What is a topical sunscreen agent?
Sunscreen can be applied to the skin to protect against ultraviolet (UV) radiation.
Ultraviolet A (UVA) (320–400 nm) makes up 95% of the UV radiation detected on the earth’s surface. UVA leads to photoageing, hyperpigmentation, and may contribute to the development of skin cancer. UVA is divided into UVA I and UVA II. UVA I (340–400 nm) is less potent than UVA II (320–340 nm).
Ultraviolet B (UVB) (280–320 nm) exposure can cause sunburn, skin cancer, hyperpigmentation, and skin inflammation. UVB makes up 5% of UV radiation reaching the Earth’s surface.
Ultraviolet C (UVC) (100–280 nm) in sunlight is filtered out by the earth's atmosphere and does not reach the surface. The wavelength of visible light si
Classification of topical sunscreen agents
Topical sunscreens can be broadly classified into two groups; chemical sunscreens and physical blocking sunscreens.
- Chemical sunscreens (also called organic filters) work by absorbing UV radiation.
- Physical blockers (also called inorganic filters) reflect visible light, and were previously believed to also reflect or scatter the UV radiation. Studies have now shown physical sunscreens absorb UV radiation.
Chemical sunscreen agents
Chemical sunscreens often contain a combination of ingredients to provide coverage against both UVB and UVA radiation.
Some chemical absorbers may degrade when exposed to sunlight (ie, they are photo-unstable) and may not perform as well as expected. These chemicals are therefore mixed with other agents that enhance the stability of the overall sunscreen product. Octocrylene and bemotrizinol, for example, are often combined 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 sunscreen agents
The two most common physical sunscreens are the metal oxides titanium dioxide and zinc oxide. They reflect photons in the visible light range, but work as chemical filters in the UV range. Titanium dioxide absorbs up to 400 nm, and zinc oxide up to 370 nm. 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 (nanoparticles) have been developed, thereby reducing the whitening appearance and increasing their effectiveness as UV absorbers.
Titanium dioxide and zinc oxide are also stable in sun and are often formulated with photo-unstable chemical absorbing sunscreen agents.
|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|
|Inorganic metal oxides|
|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|
What are the benefits of wearing a topical sunscreen agent?
Sunscreen is particularly useful for those with fair skin such as skin phototypes I, II, and III.
The benefits of wearing a broad spectrum sunscreen include:
- Avoidance of sunburn
- Reduction in risk of skin cancer
- Reduced hyperpigmentation
- Reduced photoageing
- Avoidance/reduced aggravation of photosensitive dermatoses.
Are topical sunscreen agents safe?
Physical sunscreens such as titanium dioxide and zinc oxide have an excellent safety profile. They have not been found to be systemically absorbed even when used as nanoparticles.
Chemical sunscreen ingredients have been found to be systemically absorbed. Avobenzone, oxybenzone, octocrylene, ecamsule, homosalate, octisalate, and octinoxate have all been detected systemically after regular use. There are no known harms from these ingredients being systemically absorbed; however, it does need to be further studied to confirm this.
Studies in rats have expressed concern that the 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.
Vitamin D deficiency is a concern for some with regular use of sunscreen. This has not been shown to be an issue with real life use of sunscreen in randomised controlled trials. Vitamin D supplements can be safely and easily taken if this is a concern however.
Oxybenzone and octinoxate are believed to impair coral reef growth and cause coral bleaching. Hawaii passed a bill in 2018 banning these sunscreen ingredients.
Are topical sunscreen agents safe for infants?
Infants younger than 6 months should avoid using sunscreen unless they cannot be protected by other means such as shade and clothing. Infants have a thin, less developed skin barrier and therefore are more likely to become irritated or sensitised by sunscreen ingredients as well as possible increased systemic absorption.
Physical sunscreens, such as titanium dioxide and zinc oxide, are preferred in children as they are not irritating, provide excellent broad-spectrum protection, and are not systemically absorbed.
What are the side effects of topical sunscreen agents?
The development of a rash following sunscreen use is the most common adverse effect. This is usually an irritant contact dermatitis. Less commonly this is an allergic contact dermatitis to one of its components: this may be a fragrance, preservative, or a sunscreen chemical.
If changing the product does not solve the problem, patch testing and photopatch testing may be required to determine the cause. Any new product should be tested on a small area for a day or two before applying it widely.
Contact dermatitis to sunscreen