You may have heard the shocking statistic that your skin absorbs around 60% or more of the chemicals it comes in contact with. Rest assured that this figure is completely false and your skin barrier is far more competent than the scaremongers would have you believe. However, we know that some chemicals can't be absorbed.
There is a large market for nicotine patches and other transdermal patches that deliver a controlled dose of medication through the skin and into the bloodstream. So what factors determines whether a particular chemical is absorbed, and should you be worried about cosmetic ingredients and chemicals getting through your skin?
The difference between skin penetration and absorbtion
Skin penetration and skin absorption are often used interchangeably, which may be where some of the confusion arises, but there are important differences between the two terms. Skin penetration means a molecule has passed through the outermost layer, the stratum corneum, or skin barrier. It may reach the lower levels of the epidermis, but it does not reach the bloodstream. When a molecule is absorbed it passes through the epidermis, into the dermis, and is absorbed into the bloodstream.
Journey through the skin
Imagine you could shrink down to the size of a water molecule. You’re around 0.27 nanometres tall and you’re about to try to cross the stratum corneum, which stretches out before you like a vast ocean, disappearing somewhere beyond the horizon. Remember that skin cells continually flake off and are replaced by new cells pushing up from below, so if you attempt this journey you will be swimming against the current the entire time. Too easy, you say? Well, speaking of skin cells, you’ll most likely take the intercellular path between the cells, so let’s fill the water with a few thousand cruise ships which you’ll have to navigate your way around. And don’t forget your immune cells, which patrol the waters like a bunch of ill-tempered great white sharks. Hopefully, you’re a strong swimmer.
Because of factors like molecular size, polarity and pH, add to this scenario the fact that most cosmetic ingredients are lousy swimmers. They might make it a few hundred metres before giving up and letting the current slowly carry them back out to shore. This is by design. Cosmetic ingredients are designed to work on the outer layer of skin, so ingredients are chosen that remain in this layer.
Assume you defied the odds and survived the treacherous 60 – 250km journey through the stratum corneum. You’ve successfully penetrated the skin, but don’t stop to rest just yet. From there it’s a mere 1,000km or so through the rest of the epidermis and the dermis to reach the bloodstream where you’ll have to overcome further challenges before being absorbed.
How chemicals penetrate the skin
he previous section makes crossing the skin barrier sound all but impossible, but some molecules are stronger swimmers than others. There are also ways to enhance penetration, like covering the application site with an occlusive dressing, which slows the current and helps molecules push against it¹, or with penetration enhancing molecules that disrupt the skin barrier just enough to allow a target molecule through (picture Moses parting the Red Sea). This is how transdermal patches are able to deliver medicines through the skin into the bloodstream, but even then it’s difficult and not terribly efficient². When it comes to the safety and regulation of cosmetic ingredients, regulators factor in their ability to penetrate the skin or be absorbed.
Dig a little deeper
Unfortunately, there is no onus of proof when it comes to making ridiculous claims like ‘your skin absorbs 60% of the chemicals it comes into contact with.’ Sometimes research gets picked up by someone who doesn’t fully understand it and distorted into a sensational and frightening headline that’s shared and believed without close examination. Often, as in this case, the claims seem to come from nowhere and have absolutely no basis in science. The fact is, very little is capable of penetrating the skin, and even less of being absorbed into our body. Before accepting the next sensational claim about the harmful effects of this or that chemical, we should all take a moment to consider the source of the information and dig a little deeper to find the whole story.
By Josh Townley, PhD.
Josh is a science writer with 10 years experience in the pharmaceutical and skincare world, first developing products in the R&D lab, then registering them in the regulatory department. He has a PhD in chemistry and a bachelor’s degree in forensic science.
1. Takahashi M, Machida Y, Marks R. Measurement of turnover time of stratum corneum using dansyl chloride fluorescence. J Soc Cosmet Chem 1987;38:321–31.
2. Prausnitz MR, Mitragotri S, Langer R. Current status and future potential of transdermal drug delivery. Nat Rev Drug Discov 2004;3(2):115–24.