A system determines a risk level of a respiratory attack of a user of the system. The system comprises part of a network of systems, and it reports information to, and receives information from, the network. The system comprises a local activity monitoring device for tracking the physical activity level of the user. Location information and activity levels in respect of other users of the networkof systems are received, relating to when they have suffered a respiratory attack. A warning can then be given to the user which takes account of the location of the user and the activity level or planned activity level of the user, and the received location information and activity levels in respect of other users.