It has been claimed by alternative medicine that the human body is oxygen-deprived, and that oxygen will remove "toxins" and even cure cancer. Proponents claim this practice is not only safe, but enhances health and well-being, including strengthening the immune system, enhancing concentration, reducing stress, increasing energy and alertness, lessening the effects of hangovers, headaches, and sinus problems, and generally relaxing the body. It has also been alleged to help with altitude sickness. However, no long-term, well-controlled scientific studies have confirmed any of the proponents' claims. Furthermore, the human body is adapted to 21 percent oxygen, and the blood exiting the lungs already has about 97 percent of the oxygen that it could carry bound to hemoglobin. Having a higher oxygen fraction in the lungs serves no purpose, and may actually be detrimental.