Fragrance blends are developed in laboratories by professional fragrance designers. They carefully select essential oils and other chemical compounds that sound harmonious together, and then determine the ideal percentage of each. Aroma oils can contain carrier oils, esters, alcohols, resins, and essential oils. Although they are not natural, they still retain the qualities of essential oils, which inherently have a stronger and more persistent odor with therapeutic benefits. Fragrance blends can be completely synthetic or contain up to 80% natural ingredients.