Topical sunscreen agents

Classification of sunscreens

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.

Chemical absorbers

The table below is a list of some of the common chemical absorbers available and the protection they provide against the UV range.

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 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.

Metal oxides

The two most common metal oxides are titanium dioxide and zinc oxide. They refect photons in the visible light range. Although they were previously also thought to reflect UVR, research has shown that they actually absorb it. 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 have been developed, thereby reducing the whitening appearance and increasing their effectiveness as UV absorbers. In some products, bright fluorescent colours have been added.

Titanium dioxide and zinc oxide are also photostable and are often formulated with less photostable chemical absorbing sunscreen agents.

Antioxidants

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.

Are sunscreens safe?

Potential adverse reactions of sunscreens have been studied, including:

• Systemic toxicity of chemical ingredients
• Nanoparticles
• Vitamin D synthesis
• Nitric oxide synthesis.

Daily use of sunscreens has found to be safe for humans and has not been associated with increased any-cause mortality.

However, increasing amounts of sunscreen chemicals find their way into the marine environment. Oxybenzone and octinoxate in low concentration have been suspected of being toxic to coral, causing coral bleaching, larval deformity in a laboratory study [3]. Hawaiian legislature is considering a bill that would prohibit the sale of sunscreens containing either of the two compounds as of Jan. 1, 2021 [4] [5].  

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.