What Makes a Good Sunscreen?

Too much sunlight can kill. Fortunately, awareness of skin cancer has grown worldwide, and modern sunscreens can do a lot to protect the skin from the sun’s harmful effects.

Chemical UV filters usually contain π systems that absorb ultraviolet (UV) radiation; the absorbed wavelength λ depends on the size of the π system. Early sunscreens only used filters that absorb UV-B rays (λ = 290–320 nm), which are responsible for sunburn. The first sunscreen with a filter for UV-A rays (λ = 320–400 nm), which can cause long-term damage, was developed in the 1980s.

Another class of UV blockers are inorganic compounds such as titanium dioxide. It has long been thought that they work by reflecting and scattering light. This is true for visible light, but in the UV range, the particles actually absorb the radiation. Here, the absorbed wavelength depends on the particle size. Today, a single sunscreen contains a combination of UV filters to cover the harmful UV spectrum.

While modern sunscreen lotions are already fairly well developed, there is still room for improvement at the long-wave end of the spectrum between UV-A and visible light. Additionally, not all chemical UV filters are photostable over time, and usually, a lot of substance is needed. A typical SPF 30 lotion contains up to 20 % UV filters. This also makes it challenging to formulate a sunscreen lotion that the customer will actually like and use. It needs to smell nice and be easy to apply, not feel too heavy or sticky on the skin, and preferably be waterproof. The high content of UV filters, as well as the oil needed to dissolve them, makes this much more difficult.

In the United States, an entirely different issue is hampering the development of improved sunscreens: While in other countries they are considered cosmetics, in the US they are treated as drugs. Thus, new UV filters have to be approved by the Food and Drug Administration (FDA), which is extremely cautious in this regard. The last time a new UV filter came to market in the US was 1999. Effective substances developed since, such as iscotrizinol or bemotrizinol, have been in use in Europe and Australia for years, but are not yet approved in the US, which prevents the sale of state-of-the-art products.