclear which particle size fractions and sources of particles are responsible for the health effects. METHODS Daily levels of nucleation (<0.03 microm), Aitken (0.03-0.1 microm), accumulation (0.1-0.29 microm), and coarse mode (2.5-10 microm) particles, particles with diameter <2.5 microm (PM2.5), and gaseous pollutants were measured at central outdoor measurement sites in Helsinki, Finland between 1998 and 2004. We determined the associations of particles with daily cardiorespiratory mortality and acute hospital admissions among the elderly (>or=65 years). For the analyses we used Poisson generalized additive models and for the source apportionment of PM2.5 we used the EPA positive matrix factorization method. RESULTS There was a suggestion of an association of hospital admissions for arrhythmia with Aitken mode particles and PM2.5 from traffic. Otherwise few associations were observed between various sizes and types of particles and either cardiovascular admissions or mortality. In contrast, most particle fractions had positive associations with admissions for pneumonia and asthma-chronic obstructive pulmonary disease (COPD). The strongest and most consistent associations were found for accumulation mode particles (3.1%; 95% confidence interval = 0.43-5.8 for pneumonia over the 5-day mean, and 3.8%; 1.3-6.3 for asthma-COPD at lag 0, for an interquartile increase in particles). We also found a positive association of respiratory mortality mainly with accumulation mode particles (5.1%; 1.2-9.0 at lag 0). CONCLUSIONS All particle fractions including Aitken, accumulation, and coarse mode had especially adverse respiratory health effects among the elderly. Overall associations were stronger for respiratory than for cardiovascular outcomes.