Mechanism of skin absorption

When a substance is applied topically, for it to enter the bloodstream, it must first penetrate the outermost layer of the skin, the stratum corneum, which is composed of tightly packed, hydrophobic dead skin cells. Examples of ingredients that can penetrate this layer are many, a few common examples are Sodium Lauryl Sulfate, Formaldehyde, Propylene Glycol, Phthalates, Petroleum..the list goes on. 

Let's pick one example of an innocent sounding ingredient - Mineral Oil: A byproduct of petroleum that many products use as a base. Mineral oil is carcinogenic, immunotoxic, and allergenic and can accumulate in the body and cause long-term health issues. 

Once the substance penetrates the stratum corneum, it moves through the deeper skin layers (epidermis, dermis, and subcutaneous tissue) via passive diffusion, driven by the concentration gradient from the higher concentration on the skin surface to the lower concentration in the bloodstream.

In the dermis, the substancel is absorbed by capillaries and lymphatic vessels, which transport it into the systemic circulation, allowing it to reach target tissues and organs.

Before entering the bloodstream, the chemical may undergo skin metabolism, where enzymes in the skin (e.g., N-acetyltransferase) modify its structure. This can change its bioavailability or alter its effects.

Factors influencing absorption include molecular size - smaller molecules diffuse more easily; lipophilicity- lipophilic chemicals penetrate the stratum corneum more effectively than hydrophilic ones; and skin factors like skin thickness, hydration, and integrity affect absorption rates.

This process enables some unwanted substances to reach the bloodstream often causing systemic and long term effects and accumulation.

Deodorant is one of the worst offenders here because it is applied around hair follicles, sweat glands and sebaceous glands, which act as "shortcuts" through the stratum corneum coupled with the fact that they often contain aluminium and some other nasty offenders. 

Look at your labels carefully. There are many apps where you can check the ingredients of your products.