Usability Engineering and E-Health participant of stress. The system presented a 3D immersive video with an audio narrative. The participants' heart-rate was monitored in addition to their respiration and their subjective opinion through questionnaires. From their measure- ments they found that the VR e-health and audio narrative did effect their emotions in a positive manner. Rizzo et al. (2008) describe the use of a VR e-health system to train soldiers to deal with the stress of war by providing a realistic environ- ment to expose them to the stress levels before going out to Iraq. As Rizzo et al. have pointed out the emotional state of the soldier can affect their decision making process under pressure which could have fatal consequences. The VR e-health system would habituate them to the environment to minimise their stress levels. Virtual environments can also have a social aspect with interaction with other users, for ex- ample Second Life (http://secondlife.com/). Such et al. (2008) are also developing a system for the treatment of addiction where they interact with other sufferers and are taught healthy behaviour (or finding alternatives to their addiction). It has been suggested that virtual humans in a VR e-health environment can produce the same social effects as when interacting with a human. Park and Catrambone (2007) conducted a study whereby they observed whether `social facilita- tion' is present with a virtual human as well as a real human. Social facilitation is where generally individuals do an easy task when another person is nearby better than when they are on their own. Oppositely when people do a hard task and an- other person is nearby they generally do that task less well 16 than when they are on their own. The authors found that when a virtual human is present real humans also react in the same way as if the virtual human was real in terms of social facilitation. The usability of systems can also be