Topical sunscreen agents
Based on their mechanism of action, topical sunscreens can be broadly classified into two groups, chemical absorbers and physical blockers. Chemical absorbers work by absorbing ultraviolet (UV) radiation and can be further differentiated by the type of radiation they absorb, UVA or UVB, or both UVA and UVB. Physical blockers work by reflecting or scattering the UV radiation.
The table below is a list of some of the common chemical absorbers available and the protection they provide against the UV range.
| UVA II
| UVA I
|Aminobenzoic acid derivatives|
|Octyl methoxycinnamate (octinoxate)||Complete||None||None|
|Ethoxyethyl p-methoxycinnamate (cinoxate)||Complete||None||None|
|Homomenthyl salicylate (homosalate)||Partial||None||None|
|Ethylhexyl salicylate (octyl salicylate/octisalate)||Complete||None||None|
|Other chemical absorbers|
|Avobenzone (butyl methoxydibenzoylmethane)||None||Complete||Complete|
|Ecamsule (terephthalylidene dicamphor sulfonic acid; Mexoryl SX)||Partial||Complete||Complete|
|Ensulizole (phenylbenzimidazole sulfonic acid)||Complete||Partial||None|
|Bemotrizinol (Tinosorb S)||Complete||Complete||Complete|
|Bisoctrizole (Tinosorb M)||Complete||Complete||Complete|
Chemical absorbing sunscreens often contain a combination of ingredients to get coverage against both UVB and UVA radiation. An individual compound may degrade another compound (eg octinoxate degrades avobenzone). Some chemical sunscreens are also combined with physical blockers.
Some chemical absorbers may degrade when exposed to sunlight, ie they are photo-unstable; and therefore may not perform as well as expected. Often these chemicals are mixed with other agents that enhance the stability of the overall sunscreen product. Octocrylene and bemotrizinol are often incorporated with other chemical absorbers because they are photostable and prevent the formulation from breaking down when exposed to the sun.
Another important property to consider is water resistance. No sunscreen is totally waterproof. In addition, the product can be rubbed off the skin surface, for example with a towel following bathing.
Physical blockers are effective at protecting against both UVA and UVB radiation. The two most common physical blockers are titanium dioxide and zinc oxide. These agents are the near ideal sunscreen as they are chemically inert, safe, and protect against the full UV spectrum. Their only drawback is their poor cosmetic appearance when applied to the skin. By decreasing the particle size, microsized or ultrafine grades have been developed, thereby reducing the whitening appearance. In some products, bright fluorescent colours have been added.
Physical blockers, titanium dioxide and zinc oxide, are also photostable and often formulated with less photostable chemical absorbing sunscreen agents. The reflection and scattering of the photons of ultraviolet by the physical blocker allows the chemical absorbing sunscreen to be more effective.
Antioxidants are sometimes used in sunscreens but have not been shown to be biologically active. The antioxidants degrade with time on the skin. When formulated on a sunscreen, they do not penetrate through the epidermis.
Do sunscreens affect hormones?
Studies in rats have expressed concern that application of large amounts and frequent application of oxybenzone may have endocrine effects. However, studies in humans have been reassuring with no evidence for endocrine effects in humans.
Rashes from sunscreens
Unfortunately, some people find that sunscreens irritate, and others develop dermatitis where they have applied them.
Sometimes this is because of generally sensitive skin (irritant contact dermatitis), at other times because of an allergic reaction to one of its components: this may be a fragrance, a preservative or a sunscreen chemical.
The cause can be difficult to work out, so if simply changing the brand doesn't solve the problem, ask your dermatologist for advice. He or she may organise patch tests and photopatch tests. Be careful to test a new product on a small area for a day or two before applying it widely.
- Edlich RF, Winters KL, Lim HW et al. Photoprotection by sunscreens with topical antioxidants and systemic antioxidants to reduce sun exposure. Journal of Long-Term effects of Medical Implants 2004;14(4):317-340. Medline.
On DermNet NZ:
- Sun protection
- Sun protective clothing
- How to choose and use sunscreens
- Sunscreen allergy
- Sunscreen testing and classification
- Australian Photobiology Testing Facility
- Sunscreens and photoprotection – Medscape Reference
- Sunscreens 2011 – EWG's Skin Deep
- AS/NZS 2604:2012 Sunscreen products – Evaluation and classification – Standards New Zealand
- Sunscreens – Australian Prescriber, October 2012
- Consumer medicine information and data sheets – Medsafe
- Drugs, Herbs and Supplements – MedlinePlus
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