Lemon oil is squeezed from the peel, which is why it smells like zesting a lemon rather than like lemon squash. Up to seven tenths of it is limonene, with a trace of citral doing most of the actual lemon smell. Limonene is one of the better-studied fragrance compounds and has a long safety record in food and cosmetics.^[1] The catch is that pressed peel oil carries bergapten, which makes skin burn in sunlight.^[2] Steam-distilled lemon oil has none of it, so check which one you have before putting it on skin.