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.

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. Some are also combined with physical blockers.

Some chemical absorbers may degrade when exposed to sunlight, ie: 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.

Physical blockers

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.

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.

Photoallergic contact dermatitis to sunscreen