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Chemical Absorption Through The Skin
The skin is the largest organ of the body and acts as an interface with the surrounding environment. It is a semi-permeable structure and serves as a physical and protective barrier. Various factors influence its permeability such as age, site, keratin content, thickness. The physical condition of the skin at the point of external application is another significant variable.(1) The skin of an infant or child is more permeable than that of adults. The skin over the organs in decreasing order of permeability is genitals, head and neck, trunk, arm and leg.(2)
Apart from permeability, other factors which could influence absorption through the skin are concentration, duration of contact, solubility, etc. Being semi-permeable, the skin does not allow absorption of all the material that it comes in contact with, at the same rate. In general, fat-soluble and colloidal particles are absorbed better than water-soluble and crystalline solutions.
The absorption of a particular substance could be as high as 60% of the applied amount for certain substances such as salysylic acid.(3) On the other hand, it might not absorb other substances at all. In a study, commonly used topical sunscreens were found to be absorbed to the extent of around 14% but benzophenone-3 showed sufficiently high penetration.(4) The transepidermal penetration of 14C-benzoyl peroxide was studied in the rhesus monkey. Around 45% of the applied amount was recovered from the urine.(5)
Another substance present with the active ingredient also influences the rate and amount of absorption through the skin. Some molecules act as vehicle or carrier and enhance absorption.(6) Dimethyl sulfoxide is a frequently used carrier for transport of medication through the skin. There are certain substances that increase the barrier function of the skin and could decrease absorption. In a study, tea tree oil at 5% concentration was found to decrease the absorption of benzoic acid and methiocarb.(7)
Absorption through the skin could increase or decrease in disease conditions, inflammations or abrasions of the skin. Skin abrasion allows a locally applied substance to come directly in contact with subcutaneous tissue and blood vessels. Absorption is at a much higher rate than in healthy skin. Similarly, an inflammation leaves the skin leaky and allows larger molecules to be absorbed. If a medicine is unsafe for oral consumption and prescribed for external application, it could lead to systemic complications owing to the increased skin permeability.
Thus, the mechanism of absorption through the skin is complex and needs to be considered while prescribing a topical medication. To be safe, only those substances, which have no risk when taken orally, should be used for local application on the skin.
References
1. Katz M, Poulsen B J (1971). Absorption of drugs through the skin. In: Brodies B B, Gillette J R, eds. Handbook of Experimental Pharmacology, Concepts in Biochemical Pharmacology, vol. 28. New York: Springer-Verlag, chapter 7
2. Marks R M, Barton S P, Edwards C (1988). The Physical Nature of the Skin. Lancaster: MTP Press.
3. Taylor JR, Halprin KM. Percutaneous absorption of salicylic acid. Arch Dermatol. 1975 Jun;111(6):740-3.
4. Jiang R, Roberts MS, Collins DM, Benson HA. Marks R M, Barton S P, Edwards C (1988). The Physical Nature of the Skin. Lancaster: MTP Press. Br J Clin Pharmacol. 1999 Oct;48(4):635-7.
5. Nacht S, Yeung D, Beasley JN Jr, Anjo MD, Maibach HI. Benzoyl peroxide: percutaneous penetration and metabolic disposition. J Am Acad Dermatol. 1981 Jan;4(1):31-7.
6. Bronaugh RL, Franz TJ. Vehicle effects on percutaneous absorption: in vivo and in vitro comparisons with human skin. Br J Dermatol. 1986 Jul;115(1):1-11.
7. Topical use of tea tree oil reduces the dermal absorption of benzoic acid and methiocarb. Arch Dermatol Res. Nielsen JB, Nielsen F. 2006 Mar;297(9):395-402. Epub 2005 Nov 29.
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