Wait, fragrance? Isn't that a little general? Unfortunately, yes. The soap market is a cutthroat place, and companies are cagey about revealing the ingredients that make their formulas smell just right. When you see fragrance listed as an ingredient on a skin care product, you're looking at a top-secret mix of esters, ketones, aldehydes, amines and more. This makes it difficult to construct allergy tests for fragrance because, in North America in particular, we don't even know what the ingredients in most fragrances are.
Even though fragrance doesn't actually contribute to skin cleansing, it's one of the most common contact allergens in soap. Furthermore, fragrance allergens are found in just about any cosmetic product that doesn't carry a "fragrance-free" label. And because the cosmetics industry (which is largely self-regulated in the United States) is pretty secretive about its formulas, the estimated range of cosmetic products that contain the fragrance allergens used for skin patch testing is anywhere from 15 percent to all of them [source: Storrs].