By burning too-dry fuel at a high load, modern boilers increase their PM\({}_{2.5}\) emissions 4-5 times compared to a nominal load and standard fuel. At the present time, emission factors (EF) for fine inhalable particles, with diameters that are generally 2.5 micrometres and smaller than PM\({}_{2.5}\), are being monitored. Older published data reported the emission factor for dust particles (without size limitation). The difference between the emission factors for dust particles and PM\({}_{2.5}\) particles was observed by [PERSON] et al. [9]. The emission factor values for PM from logwood combustion are in the range of 0.71-6.2 g/kg with an average value of 2.1 \(\pm\) 1.3 g/kg, and for PM\({}_{2.5}\) particles, they are in the almost identical range of 0.5-6.0 g/kg with an average value of 1.8 \(\pm\) 1.3 g/kg. Measurements from Norway, where the highest wood store emissions factor for birch and spruce was found to be 36 g/kg [14], demonstrate a high variability of emission factors when burning wood biomass. Under standard conditions and at a steady state (nominal load), the emissions from the combustion of PM\({}_{2.5}\) reach up to 84 mg/MJ (1.34 g/kg of fuel for 16 MJ/kg in gross calorific value). This value increased to 105 mg/MJ when the ignition phase is included in the measurement. During burning wet log fuelwood, the value increases by 27% to 568 mg/MJ [10]. Differing values of emission factors have been published. For example, according to the EMEP/EEA Guidebook [15], the emission factor ranges are as follows: for modern boilers (advanced or eco-labelled) the PM\({}_{2.5}\) average value is 93 mg/MJ ranging from19 to 33 mg/MJ, for traditional boilers 470 (235-940) mg/MJ, pellet boilers 29 (9-47) mg/MJ, and for older boilers 740 (370-1480) mg/MJ, equivalent to approximately 23.6 g/kg of fuel with a gross calorific value of 16 MJ/kg [15]. For old boilers, [PERSON] et al. [16] report an even higher emission factor value for dust particles (73-2200 mg/MJ). The PM emission factor is significantly higher in briquette combustion than with fuelwood, the value of which is 7.76 \(\pm\) 6.02 g/kg [17]. The PM emission factor for the combustion of bituminous coal briquettes ranges from 0.17-26 g/kg to 0.13 g/kg for PM\({}_{2.5}\). When burning bituminous coal briquettes,more significant differences were found for the average emission factor value for PM (3.5 \(\pm\) 6.5 g/kg) and for PM\({}_{2.5}\) 7.7 \(\pm\) 5.8 g/kg [9]. A substantially higher emission factor for bituminous coal (14.4 g/kg) is reported by [PERSON] [11]. The EF value for anthracite coal was reported to be only 0.5 g/kg, which is related to a low volatile flammable substance content and a high fixed carbon content, which stabilised combustion during the heating period [18]. The EF PM\({}_{2.5}\) values for old boilers indicate that the amounts of particles produced may be higher during burning biomass than when burning bituminous coal [9; 16]. Emission factors represent a significant step for an objective comparison of the technical parameters of the combustion equipment for standardised fuels. When burning biomass, some elements go into a gaseous phase depending on the combustion temperature; therefore, they are not included in the EF. The resulting environmental load of burning biomass will be higher. During wood combustion, 85-100% of Cl and approximately 50-70% of S are already released at 500 \({}^{\circ}\)C into the gaseous phase [19]. The release of Na, K, Zn, and Pb is influenced by the type of biomass and temperature. Zn and Pb start to be released at 500 \({}^{\circ}\)C, and at 850 \({}^{\circ}\)C, more than 65-85% of the total element content is released into the air [20]. When burning biomass, amounts of trace elements are released in the following order: As \(>\) Cd, Pb, Zn \(>\) Cu [20]. Of the major elements, 70-90% of K and Na are released at a higher temperature (800-1050 \({}^{\circ}\)C) [19]. At 1000 \({}^{\circ}\)C, 94 to 99% of K and 97-99% of Na are released depending on the original concentration of the element in biomass [21].