The Science of Picking the Right Sunscreen

This week marks the summer solstice in the northern hemisphere—the longest day of the year. For many of us, that means we are starting to spend more time in the sun. With the large quantity and variety of sunscreens available, it can be a challenge to pick the best one. Does the SPF really matter? Should I go organic? What about the warnings on possible carcinogens—cancer causing agents—in sunscreens? Is it better to avoid sunscreen altogether?

Luckily, science can help us with some of these answers. Here are 8 science-based tips for choosing the best sunscreen to protect your skin from the summer sun.

1. Choose a “broad spectrum” sunscreen.

As an astronomer, I know well that clouds are able to block some ultraviolet radiation from reaching the Earth’s surface. That is why we have to launch our UV telescopes into space. However, clouds can’t block all of the UV radiation that the Sun emits so we still have to protect ourselves here below the atmosphere. The spectrum of UV light that reaches us is split into two kinds of radiation: UVA and UVB. UVA rays are blamed for premature aging of skin like wrinkles and age spots, while UVB rays are what give us those nasty sunburns. Both are known to cause skin cancer. Sunscreens are only rated for their protection against UVB rays (more on that next) so choosing a sunscreen that is labeled “broad” or “full” spectrum means that it is proven to protect against UVA as well as UVB rays.

2. Use at sunscreen with at least an SPF 30.

SPF, short for “sun protection factor” quantifies how much solar energy in the form of UVB radiation is required to burn sunscreen-protected skin relative to unprotected skin. It does not, contrary to a common misconception, indicate how many hours the lotion will keep you burn-free while under the Sun’s rays.  In 2011, the Food and Drug Administration released a mapping between SPF value and sun protection. Specifically, an SPF of 15 protects against 93.3% of UVB radiation while an SPF of 30 will filter out 96.7% of UVB rays. An SPF of 50 correlates with protection against 98% of UVB radiation. Thus, the SPF scale is not a linear one. So, while there is some increase in protection to be gained by swapping out your SPF 30 for an SPF 50 lotion, the tiny percentage increase may not be worth a much larger price tag.  

3. Consider the context when reading warnings on sunscreen ingredients.

Organizations that rank sunscreens, like for example the Environmental Working Group, often – and importantly - call attention to potentially problematic ingredients found in sunscreens. However, those rankings don’t always reflect the various circumstances under which those ingredients were dubbed as problematic. Are they shown to be dangerous just when ingested? Are they found to cause cancer in humans, mice, or fish? The rankings also don’t balance the (usually minor) ingredient-related risks with the alternative: the risk of developing skin cancer from using no sunscreen at all. In that case, the choice should always be to wear sunscreen. One possible exception, however, may be the use of spray sunscreens. Consumer Reports has recommended that the sprays not be used on children, at least until more research can be done, as they can be problematic if inhaled. This is unfortunate as anyone who has tried to lotion up a moving toddler knows all too well. Adults also, should not use sprays on the face.

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4. Organic and inorganic filters in sunscreen work differently.

Organic filters typically absorb the incident UV radiation and then convert it to heat while inorganic filters instead reflect UV light so it bounces off the skin. While there is often a sense that organic options tend to be safer, inorganic sunscreens are actually usually less irritating to the skin. Most sunscreens use both organic and inorganic filters to block UV rays.

5. Buying sunscreen is not enough.

Of course, sunscreen only works if you apply it correctly. Personally, I prefer to buy the less oily versions because they feel better on my face and body, meaning that I am more likely to apply them. The doctors at the Mayo Clinic recommend applying sunscreen to dry skin fifteen minutes before going outside and then again every two hours while you’re exposed.

6. Choose water-resistant sunscreen.

Sunscreens marked as water-resistant are typically rated for protection against UV rays for 40-80 minutes of swimming or heavy sweating. That means we still need to reapply more frequently if we aren’t keeping dry. Water, along with sand and snow, also reflect sunlight making it even more important to make sure we are properly protected.

When you can, limit sun exposure. Find a spot in the shade, or avoid the beach during peak hours when the sunlight is most direct.

7. Don’t let your sunscreen expire.

Sunscreens are usually rated to last for three years so you can use your leftover sunscreen from last summer but you’re better off tossing that bottle that has been sitting in the drawer for too long.

8. Don’t rely on sunscreen alone.

The Mayo Clinic also recommends that we not become too reliant on sunscreen. When you can, limit sun exposure. Find a spot in the shade, or avoid the beach during peak hours when the sunlight is most direct.

Until next time, this is Sabrina Stierwalt with Everyday Einstein’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Everyday Einstein on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.com.