The oil found in a plug-in air freshener is typically a type of scented fragrance oil. Fragrance oils are made with a combination of ingredients which are primarily synthetic or man-made. Plug-in fresheners made by smaller, independent fragrance companies may contain natural essential oils.
Fragrance oils are less expensive than essential oils which probably explains why most commercially available plug-in air fresheners are made with fragrance oils. Since some fragrances are not extractable from nature, fragrance oils afford the creation of a wider variety of olfactory-pleasing scents than essential oils.
The U.S. Food and Drug Administration (FDA) permits specific ingredients to be kept secret to protect fragrance oil producers from having their scents copied. Basic ingredients commonly found in fragrance oils include carrier oil, scented alcohols, scented aldehydes (organic compound) and scented esters (complex compound). According to S.C. Johnson & Son, Inc., their Glade® Plug-Ins® Scented Oil air fresheners contain fragrance oils "infused" with real essential oils.
Essential oils are extracted from the roots, leaves, stems, flowers, bark or other parts of specific plants, most commonly through a distillation process using water or steam. They are highly concentrated and must be added to carrier oil. Essential oils such as lavender, nutmeg, and rose are often used for aromatherapy and other therapeutic applications.