My great-grandmother was a big proponent of wearing dark-colored, long-sleeved shirts in the middle of North Carolina’s scalding summers. Her reply whenever I’d ask her why she’d chosen that gardening outfit on a 95-degree day was always the same: “What keeps out the cold will keep out the sun.” 

Her ancestral wisdom was spot on. Ultraviolet radiation, of which the sun is a primary source, is thought to be a leading cause of skin problems in people—including wrinkles, sunburn, decreased immune function, irritation, and certain forms of cancer. Historically, humans have found ways to protect themselves from the sun. Indigenous populations in Alaska constructed snow goggles out of bone or wood to protect their eyes from UV rays reflecting off the snow. In Myanmar, thanaka, a paste of crushed tree bark, is still used. 

Consumer-wise, today there are two primary sun protectants on the market: sunscreens and UPF materials. The second is designed using “various weaving methods, dyes, and photo-protecting chemicals to impede ultraviolet light from penetrating through the fabric and damaging the skin,” says Travis W. Blalock, an associate professor of dermatology at Emory University School of Medicine. 

[Related: Your summer guide to sunscreen, from SPF to not-so-magic pills]

Basically, UPF is a grade given to clothing and other textiles specially designed to block UV rays from reaching the skin. According to outdoor retailer REI, even a plain white t-shirt provides a UPF rating of around 5, which isn’t much, but is better than your birthday suit.

While SPF and UPF products will defend your body from the sun, their levels of protection are not determined in the same way, explains Shadi Kourosh, the director of community health for the department of dermatology at Massachusetts General Hospital. A product’s SPF rating is based on how long someone can be in the sun with sunscreen on before their skin starts to redden, relative to how long they can be in the sun without it. For instance, if someone can be in the sun for 30 minutes before they start to burn, properly applied SPF 30 would allow them to stay outside for 30 times longer.

In comparison, UPF ratings are established by the percentage of UV rays that penetrate the material. If a shirt has a rating of UPF 50, it is thought to block 98 percent of ultraviolet A (UVA) and ultraviolet B (UVB) rays from reaching someone’s skin. (SPF only measures protection against UVB rays.) Officially rated UPF products range from 15 up to 50+.

I asked both Kourosh and Blalock about the benefits of UPF, how to tell if a product will offer good sun defense, and if SPF ratings serve as an effective guide for choosing the proper level of protection. Both interviews have been edited and condensed for clarity. 

Is UPF clothing and gear as effective as sunscreen?

Blalock: UPF is just one component of protecting the skin from the harmful effects of ultraviolet light. It does an amazing job of protecting the skin that it covers—however, I recommend using a broad spectrum sunscreen with an SPF greater than 30 to apply to areas not covered by UPF clothing. As a parent and a doctor, I am acutely aware of the benefit of UPF clothing, which does not have to be reapplied and doesn’t wash off during swimming. Sunscreen does have to be reapplied after a designated period, and spots could be missed if it is not applied uniformly. 

Kourosh: Even if you’re wearing a UPF fabric that blocks 99 percent of the sun’s rays, if you’re out for long enough, some of those rays still might get through. One concept discussed in the medical community, especially among dermatologists, is the percentage of body surface area covered by a garment and the weight of the garment itself. So it’s about how good the fabric is at blocking the sun, and how much of the body it covers. And there are other factors that affect its effectiveness—like the clothing should be loose rather than tight, and it should not be wet. 

So SPF is the superior choice between the two?

Blalock: I don’t typically think of SPF as being superior given that UPF and SPF focus on different aspects of photoprotection. I think of these concepts and measurements as complementary instead of comparative. However, as a practical matter, it is generally believed that UPF clothing may block out UVA more effectively than some sunscreens. 

How is the UPF level determined? 

Kourosh: While national and international health agencies like the Food and Drug Administration and the World Health Organizations recommend UPF as one of the pillars of sun prediction, there is no global standardization. Australia and New Zealand have done the best job of establishing guidelines and standards and having agencies for enforcement. But in the US, it’s not enforced, so that’s another reason why it’s a good idea to go for the maximum protection, because that gives you the best chance of getting the protection you’re hoping for. Australia and New Zealand also have a protection rating system corresponding to the percentage of rays that make it through the fabric.

What’s the right amount of sun protection then?

Kourosh: Usually, the protection we’re getting from either rating is less than we think. The estimate of SPF or UPF protection is based on the perfect world of lab settings. We’re probably outside for longer periods or in situations where the sun exposure is very intense, like at the beach. So we cannot assume that the conditions in which the testing was done are the same as what we are encountering in the real world. This is why I recommend that people get the maximum levels they can find on a product.

Blalock: My general advice to patients starts with an understanding that we know the negative impacts of the sun on your skin. Ultraviolet light can increase your risk of skin cancer, cause sunburns, and accelerate signs of aging, like wrinkles and spots. Thus, the more informed you are, the more likely you can make educated choices about protecting your skin. I recommend selecting sun protection that you are willing to use consistently. The skin is not protected by sunscreens or UPF clothing that aren’t used. 

How can people check if their UPF products are legitimate? 

Blalock: The easiest way for consumers to know is to purchase from a manufacturer that clearly indicates a UPF designation on the label. While certain types of fabrics are better at preventing ultraviolet light from getting to the skin—dark or bright colored clothing, densely woven fabrics, and loose-fitting clothing—there are no reliable ways for the consumer to know this unless they’re labeled as having a confirmed UPF. Companies that place this label on their clothing commonly do laboratory testing to evaluate sun- protective capabilities. This takes a lot of the guesswork out of the equation for consumers.

When should people wear or use UPF products?

Blalock: Minimizing UV damage to your skin is advised when sun exposure is likely to be high. You can monitor the UV index through your local weather report, or just be aware that the time with the most exposure is typically from mid-morning around 9 am to late afternoon around 4 pm. And there’s little downside to wearing or using sun protection outside of these times. Thankfully, as UPF clothing has become more mainstream and fashionable, I’m hoping we’ll see more of them worn at all times.

Which activities does UPF work best for?

Blalock: The big concern I hear most commonly regarding sunscreen is the need for reapplication. That might be common as people focus on their specific activities, like swimming in the ocean, engaging in athletics, or even going on a long hike. UPF clothing that is comfortable and not too uncomfortable provides the ability to engage in meaningful life activities without worrying about the reapplication requirements of sunscreen. 

Kourosh: Another issue that’s becoming increasingly important in the medical community is occupational exposure—so people who work in certain professions where they’re chronically exposed to heat and sunlight. Sunlight and heat are capable of causing certain skin problems, and that puts workers at risk. Some countries, like Germany, now have regulations around what we could call personal protective equipment against UV exposure, which employers must provide. 

There are also people who work in environments with snow, open water, white sand, asphalt concrete, or polished metal. These are reflective surfaces that intensify a person’s exposure to UV rays. They should opt for maximum-protection clothing and sunscreen, and seek shade as often as possible.