Because of the high concentration of negative oxygen ions in the forest, negative oxygen ions are called "air vitamins" and "air longevity factors", which refer to oxygen ions with one or more negative electron charges, which are beneficial to human physical and mental health. It mainly affects people's physiological activities through their nervous system and blood circulation. Air is mainly composed of nitrogen, oxygen, carbon dioxide and water vapor. Nitrogen has no affinity for electrons, only oxygen and carbon dioxide have affinity for electrons, but the oxygen content is 700 times that of carbon dioxide. Oxygen molecules in the air combine with free electrons to form negative oxygen ions. The high concentration of negative oxygen ions in forest is due to the photoelectric effect formed by tree tip discharge and photosynthesis of green plants, which ionizes the air and produces a large number of negative oxygen ions.