Q: Why should we use sunscreen?

Sunscreen’s purpose is to prevent damage caused by UV (ultraviolet) rays from the sun.  Visible light has a wavelength of 400 to 780 nanometers (violet to red).  The ultraviolet range is composed of wavelengths from 10 to 400 nm- that is, the wavelength range right below visible violet light, thus the name ultraviolet.”  (The region directly below 10 nm is composed of x-rays.)

uv chart

The UV region is further broken down into subtypes.  UVA rays have wavelengths of 315 to 400 nm.  UVB rays range from 280 to 315 nm, UVC rays cover 180 to 280 nm, and wavelengths of 10 to 180 nm are referred to as vacuum UV.  Vacuum UV rays are blocked by air particles and UVC rays are stopped by the Earth’s ozone layer, so neither affects the average human.  (Only astronauts- those who aren’t shielded by the Earth’s atmosphere- need to be shielded from UVC and vacuum UV rays.)  UVB rays reach the outer layer of human skin, while UVA rays can penetrate deeper.

UV rays carry more energy than visible light and can damage DNA (deoxyribonucleic acid), the molecules that contain our genetic code.  DNA is composed of four nucleobases: adenine, guanine, thymine, and cytosine.  The sequence of these four nucleobases and how they’re paired makes up the DNA code for the cell, sort of like how code dictates a computer program.  Unfortunately, DNA can be damaged by UV exposure.  One possible way DNA can be damaged involves the UV ray breaking apart pairings between cytosine and thymine.  This could allow two thymine bases to form a covalent bond, sticking them together to create a thymine dimer, messing up the original genetic sequence.  While cells are capable of repairing damaged DNA, mistakes can be made and a repair may be missed.  If the damage is significant and persists, the cell can’t work properly.  Damage could cause cell death, or it could cause the cells to become cancerous.

DNA damage

Humans are equipped with some natural protection against UV damage.  Melanin, a pigment in cells, absorbs UVB rays and dissipates them as heat, preventing them from damaging DNA.  Exposure to UV light can cause cells to produce more melanin, thus creating a tan.  Melanin production varies from person to person.  Some people may have naturally high levels of melanin or can synthesize it fairly well.  Others may not have low levels of melanin or none at all.  Some form concentrated spots like freckles or moles which contain large amounts of melanin relative to the surrounding skin.

However, melanin isn’t a perfect protection, and there are lots of complications from too much UV exposure.  For example, overexposure to higher energy UVB rays can cause sunburn, the result of increased blood flow, swelling, and pain as the body tries to heal from the damage.  With bad sunburn, cell death may occur, which is one of the reasons why skin can peel or flake off.  UVA rays, which penetrates deeper into the skin, can disrupt connective tissue, causing the skin to lose elasticity and wrinkle.  And of course, accumulated exposure and DNA damage could lead to increased risk of skin cancer.


To avoid this damage, we can protect our skin by staying in the shade and wearing hats and clothing.  We can also cover exposed skin with sunscreen.

Sunscreens can be lotions, sprays, gels, or waxes.  There are two types of sunscreens: chemical and physical.  Chemical sunscreens absorb UV rays, while physical sunscreens block UV rays.

Chemical sunscreens contain organic (carbon-containing) compounds, which absorb UV rays and release the energy as heat.  Different organic compounds are better at absorbing certain wavelengths than others, so a mix of various compounds can provide a broader spectrum of protection.  For example, avobenzone absorbs UVA rays; octyl salicylate absorbs UVB rays; and oxybenzone absorbs both UVA and UVB rays.  (These are just a few examples of the FDA-approved sunscreen ingredients.)  Some of these compounds are photostable and won’t break down from UV exposure, but other compounds will break down over time with exposure.

Physical sunscreens include formulas that contain inorganic compounds zinc oxide or titanium dioxide, which act as physical blockers to prevent UV rays from damaging DNA.  Instead, the rays are reflected or scattered away, kind of how white paint reflects light.  (In fact, both zinc oxide and titanium dioxide are used as pigments in white paints.)   Older formulas were often thick and chalky in appearance, but newer formulas have scaled down the inorganic compound particle size to make it more visually appealing.

How do we know our sunscreen works?  Other than just testing it out and hoping we don’t get burned, we can look at the sunscreen’s SPF, or sun protection factor.  SPF indicates the effectiveness that the sunscreen blocks UVB radiation.

Fraction UVB Unprotected = 1 ÷ SPF

Fraction UVB Protected = 1 – (1 ÷ SPF)

The higher the SPF, the more protection provided.  So, for example, an SPF of 30 means that 1/30 of the UVB radiation penetrates the skin; or 29/30 is blocked.  Below is a chart showing the relationship between SPF and the percentage of UV radiation blocked.  No SPF can block 100% of UVB rays- that would have to be an SPF of infinity.

spf percent relation chart

Furthermore, SPF is for measuring UVB blockage; there is no metric for UVA blocking power.  In the US, sunscreen formulas that contain UVA blocking agents are labeled as “broad spectrum” sunscreen.

You may have heard that SPF can be used to figure out how long you can stay in the sun until getting burned.  The idea is simple- you take the number of minutes it would take for you to get burned without protection and then multiply it by the SPF number to get the number of minutes you can be in the sun with sunscreen without getting burned.

minutes to burn without sunscreen  X  SPF

=  minutes to burn with sunscreen

However, this is not strictly true.  Under ideal conditions, yes, this relation works.  But if we take SPF 70, for example, and say that it takes 15 minutes for you to burn without sunscreen, you wouldn’t have to worry about reapplying for 17.5 hours.  The problem is that sunscreen rubs and wears off, especially in water or when exposed to sweat.  Like mentioned previously, some of the organic compounds break down when exposed to UV rays, meaning they have to be replaced by reapplication.  UV exposure also varies over time depending on the day’s conditions- UV rays are generally at their peak on clear sunny summer mid-days.  (Though that’s not to say you can’t be burned on cold cloudy days- you certainly can, as UV rays can penetrate through the clouds, it just might take a little longer.)  An hour exposure at 10 AM in the morning is different than, say, 10 PM at night.  Additionally, SPF is calculated from perfect use and people generally don’t use as much sunscreen as what’s used in testing.  In summary, it’s best to reapply sunscreen periodically to ensure continuous coverage.

sunscreen bottles

So what sunscreen should you use?  That’s up to you.  Some people may have allergic or other adverse reactions to certain types of sunscreens, so you just have to find one that works the best for you.  The CDC recommends that you pick a sunscreen that provides broad spectrum (UVA and UVB) protection with an SPF of at least 15.  You should reapply periodically and sit in the shade when possible.

Keep calm and science on.


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