Hot-spring water is one of Iceland's primary natural resources. It is used for space heating, to produce electricity and is utilized in various industries, including greenhouse farming. Icelanders were quick to realize the medicinal benefits of thermal water, which is partly responsible for their longevity and good health. One of the main reasons for this longevity is because Icelanders regularly visit the pools to improve their overall well-being. Swimming in thermal water under a bare sky while breathing in clean, fresh air is one of the best exercises available anywhere. This is particularly true for asthma and heart patients. And since swimming strengthens the body and makes all physical movements much easier, it is especially suitable for older people, children and teenagers. Most important, however, is the delightful feeling one gets from moving and stretching in a pool that is kept at a constant temperature of 29°C (84°F). Surveys show that 94% of foreigners who visit the thermal baths and pools in Reykjavík said that the experience had a positive effect on their health and well-being.