Sunscreen
As I’m writing this, it has been over a year since the world grinded to a halt because of the COVID-19 virus. During the past year we’ve seen entire countries lockdown and stay-at-home orders enforced to prevent the spread of the virus and protect those who are most vulnerable. Although they are tough, these public health measures are important for the safety of our communities. For those of us who are lucky enough to avoid COVID-19, these restrictions have presented us with a new set of challenges: stir-craziness and slight case of the travel bug.
As the weather warms up and we begin to dream of beaches and sun kissed getaways, let’s talk about the chemistry of sunscreen. Why is sunscreen important? It shields our skin from harmful ultraviolet (UV) light emitted by the sun. Because UV light contains a lot of energy, the light can penetrate skin and cause harm to skin cells. The high-energy light can cleave bonds in your DNA within your skin cells and cause irreversible damage, the worst example being skin cancer. Sunscreens are labeled with an SPF (Sun Protection Factor): the higher the SPF the less UV light that interacts with your skin.
High-energy UV light can penetrate skin and alter genetic material within cells and cause irreversible damage.
Your body’s natural defense against UV light are molecules called melanin. Melanin, which is related to the pigmentation of your skin (the more melanin in your body the darker your skin), is produced to absorb UV light and prevent damage. When your skin is exposed to UV light, more melanin is produced in the affected area which causes the pigmentation of the skin to change - the skin will appear tan or more red. While melanin operates under the same principles as sunscreen, its SPF is much lower than sunscreen (SPF of melanin ranges from ~ 1-10 depending on the person). This illustrates the need for sunscreen regardless of the amount of melanin your body produces.
Melanin is produced to absorb incoming UV light and protect the skin cells.
The first sunscreens were originally made from metal oxides such as zinc oxide (ZnO) and titanium oxides (TiO). Characterized by their bright white colours, these inorganic compounds act as a physical barrier on your skin. Metal oxide sunscreens are effective since they can both reflect and absorb UV light. While the smooth, metallic surface can reflect UV light, most of it is absorbed by the metal oxide. The metal oxide acts as a semiconductor and as such, its valence electrons can absorb harmful UV rays. The metal oxide then transforms the energy into heat or infrared light which effectively blocks UV light from interacting with your cells. The SPF of these so-called physical sunscreens are typically around 50, making them extremely effective. However, since the metal oxides sit on the surface of your skin, they do have a tendency to rub-off easily.
The more popular type of sunscreens are ones that absorb into the skin, or chemical sunscreen. These sunscreens will contain organic compounds including oxybenzone and avobenzone. These organic ingredients can be absorbed into the skin and are designed to preferentially interact with UV rays. The chemical bonds within these molecules will absorb UV light and dissipate it as heat. This process will break the chemical bonds and disintegrate the organic molecule - hence why you have to continuously reapply these sunscreens. The bonus of chemical sunscreen is that it absorbs into your skin so you don’t have to worry about it rubbing off. Unfortunately, recent studies have shown that some active compounds in chemical sunscreens are major pollutants in aquatic ecosystems. For example, scientists have discovered oxybenzone from sunscreens are major pollutants of coral reefs. As such, there is a push for “greener” chemicals and a resurgence in the popularity of physical sunscreens.
So next time you pack your sunscreen while preparing for a day at the beach, remember the fascinating chemistry behind sunscreen and why it’s so important! Also, feel free to use the word “oxybenzone” around your friends, you can trick them into thinking your smart - smartER than you already are.