Phthalates (DEP, DMP, DBP)
This group of chemicals is used in hundreds of products to increase flexibility and softness of plastics. They're mostly used as solubilizers in cosmetics and personal care products.
They're toxic, classified as "probable" carcinogens, and linked to many allergic reactions. They're also known to be endocrine disruptors and are associated with reproductive birth defects in males and females, as well as increased risk of breast cancer.
Banned in the European Union and Asia, they're still commonly used by the larger cosmetic companies in the US.
Since they're often added as a fragrance ingredient, they're not listed on labels. This is one of the loopholes allowing companies to avoid the disclosure of toxic ingredients in their products. One more reason to purchase products only from companies you know and trust.