Yes...essential oils are used within aromatherapy. And yes, they can enter the bloodstream...such as with massage. Usually, aromatherapy is effective by stimulating the olfactory nerve which controls certain part(s) of the brain (if memory serves) but it is also used in massage oils, etc. that do indeed enter the blood stream topically. Therefore, there would be many instances where it could be unsafe...such as certain essential oils are contraindicated in pregnancy and for people with other conditions such as seizure disorders and so on. Therefore, always use with caution and as indicated and be aware of any health issues where it shouldn't be used. Having said that, aromatherapy is an awesome tool/resource when used correctly and safely. Hope this helps. 