Page 10 - Science Focus (Issue 016)

防曬霜的科學

30
SPF 30
By SPF 30
Chantelle Sullivan
蘇盈安

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