Feel the burn? Explaining the science of sunscreen

Woman on the beach applying sunscreen

Summer sunshine constantly reminds us of the importance of sunscreen. But ever wonder how it works? Or why you still get burned even if you’ve put it on? (Spoiler alert: You’re probably not using enough. Most adults need about a shot glass-worth of sunscreen to cover their body.) We spoke with Oluwakemi Onajin, MD, a dermatologist and dermatopathologist at the University of Chicago Medicine, about the science behind sunscreen and the best ways to protect your skin from the sun.

What is SPF?

Onajin: SPF, or sun protection factor, measures how well sunscreen protects against UVB rays, which cause sunburn and play a major role in developing skin cancer. Sunscreen manufacturers determine the SPF number based on how long it takes for the sun to redden the skin with sunscreen versus without. That means if it takes 10 minutes for skin to become red without any sunscreen, using an SPF 30 sunscreen should prevent redness for 30 times longer — about five hours. That said, no sunscreen, regardless of how high the SPF, is effective for more than two hours without reapplication.

Can you explain the science behind sunscreen?

Sunscreens are classified as both organic, or chemical, and inorganic, or physical. Organic compounds, such as oxybenzone and octinoxate, work like a sponge to absorb UV radiation. Inorganic compounds, containing zinc oxide and/or titanium dioxide, shield the skin to reflect and scatter UV radiation. A broad-spectrum sunscreen contains compounds that absorb and/or reflect UVB rays in addition to UVA rays, which cause skin aging and wrinkles. Many types of sunscreen available in the U.S. combine organic and inorganic compounds to provide this kind of broad-spectrum protection.

What’s the minimum SPF you should use? Is a higher SPF really that much better for you?

A broad-spectrum sunscreen with SPF 30 or higher. We know that SPF 15 blocks about 93 percent of UVB rays, SPF 30 blocks 97 percent and SPF 50 blocks 98 percent. There’s no available sunscreen that blocks all UV rays, which is why it’s important to practice other sun protective measures. While sunscreens with higher SPF block slightly more UV rays, they last the same amount of time as a lower-number SPF sunscreen and should be reapplied after two hours.

How much sunscreen should I use and when should I apply it?

To cover the body, most adults need approximately one ounce — enough to fill a shot glass. You should apply sunscreen to dry skin 15 to 30 minutes before going outside. When outdoors, reapply every two hours, or after swimming, toweling off or sweating. Sport or water-resistant sunscreen will stay on wet skin for 40 to 80 minutes. It’s best to practice sun protection daily, even on cloudy or cool days, regardless of season. Sand, water and snow reflect sunlight.

What kind of sunscreen is best for me?

Organic sunscreens are usually easier than inorganic sunscreens to rub into the skin without leaving white residue. People with sensitive skin should use inorganic sunscreen, since it’s hypoallergenic and less likely to irritate your skin. Creams are best for dry skin. Lotions are thinner, less greasy, and generally preferred for large body areas. Gels are good for hairy parts of the skin such as the scalp. Sprays are sometimes preferred because they’re easier to apply, especially on children. However, it’s difficult to know if you’re applying enough to cover a specific area. Spraying into your hands first helps to make sure that you’re adequately covered as well as to avoid inhalation. You could also consider applying cream initially, then using spray when re-applying.

How effective is sun-blocking clothing?

Clothing is an additional protective tool that provides a physical barrier to UV rays. The degree of protection provided by clothing is defined by the ultraviolet protection factor (UPF) and determined by what it’s made of and how it’s made. A shirt with a UPF of 50 allows just 1/50th of rays to reach your skin versus a thin white cotton t-shirt with a UPF of 5.

If I do get sunburned, what are the best remedies?

Aloe vera-based gels can provide pain relief. Treat sunburns as soon as possible by avoiding continued exposure, taking a cool bath, putting on over-the-counter hydrocortisone cream, taking an oral NSAID such as ibuprofen and staying hydrated. Do not pick at a peeling sunburn; instead, apply a fragrance-free moisturizer. If you have a sunburn with severe blistering on a large part of your body, severe pain and symptoms such as a fever, headache or vomiting, you’ll want to seek medical care immediately.

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