Experimental studies have shown that aromatic compounds (essential oils) produced from trees can lower the production of stress hormones, reduce anxiety, and increase the pain threshold. When inhaled, aromatic plant essential oils increase the antioxidant defense system in the human body.
Studies have also shown an association between higher amounts of volatile aromatic compounds in the air and improved immune function. Specifically, higher levels of volatile aromatic compounds cause increased production of anti-cancer proteins in the blood as well as higher levels of the front-line immune defenders called natural killer cells (NK). Adults who have higher NK activity tend to have a lower frequency of colds and flu.