To understand how substances in skincare products make their way into our system, it’s necessary to know the skin’s structure. According to Dr Chan Yung, a dermatologist at Apex Dermatology Institute in Tsim Sha Tsui, the skin is composed of several layers – the epidermis, the dermis (where the blood vessels, hair follicles and sweat glands are) and a deeper level called the hypodermis.

“The epidermis is the first barrier between us and the outside world,” Dr Chan says. “It is lipophilic, or oil-loving, and hydrophobic, or water-hating. Water cannot penetrate this layer, therefore water-based products such as toner or shampoo are unlikely to be absorbed by the skin – this is also why we can take a bath and swim. Oils, on the other hand, can penetrate this layer but not too deeply, as the epidermis contains a lot of water, and oil and water don’t mix. Therefore, to facilitate the absorption of oils into the skin, they must first be emulsified. This process involves combining oil and water using an emulsifier. Most serums, creams and lotions are emulsions.”

But, it’s a long journey for those ingredients to find their way past your skin. Furthermore, Dr Chan says that not all the substances in these products end up entering the blood vessels in your dermis. “Our skin contains various enzymes that may break down these foreign substances,” he says. “Certain factors, such as the size and solubility of these molecules also affect their absorption. In addition, some products, like sunscreen and antioxidants are designed to stay on the surface of the skin to remain active. In the end, some substances may make their way to the blood vessels in the dermis and then get absorbed into the bloodstream, but it’s a small amount that’s really quite difficult to quantify.”

The harmfulness of those absorbed substances depends on a few factors. Of course, if the ingredients are toxic and enter your system in large quantities, they can cause harm, but Dr Chan says the human body can filter out smaller quantities.

“Since some chemicals can enter your bloodstream through topical application, it’s best to avoid products that contain harmful ingredients such as parabens, sodium lauryl sulphate, a preservative called methylisothiazolinone and heavy metals,” says Dr Chan. “My advice is to read the label when buying skincare products – even organic or ‘chemical-free’ ones. Such descriptions may sound appealing, but some of these products may contain harmful preservatives.”