One of the most important concepts behind the Scientific Method is called falsifiability.  A claim is only legitimate if it could be proven false (but is not).  

For example, if I say "I have proven that vitamin Q prevents heart disease," that is a falsifiable claim.  It would be relatively easy to prove that the mythical vitamin Q does NOT prevent heart disease.  This makes my claim legitimate, assuming that I can make my case of course.

However, if I say "Vitamin Q was planted on Earth by space aliens two million years ago," that is NOT a falsifiable claim.  There is no way to prove that vitamin Q was NOT planted here by space aliens.  Since this claim cannot be proven false, it has no scientific legitimacy.

Next, any medical claim has to overcome the placebo effect.  The placebo effect is amazingly powerful, between 30 and 60 percent depending on the topic.  A placebo pain killer can be effective for 60% of the people who take it.  A placebo antibiotic may "only" be effective for 30% of the people who take it.

This is why scientific studies have to use a control group.  In order to rule out the ridiculously huge placebo effect.  A control group gets treated the same way as the real test subjects, but they don't get the actual medicine.  For example you may have 100 people in your study, 50 of whom get an aspirin, and 50 of whom get a sugar tablet that looks just like an aspirin.  That way you can compare the results of the aspirin compared to the results of the placebo.

On to aromatherapy, then.  The New York Times has an article about aromatherapy bars which hovers between "gushing" and "skeptical."  To her credit, the author clearly raises an eyebrow at some of the claims made for aromatherapy.

Aromatherapy has two big problems: it's not falsifiable, and there's no way to rule out the placebo effect.

If I tell you that "the smell of peppermint is invigorating," there's no way to falsify that claim.  You can't prove (in any meaningful fashion) that I am not being invigorated.  Mostly because terms like "invigorated" are so very vague.  How do you measure peppiness, or a sense of purpose?  You can't, which means that you can't measure if they change - or not.

Aromatherapy is also unique in that there is absolutely no way to form a control group.  You can't have one group of people smell peppermint, and another group of people smell something that they think is peppermint-scented but really is not.  You can't trick someone into thinking they're smelling peppermint oil.  You either are or you aren't.

As a third nail in the coffin, most of aromatherapy's claims relate to conditions (like stress, and happiness) which are extremely susceptible to the placebo effect.  I bet if I told you that some other pretty smell (freshly cut grass, or pine needles) reduced stress, you'd find those smells relaxing.  Just calling something "relaxing" is enough to make it so.

All of which isn't to say that you shouldn't bother with pretty smells.  I have a whole collection of perfumes and essential oils.  They smell pretty!  What's not to like?  (In fact, I applied a sample of Bond No. 9 "Chinatown" before I started writing this.)  But don't get suckered in by any claim stronger than "it smells pretty."

Photo credit: Flickr/Crystalwood Naturals