Plants and trees produce phytoncides that enhance the activity of special immune cells called Natural Killer cells (NK cells) as well as anticancer proteins. NK cells fight viral infections and destroy cancer cells. Studies have found that the immune boosting effects last for 7 days after trips to the forest.
It has also been shown that these aromatic plant molecules can decrease the production of stress hormones and enhance production of neurotransmitters involved with cognitive function.
Interestingly, you can get the same immune boosting effects from aromatic phytoncides from essential oils…. I think it is much nicer to get it from a real walk in nature, but perhaps if you don’t have time or the weather is not nice then essential oils might be a cool alternative.