### Current State of Knowledge on Particulate Matter Emissions from Road Transport
The present study concerns an assessment of the effects of the motor vehicles category under specific traffic conditions on the emissions of different particulate matters. Therefore, the aim of this study is to investigate the environmental risks of particulate matter from road transport on the basis of the official results of the pollutant emission inventory, in accordance with the methodology recommended by EMEP/EEA (European Monitoring and Evaluation Programme/European Economic Area) [8]. A recommended tool in the form of the COPERT 5 software was used to determine the national annual emission of dust from road transport [7]. The authors decided to verify the research hypothesis in regards to the significant impact of traffic inside and outside cities, as well as on highways and expressways, on particulate matter emissions. The second research hypothesis concerns a significant share in the emission of particulate matters from sources other than the exhaust systems of internal combustion engines. Methodology for Studying Environmental Risks of Particulate Matter from Road Transport Based on Emission Inventory  
The study of environmental hazards associated with the emission of road transport particulate matter was carried out on the basis of the results of the official inventory of pollutant emissions from road transport in Poland in 2018, prepared by the National Centre for Emissions Management (KOBIZE), the Institute of Environmental Protection--National Research Institute [29]. The inventory of emissions from road transport was carried out in compliance with the European Monitoring and Evaluation Programme (EMEP)/European Environmental Agency (EEA) methodology [8] with the use of COPERT 5 software [7]. Motor vehicles are classified into categories. In a philosophical sense, a category (comes from the Greek \(\kappa\alpha\pi\gamma_{0}\rho\ell\
u\)--kategorie--to adjudge) is a concept introducing a structure: a class of objects having certain interrelated characteristics. The motor vehicle categories are divided according to various criteria, primarily [1; 24; 3; 4; 7; 8; 11; 24; 29]:  
* motor vehicle purpose of use;
* motor vehicle conventional size and propulsion engine;
* characteristics of the motor vehicle and its propulsion engines as regards, among others, the engine cycle, detailed technical solutions and performance, primarily with respect to pollutant emissions;
* power fuel for the motor vehicle's internal combustion engine;
* technical level of motor vehicle and its propulsion engine. The elementary category of road vehicles [1; 24; 3; 4; 24] comprises motor vehicles that all share the same essential characteristics (e.g., passenger cars with a compression-ignition engine with a displacement of more than 2 dm3, Euro VI stage, powered by diesel fuel). The cumulative category of road vehicles [1; 24; 3; 4; 24] comprises motor vehicles that do not all share the same essential characteristics (e.g., passenger cars with spark ignition engines). The most cumulative motor vehicle category includes all motor vehicles. According to the procedure used in the inventory of pollutant emissions from road transport, national annual emissions of particular pollutants are determined for the elementary categories of motor vehicles. The subject of research presented in this study is the emissions recorded for the cumulative categories of motor vehicles, including [1; 24; 3; 4; 7; 8; 11; 24; 29]:  
* passenger cars;
* light commercial vehicles;
* heavy duty trucks;
* buses;
* mopeds and motorcycles (and microcars and quads). An example of differentiating motor vehicles according to detailed characteristics is the division of the category of buses into city buses and coaches, and the category of trucks into rigid and articulated. Due to the cycles of the internal combustion engines, engines are divided into two-stroke and four-stroke.