By Chantelle Sullivan 蘇盈安
To catch the tail end of summer, visits to the beach and outdoor activities are a definite must on the list of things to do – and sunscreen is something we often take for granted in situations like these. We are often simply told that the higher the SPF the better. But what does the SPF represent?
SPF stands for Sun Protection Factor. Most sunscreens on the market have an SPF between 15 and 50. Those that are higher than 50 have not been confirmed to be more effective at shielding the skin from ultraviolet (UV) radiation. In addition, no sunscreen is 100% effective in protecting the skin from being penetrated by UV. The SPF number gives a rough indication as to how long the skin stays protected after application. For example, a sunscreen with SPF 30 prevents skin from burning for 30 times longer than usual. It is important to note that even high SPF sunscreens need reapplications to ensure sufficient protection.
You may have also noticed sunscreens being advertised as “broad spectrum” – meaning that they protect against both UVA and UVB radiation. But what are they? They are the ultraviolet rays emitted by the sun, which are categorized into three subgroups: UVA, UVB and UVC.
UVA has a wavelength ranging from 315-400 nanometers (nm; i.e. 10-9 m) and accounts for 95% of solar ultraviolet radiation that reaches the surface of Earth. It can penetrate past the skin into connective tissue and cause indirect damage to DNA through the generation of reactive oxygen species (ROS). The energy within ultraviolet radiation gives it the ability to “knock off” an electron from molecules such as water, resulting in a highly reactive and unstable molecule, also known as a free radical. Their toxicity to our cells is due to their unpaired electron. This molecule can then attack important biomolecules such as DNA, leading to base mutations. Therefore, ROS generation can increase the probability of obtaining harmful mutations in the skin and connective tissue, which has the potential to develop into skin cancer.
The second type of ultraviolet radiation, UVB, with a wavelength from 280-315 nanometers, does direct damage to DNA by inducing a bend or kink to the double helix. It accounts for 5% of total UV radiation that reaches Earth. A majority is absorbed by the atmosphere, but it is the main culprit behind sunburn. This subtype of UV radiation also causes an increase in melanin (a dark pigment) production by the skin upon exposure.
Fortunately, the third subtype UVC, with the shortest wavelength of 100-280 nm, does not pose a threat against our skin since it is fully absorbed by the ozone layer.
Sunscreens contain both inorganic and organic chemicals as the main active ingredients to protect our skin against the sun’s UV radiation. Most inorganic chemicals act as a physical barrier and can reflect the UV rays that hit the surface of our skin. The most common types of inorganic chemicals used in sunscreen are titanium dioxide and zinc oxide. They are added to sunscreen in the form of nanoparticles of 20-40 nm. Their high refractive index coupled with their ability to scatter both UVA and UVB rays makes them the perfect candidate for use in sunscreens.
On the other hand, organic chemicals such as oxybenzone absorb ultraviolet radiation and dissipate it as heat which is a rather harmless form of energy. However, such active ingredients vary in photostability, which is their stability when exposed to UV. Therefore, certain organic chemicals are prone to breakdown as they absorb UV radiation within their bonds. Because of this phenomenon, and to replenish the sunscreen that has been washed away by water and sweat, it is crucial to reapply every few hours to ensure that enough active chemicals are present on the surface of your skin to protect it from UV damage.
|Did You Know?
Organic compounds are generally defined as carbon-compounds that usually contain C-H or C-C bonds (e.g. CH4, CH3COOH), whereas inorganic compounds usually do not contain C-H or C-C bonds (e.g. ZnO, TiO2), although they can still contain carbon atoms (e.g. CO, CaCO3).
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