Worldwide more than 230 million people suffer from asthma. Reliable and timely guidance for indi-viduals to minimize their risk for asthma attacks is not available. This is largely due to the fact that asthma symptoms are often caused by multiple environmental and personal factors. Many of them are neither captured nor systematically analysed. This is addressed by the project ActOnAir. It aims at a comprehensive capture of health factors and the environmental exposure of individuals, as well as a subsequent analysis in real-time. For this purpose the ActOnAir system provides a mobile sensor box for data collection, a sensor data integration and processing platform, a data mining component and a smartphone application for patients. This contribution outlines the design objectives of the ActOnAir system and discusses corresponding key requirements. The related system architecture is introduced and first results from a prototype implementation are sketched.