Particle and gaseous emissions of four different wood chip-fired district heating units in the size range 5-15 MW were studied. Three of the units were fire-tube boilers based on rotating grate combustion technology, and one was a gasification combustion boiler. All of the units were equipped with cyclones to remove coarse particles from the flue gas. In addition, two of the rotating grate boilers were equipped with single field electrostatic precipitators (ESP), and one with a condensing flue gas scrubber. The particle filtration efficiencies of these particle removal appliances were measured. The grate boilers produced on average 211-483 mg/MJ of total suspended particles (TSP) and 55-92 mg/MJ of fine particles (PM1) upstream of the secondary particle filters. These levels of TSP and PM1 were clearly higher than those produced by the gasification combustion boiler (on average 50 mg/MJ TSP and 13 mg/MJ PM1). The ESPs were found to remove around 95% of the PM1s and 93% of TSP, leading to relatively low particle emissions. The lowest collection efficiency (for particles below 10 mu m) was achieved in the size range 0.2-1 mu m. The condensing flue gas scrubber removed on average 44% of PM1 and 84% of TSP. In all of the plants, the fine particles mainly consisted of potassium salts. In addition, they contained small amounts of organic material and various metals, of which zinc was the most abundant. In general, the study shows that small (<20 MW) biomass-fired boilers, not equipped with effective particle filters, are an important source of fine particles introduced into ambient air and they have a high potential for reducing the emissions with currently available flue gas cleaning technology.