Airborne particulate matter (PM10) samples were collected daily, indoors and outdoors, in a primary school at Aveiro, Portugal, from February 28 to May 27, 2011. The carbonaceous content (organic and elemental carbon) was determined by a thermo-optical technique. The organic speciation of PM10 was performed by gas chromatography-mass spectrometry. Mean PM10 levels of 107 and 36 µg/m3 were obtained in the schoolroom and outdoors, respectively. On average, organic carbon accounted for 30.0% of the mass of PM10 indoors, whereas a lower mass fraction of 21.3% was found outdoors. The lack of correlation between indoor and outdoor organic carbon and the much higher indoor levels suggest significant contributions by indoor sources. The most abundant organic compound classes were acids, sugars, polyols and n-alkanes. Infiltration of outdoor particles leads to contamination of the schoolroom with vehicle emissions, as well as emissions related to the burning of biofuel in nearby restaurants and bakeries. However, the much higher indoor concentrations than in the outdoor air for the majority of compounds suggest that the origin of much of the particulate matter is from within the school building (due to school activities and materials, skin debris, microorganisms, and so on), and this also includes the formation of secondary organic aerosols. Based on the concentrations of polycyclic aromatic hydrocarbons, a negligible cancer risk was estimated in relation to the air within the school.