Outdoor particulate matter (PM10) is associated with detrimental health effects. However, individual PM10 exposure occurs mostly indoors. We therefore compared the toxic effects of classroom, outdoor, and residential PM10. Indoor and outdoor PM10 was collected from six schools in Munich during teaching hours and in six homes. Particles were analyzed by scanning electron microscopy and X-ray spectroscopy (EDX). Toxicity was evaluated in human primary keratinocytes, lung epithelial cells and after metabolic activation by several human cytochromes P450.We found that PM10 concentrations during teaching hours were 5.6-times higher than outdoors (117 ± 48 lg/m3 vs. 21 ± 15 lg/m3, P < 0.001). Compared to outdoors, indoor PM contained more silicate (36% of particle number), organic (29%, probably originating from human skin), and Ca-carbonate particles (12%, probably originating from paper). Outdoor PM contained more Ca-sulfate particles (38%). Indoor PM at 6 lg/cm2 (10 lg/ml) caused toxicity in keratinocytes and in cells expressing CYP2B6 and CYP3A4. Toxicity by CYP2B6 was abolished with the reactive oxygen species scavenger N-acetylcysteine. We concluded that outdoor PM10 and indoor PM10 from homes were devoid of toxicity. Indoor PM10 was elevated, chemically different and toxicologically more active than outdoor PM10. Whether the effects translate into a significant health risk needs to be determined. Until then, we suggest better ventilation as a sensible option.