The Food and Drug Administration (FDA) includes the essential oil, oleoresin and extracts of lemon on the list of substances considered Generally Recognized as Safe (GRAS) for use in human food. Lemons are also on FDA's list of the 20 most frequently consumed raw fruits in the United States.

The safety of cold pressed lemon oil (Citrus Limon(Lemon) Fruit Oil, Citrus Limon (Lemon) Peel Oil) has been evaluated by the Research Institute for Fragrance Materials Expert Panel (REXPAN). Based on this evaluation, an International Fragrance Association (IFRA) Standard has been established. The IFRA Standard restricts the use of cold pressed lemon oil in fragrances because it contains some compounds that may cause skin effects when exposed to the sun.
Link to the IFRA Code of Practice:
http://ifraorg.org/en-us/standards_restricted/s3/p3

Botanical and botanically derived ingredients used in the formulation of cosmetics are generally mild and safe. Prior to marketing the finished cosmetic product, the safety of each ingredient must be substantiated in accordance with 21 CFR 740.10. Safety substantiation of cosmetic ingredients may include tests for ocular and skin irritation as well as allergenicity, phototoxicity, photoallergenicity and mutagenicity, depending on the application or intended use. There is a considerable body of information about the safety of botanical ingredients and a well established history of use. These resources are consulted to ensure the safety of these materials as they are used in cosmetics.