How much of our skin care or cosmetics are absorbed into our body?

Our skin is a complex organ that protects us from external environment. Our skin does not only protect us but it also regulates our body temperature, water loss, defense against microbes and repair. The skin is composed of epidermis the outer region and the dermis, the inner region.

The outermost top layer of epidermis is stratum corneum or horny layer of the skin. This layer functions as the main barrier to regulate the entry of external chemicals or microbes.  The epidermis can metabolize the chemicals, which can pass through the stratum corneum.  The permeability of the skin is inversely proportional to the thickness of stratum corneum.  Soles and palms have the least permeability, back, absomen, legs and arms skin have medium permeability, yet underarms and genital (perineal) areas have the highest permeability.  Stratum corneum thickness is less in newborns and children compared to adults. Therefore, children’s skin could absorb 40-50% more than adults.

 The inner region of our skin, dermis, provides physiological support to epidermis. Dermis has blood vessels, sensory nerves and the lymphatic system in the skin. Some chemicals or compounds that could not be metabolized by epidermis can pass into the blood stream (i.e. caffeine).

Skin care or cosmetic ingredients are absorbed into our body?

A study from University of Pittsburgh Graduate School of Public Health showed that the skin absorbed an average of 64% of the total contaminant dosage from total contaminants found in public drinking water (1). As we discussed above different parts of the body have different permeability depending on the thickness of the skin. For instance, it has been recommended a 100% absorption estimate for the permeability of underarms and genital areas with thin stratum corneum (2).  Skin on our face and eyes are thinner compared to skin on our arms or legs.  Of note, a study showed that skin on the face to be 2 to 6 times more permeable compared to other body surfaces. This change on permeability of face is due to various factors, which is discussed below.

Studies also demonstrate that the absorption of the skin changes depending on the compound. For example, caffeine is absorbed through the skin at 40-50%.  After topical application, caffeine was detected in the blood of the volunteers after 5 minutes reaching to maximum levels after 1 hour of exposure (3).

An effective anti-aging agent, niacinamide (Vitamin B3) has been shown to be absorbed into human skin (4). Soy isoflavones genestein and daidzein, weak estrogenic compounds used for anti-aging purposes are absorbed through stratum corneum and can reach viable layers of the epidermis and dermis.  Interestingly the lipophilic nature of stratum corneum could alter the absorbance of genestein and daidzein (5, 6).

Synthetic fragrances (i.e. with phthalates) are a point of concern due to systemic levels reached through inhalation as well as dermal absorption.  For fragrance ingredients, a commonly used approach in cosmetic chemistry is to assume 100% absorption (7).

What are the factors that change skin absorption?

The absorption through is dependent on the site of application on the body, the thickness of the skin at the site of application, the skin condition, and on the hydration state of the skin.

As I explained above, the thickness of the skin depends on the site on the body for example skin of underarms is thinner than legs or facial skin is less thicker than the skin on our arms.

Skin permeability can be enhanced by:

1. Physical factors: i.e. temperature, weather, sunlight,
2. Chemical factors: i.e. detergents, acids, alkalis, solvents,
3. Pathological factors: i.e. mechanical damage to the skin, skin diseases, allergies.

Our skin condition also have an important impact on the skin permeability. Disruption of the barrier function of the skin will alter the permeability of the skin and could result in increased permeation of chemicals (8).

Skin temperature can alter the chemicals’ absorbance (9). For instance, increased skin temperature has been shown to upregulate penetration rate of the chemicals (10). Increased skin temperature could occur with fever or during hot summer days with changing weather temperature.

Another interesting point is the reservoir function of the skin. During reservoir function, skin can act as a depot for some chemicals. Instead of passing completely through the skin, the chemical or substance could partially remain in the skin, which can behave as a reservoir. These chemicals in the reservois can be released or not released after a time (11,12). The continuation of reservoir can depend on the skin temperature, humidity, the nature of the chemical and the vehicle in which the chemical is found.

Summary

Overall, it is evident that what we apply on our skin could end up in our bodies. Therefore, it is more prudent for us to be more careful about what kind of products we use and pay extra attention to the ingredients in our skin care and cosmetic products.  More and more people prefer natural, nature derived or organic ingredients in their personal care and makeup items.  Selecting natural or organic ingredients immediately cuts out a lot of preservatives such as parabens, harsh detergents (i.e. SLS), synthetic fragrances that can have phthalates and synthetic coloring agents, which are suspected to have adverse effects on human health. Products with natural or organic ingredients require more work during formulation and preservation and avoid cost-saving short cuts due to the nature of the ingredients.  While it is possible not all nature derived ingredients are good for the skin, extra effort has been performed to select the best ingredients and obtain the most effective natural or organic products.

Dr.Ebru

Scientific References 

1-Brown HS, Bishop DR, Rowan CA. The role of skin absorption as a route of exposure for volatile organic compounds (VOCs) in drinking water. Am J Public Health. 1984 May;74(5):479-84.

2-Kasting GB, Miller MA. Kinetics of finite dose absorption through skin 2: volatile compounds. J Pharm Sci. 2006 Feb;95(2):268-80.

3-Otberg N, Patzelt A, Rasulev U, Hagemeister T, Linscheid M, Sinkgraven R, Sterry W, Lademann J. The role of hair follicles in the percutaneous absorption of caffeine. Br J Clin Pharmacol. 2008;65:488–92

4-Levin, J., Momin, S.B. How Much Do We Really Know About Our Favorite Cosmeceutical Ingredients? J Clin Aesthet Dermatol. Feb 2010; 3(2): 22–41.

5-Metfort I, Heilmann J. In vivo skin penetration studies of chamomile flavones. Pharmazie. 1994;49:509–511.

6-Saija A, Tomanio A. Influence of different penetration enhancers on in vitro skin permeation and in vivo photoprotective effect of flavonoids. Int J Pharm. 1998;175:85–89.

7-Robinson, M. K., Gerberick, G. F., Ryan, C. A., McNamee, P., White, I., and Basketter, D. A. (2000). The importance of exposure estimation in the assessment of skin sensitization risk. Contact Derm. 42, 251–259.

8-Wiechers JW. The barrier function of the skin in relation to percutaneous absorption of drugs. Pharm Weekbl Sci. 1989 Dec 15;11(6):185-98.

9-Bunge A & McDougal J (1999) Dermal uptake. In: Olin SS ed. Exposure to contaminants in drinking water: estimating uptake through the skin and by inhalation. Boca Raton, FL, CRC Press, pp 137–181.

10- Scheuplein RJ & Blank IH (1971) Permeability of the skin. Physiol Rev, 51(4): 702–747.

11- Roberts MS, Cross SE & Anissmov YG (2004) Factors affecting the formation of a skin

reservoir for topically applied solutes. Skin Pharmacol Physiol, 17(1): 3–16.

12- Vickers CF (1972) Stratum corneum reservoir for drugs. Adv Biol Skin, 12: 177–189.

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