Atmospheric pollution poses a significant threat to human health. These effects on human health are well documented in outdoor environments. However, in the Western world, people currently spend most of their time indoors. Therefore, studying the relationship between outdoor and indoor pollutant concentrations is essential. In this study, outdoor and indoor concentrations were measured in an unoccupied, mechanically ventilated building in Finland equipped with air-filtration and heat-recovery systems. Sampling was performed using a switching valve system that alternated sampling location between the building's air intake before filtration and the exhaust air vent, enabling near-simultaneous measurement of indoor and outdoor pollutant concentrations. The measurement campaign was conducted between 13^th and 27^th of February 2025. During the measurement period, a strong long-range transport (LRT) episode was observed, providing a good opportunity to study the effects of LRT on indoor pollutant concentrations. The measured variables included the chemical composition of aerosol particles, black carbon concentration, particle mass and number concentrations, and particle size distributions. Also, the effect of the nearby airport on the pollutant concentrations was discussed. The airport was found to have a large effect on particle number concentrations but almost no effect on particle mass or black carbon concentrations. The most abundant chemical component of aerosol particles both indoors and outdoors was organic matter. However, outside, the second most abundant component was nitrate, while indoors, more sulfate was measured. For the measured variables, the indoor-outdoor (I/O) ratios were calculated under normal and LRT conditions. Most of the I/O ratios remained stable between normal and LRT conditions. Exceptions to this were particulate matter and black carbon concentrations, which had higher I/O ratios during normal conditions, while particle number concentration had notably higher I/O ratios during LRT. The notably higher IO ratio of particle number concentrations was most likely due to the increase in the geometric mean diameter of particles. Notably, the ratios varied between the different measured components. The lowest I/O ratio of 0.20 ± 0.08 was observed for nitrate during the LRT.  The low I/O ratio of nitrate might be related to the increasing temperature of aerosol particles while being transported indoors and therefore to increased evaporation. Under LRT conditions, the indoor black carbon concentrations increased to levels similar to those at street canyon measurement sites in Finland. The nearby airport was found to have a notable effect on the particle number but not particle mass concentrations or BC concentrations.