The Theory of Nothing

How does a sunscreen work?

How does a sunscreen work?

Most everyone is familiar with sunscreens. They're those creamy lotion preparations that one smears on exposed skin in order to avoid a bad sunburn. They've become more important nowadays because of the danger of skin cancer, also known as squamous cell carcinoma and melanoma. The reason that the sun causes skin cancer is because of UV radiation. As you might know there are two UV bands: UV-A (400 to 313 nm) and UV-B (315-280nm). Those are the wavelengths in nano-meters. The more dangerous of the two is UV-B. However, UV-A can penetrate further.

A protection scale has been created for sunscreens. It's called SPF or Sun Protection Factor. A SPF of 30 means that only 1/30 of the burning UV can get to the skin. In other words it would take thirty times longer for you to get sunburn than if you didn't use the sunscreen. As you might imagine there is a lot of confusion about this scale. The measurement is made with human subjects, which could result in wide variations. Some people burn faster than others. The problem is that there is no uniform measure of UV-A protection. The SPF doesn't take it into consideration. There are sunscreens that claim to have a SPF of 100, but this is deceiving. No sunscreen is a hundred percent effective.

A sunscreen is essentially a lotion that contains a UV absorbing chemical, but it's important to note how well the chemical absorbs UV-A because it can penetrate further into the skin and cause cancer. The FDA (Federal Drug Administration) has approved 16 chemicals for this purpose, but only 8 are currently used: oxybenzone, avobenzone, octinoxate, octisalate, homosalate, octocrylene, titanium dioxide and zinc oxide. Only two of these are good for UV-A protection: avobenzone and zinc oxide. The reason I list these is so that you can check the ingredient list of a sunscreen container to know whether it's effective in blocking both UV-A and UV-B rays. Many chemicals are no longer used because they cause irritation or are unpleasant. Dioxybenzone, for example, turns your skin blue upon UV exposure. Yikes!

One of the most used sunscreen ingredient, oxybenzone, is also accused of causing endocrine disruption, reproductive toxicity, allergies, and organ system toxicity. It's also absorbed through the skin and ends up in the body, suggesting long-term effects. Despite this The FDA says that the risks of cancer from sun exposure outweigh these possible hormonal effects.

I might add that The European regulators, who are known to be more cautious about chemical exposure, list 27 chemicals suitable for sunscreen use. Some of the chemicals they use could be more effective against both UVA and UVB exposure. For some reason, the American regulators are blocking (no pun intended) approval of these more effective chemicals, including: amiloxlate, bemotrizinol, bisoctizole, drometrizole, bisoctizole, drometrizole trisiloxane, ecamsule, enzacamene, iscotrizinol, and octyl triazone.

As a chemist, what I note about these chemicals is a commonality of structure. They are what are called aromatic (contain double bonds that interact in a unique manner) organic chemicals, which means they contain benzene rings. Benzene (C6H6) is a hexagonal ring shaped molecule with three alternating double bonds. When you connect benzene rings by means of carbonyl groups (Carbon connected with a double bond to oxygen) you get molecules that can absorb UV radiation. Add in a triazin group (a hexagon ring with three nitrogen atoms) and you have a more effective UV absorber. For example Ethylhexyl Triazone is a molecule with several benzene carboxylic acid groups connected around a triazin.

You don't have to be a chemist to remember that a broad-spectrum sunscreen like bemotrizinol absorbs both UV-B and UV-A radiation, the A-version of which can penetrate down through the epidermis and the dermis to cause skin cancer. So be careful out there in the summer sun or even on a cloudy day. There's still a lot of UV radiation (40%) getting through the clouds.

Thanks for reading.