The paper presents preliminary analysis of measurements of the mass concentrations of PM2.5 in the indoor environment, obtained with the use of two optical instruments: (Grimm device and low-cost sensor) and a sampler based on the gravimetric method (SKC). It was found that the measurement of PM2.5 using an optical device with active sampling underestimates the actual mass concentration of this mode (PM2.5), while measurement using an optical device with passive sampling of air overestimates the concentration of PM2.5. It has been shown that the physical relationship between the mass concentrations of airborne particles obtained with an optical sensor (