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The paper presents a comparison of outdoor and indoor air pollution by means of magnetic properties of dust gathered outdoors and indoors on air filters. Outdoor dust (particulate matter smaller than 10 μm, PM10) was collected by means of a PM sampler that was placed in the yard of the building of the Institute of Geophysics, Polish Academy of Sciences (IG PAS), in Warsaw, Poland. Indoor dust samples were collected with the same type of PM sampler, placed inside this building. The magnetic methods were applied for the estimation of concentration and magnetic mineralogy of the magnetic particles present in PM. Magnetic measurements, such as low and high-temperature dependence of volume magnetic susceptibility κ(T), hysteresis parameters, and frequency dependence of mass magnetic susceptibility χfd show that outdoor and indoor PM contain ultra-fine-grained magnetite-like minerals, particularly in a size range < 30 nm. This pointed to the common outdoor sources of pollution intruding inside the building through windows, ventilation systems, and partly carried on dresses by people. The mass of PM10 collected outdoors is of the same range as the mass of PM10 from inside the building. The mass susceptibility of indoor filters, however, is about 1/3 of the mass susceptibility of outdoor filters. The relationship between volume susceptibility and mass indicates a relatively homogenous composition of indoor dust but a varying composition of outdoor dust, with a variable influx of nonmagnetic exogenous particles. • Magnetic susceptibility was much lower for the indoor PM than for outdoor. • Both sources contained mainly magnetite with higher concentration outdoors. • Only indoor PM showed a linear correlation between susceptibility and mass. •. [ABSTRACT FROM AUTHOR]