Essential oils have high concentrations of lipophilic compounds which have little trouble crossing membranes. As luck would have it chapter 7 "The safety issue

Essential oils have high concentrations of lipophilic compounds which have little trouble crossing membranes. As luck would have it chapter 7 "The safety issue in aromatherapy" of Aromatherapy Science (ISBN: 0 85369 578 4) is a freely available sample chapter (pharmpress.com)