Among 17 homes in Switzerland, the relationship between indoor and outdoor levels for particulate matter less than 10 microns (PM10), particulate matter less than 2.5 microns (PM2.5) and NO2 was investigated. In 10 homes, the inhabitants also participated in conducting personal measurements. All homes were naturally ventilated. In homes without any indoor sources and where human activity was low, PM10 I/O ratios amounted to approximately 0.7. Of the indoor sources, smoking had the highest influence on I/O ratios (> 1.8). In homes not containing any apparent source, 'human activity' was an important factor accounting for high indoor levels. However, this factor is difficult to quantify. Personal exposure was in most cases higher than the indoor and the corresponding home outdoor levels. For NO2, gas-cooking was found to have the strongest influence on elevated I/O ratios (> 1.2) whilst for other homes, the I/O ratio was less than 1.