The so-called forest bath refers to the inhalation of fresh air in the forest. Many respiratory diseases stem from a deficiency of oxygen and air pollution, and these phenomena may cause heart problems. The air in the forest is clean, moist and has a higher oxygen content. In addition, several volatile substances excreted by trees stimulate the cerebral cortex and relieve nerve tension. Some trees, such as pine, cypress, lemon trees and eucalyptus, also secrete substances that kill bacteria. Forests also contain higher levels of negative oxygen ions. The forest bath is best during the day because the oxygen content is higher and the dust level is much lower inside the forest.