We know that our skin is our biggest organ. The skin is grouped into several layers, the epidermis, the dermis, and the hypodermis. These layers act as barriers to the blood vessels located under the hypodermis layer. The skin does a good job in protecting what makes it to the blood vessels, but chemicals still get in.
Penetration rates of chemicals into the skin are affected by the body part they are applied to. The skin on the soles of our feet is very thick, whereas the skin on our eyelids is very thin. Absorption rates on our face and scalp are 5-10 times higher than on other parts of the body. Absorption also depends on molecule size, chemical solubility, the means in which its transported (cosmetic), and whether or not the chemical reacts with the enzymes in your skin.
Enzymes in the skin can activate and inactivate many drugs and foreign compounds. This means our skin protects us from exposure to certain chemicals while actually intensifying the effects of others. There are actually enzymes that activate chemcials and make them more toxic!
You have likely heard the saying, you are what you eat. And the same theory holds true for your skin. This is why it's so important to read labels and become educated about what ingredients to avoid when selecting cosmetics.