phytoncides, antibacterial and antimicrobial substances that trees and other plants release into the air to help them fight diseases and harmful organisms. When humans breathe in these substances—typically by spending time in nature—their health can improve. Across several studies, phytoncides have been shown to boost immune function, increase anticancer protein production, reduce stress hormones, improve mood, and help people relax. 

Moreover, vagal nerve stimulation has been shown as an effective treatment for major depression [25,26], while relaxation techniques based on cardio-feedback are used for managing negative emotions and psychological symptoms [27,28].