Background: Combined heat and power production (CHP) is an efficient way of energy supply widely used in Finnish population centers. EU´s climate policies recommend increased use of biomass in energy production in order to reduce CO2 emissions. However, health impacts on general population may vary depending on the fuel due to differences in fine particle (PM2.5) emissions. Aims: Here we compared two scenarios of fuel use, business as usual (BAU) and increased biomass use (IBU), in Haapaniemi CHP power plant (total fuel power 404 MW) of the city of Kuopio, Finland. According BAU fuels are peat 84%, heavy oil 12% and biomass 4%, whereas according IBU the fuels are peat 49.5%, heavy oil 1% and biomass 49.5%. Methods: The annual amounts of energy produced per fuel were calculated using an on-line city-level energy balance tool adjusted for Kuopio (http://en.opasnet.org/w/Energy_balance_in_Kuopio). Respective PM2.5 emissions were calculated using appropriate emission factors for CHP plants of this size. The attributable all-cause deaths due to PM2.5 emissions were calculated for Finnish population using an appropriate intake fraction. Results: Annual PM2.5 emissions were 64 t (12-116) and 59 t (9-110) for BAU and IBU, respectively, causing 0.12 (0.02-0.22) and 0.11 (0.02-0.21) attributable deaths in Finland, respectively. In addition, actual PM2.5 emissions for year 2010 (42.4 t; fuels as in BAU) and an estimate for 2014 (19.8 t; fuels as in IBU) were used in an independent estimation of health impact, giving the result of 0.08 and 0.04 attributable all-cause deaths in Finland, respectively. Conclusions: Increasing the use of biomass from 4% to 49.5% of total fuel amount in Haapaniemi CHP plant may slightly reduce PM2.5 emissions. The used on-line energy balance tool gives comparable results to the actual measured values. From health point of view – when considering all-cause mortality due to PM2.5 emissions - replacing peat with biomass in large CHP plants seems neutral.