If an individual patient uses these oils and feels better, there's no way we can prove it doesn't improve that person's health.
The study, published in the medical journal, Psychoneuroendocrinology, tested the idea that pleasant aromas not only make us feel good, but also have a positive impact on health. While an entire industry has been built upon this hypothesis, it has been rarely subjected to rigorous scientific examination.
"We all know that the placebo effect [the idea that an inactive agent such as a sugar pill may trigger the same beneficial effects of an active agents] can have a very strong impact on a person's health; but beyond that, we wanted to see if these aromatic essential oils actually improved human health in some measurable way," explained Janice Kiecolt-Glaser, professor of psychiatry and psychology at Ohio State University and lead author of the study.
The study focused on lemon and lavender because they are the most popular scents used in aromatherapy. It used 56 healthy volunteers, who were tested to make sure they had a working sense of smell.
Each volunteer took part in three half-day sessions, during which they were exposed to both scents. Participants were monitored for blood pressure and heart rate during the experiments, and the researchers took regular blood samples. Researchers tested the volunteers' ability to heal by using a standard test where tape is applied and removed repeatedly on a specific skin site. The scientists also tested the volunteers' reaction to pain by placing their feet in freezing water. Lastly, volunteers were asked to fill out psychological tests to measure mood and stress levels three times during each session.
The blood samples were later analyzed for changes in biochemical markers that would indicate any affects on the immune or endocrine system. Levels of two cytokines — compounds that play a key role in immune response — were checked, as were those of stress hormones such as cortisol and norepinephrine.
The results: While lemon oil put subjects into a better mood, lavender oil did not, the researchers said. And neither smell had any affect on stress, pain control or wound healing.
"This is probably the most comprehensive study ever done in this area, but the human body is infinitely complex," explained William Malarkey, professor of internal medicine at Ohio State, "If an individual patient uses these oils and feels better, there's no way we can prove it doesn't improve that person's health.
"But we still failed to find any quantitative indication that these oils provide any physiological effect for people in general."