To get the most from your sunscreen, follow label instructions, which, according to FDA guidelines, read as follows on all back labels:

  • Apply liberally 15 minutes before sun exposure.
  • Reapply a) after 40 or 80 minutes of swimming or sweating, b) immediately after towel drying and c) at least every 2 hours.

This article gives you the thinking behind these instructions.

For advice on what sunscreen to use, see the REI Expert Advice article, Sunscreen: How to Choose.

When to Apply and How Much

When to apply? 15 minutes before sun exposure begins. “That allows it to bind to the stratum corneum, the outer layer of the skin,” says Dr. John Wolf, Chairman of the Department of Dermatology at the Baylor College of Medicine. “Once it’s bound there, it’s going to last longer and be harder for it to get rubbed off or washed away.”

How much to use? Be generous: To cover all exposed skin, a common guideline is about 1 oz.— a golf ball-size blob, or enough to fill a shot glass. Over how much area? That’s a gray area. A tall person wearing shorts might need more to cover exposed legs (particularly the backs of knees and calves). Skin-care pros are adamant: Don’t skimp. Applying sunscreen too thinly weakens its effectiveness. A too-thin layer of SPF 30 may deliver only SPF 5 or SPF 10 results.

“I tell my patients to put on a significant amount,” says Dr. Wolf. “What is a significant amount? I think the best advice is to slop or slather it on, which is what the Australians advise in their campaigns against melanoma [a skin cancer widespread in Australia].

When to reapply? The FDA’s tip: every 2 hours. Yet Dr. Wolf acknowledges outdoor people are notoriously lax about reapplying sunscreen.

“Probably virtually no one puts sunscreen on every 2 hours the way we tell them,” he says. “Almost everyone puts too little on, and almost no one applies it often enough.

“The fact you’re not getting burned doesn’t mean you’re not getting damage to the DNA in your skin," Dr. Wolf says. "Sunburn is just one evidence of damage. UVA penetrates your skin deeper and can affect your immune system."

Kids and Sunscreen

Under 6 months? Shield the skin of infants with shade, not sunscreen. Pediatricians believe infant skin can too easily absorb sunscreen ingredients.

Older than 6 months? Any sunscreen will work. Kids often prefer sprays and mists over rub-on lotions. Kid-targeted sunscreens offer packaging or fragrances that can appeal to young users, but their formulas may vary little from grown-up versions.

“I keep a big basket of sunscreens out by our pool, and I’ve noticed the kids really like the sprays,” Dr. Wolf says. “But you may waste a lot. One day I looked over and saw my niece using a spray and she was enveloped in this cloud. Half probably just dissipated into the air.”

Medical pros recommend everyone, adults and kids, avoid inhaling the mist from spray sunscreens.

Beyond Sunscreen

What about clothing? Most dermatologists say shade and clothing are your best defense against UV rays. Sunscreen is for unavoidably exposed areas: face, neck and hands.

To optimize protection, take shade breaks (particularly between 10am and 2-4pm) and cover up:

Hat: Brim styles vary. Wrap-around brims don’t suit everyone’s fashion sense, but they protect your ears. Another option: a UV-protection Buff

Sunglasses: Quality lenses block 100% of UVA and UVB rays. (Cheap lenses may be nothing but dark plastic with no UV-blocking ability.) Wrap-around lenses block peripheral light.

Protective clothing: UPF-rated fabrics use tight weaves or darker colors to shield skin. Desert nomads have relied on tunics and headcloths for sun protection for centuries. It’s still a good strategy.

Lip balm: It’s sunscreen compressed into a waxy stick. “Lip balm stays on lips better than if you just rubbed cream on them,” says Dr. Wolf. It tastes better, too."

Learn more about sun-shielding clothing in the REI Expert Advice article, Sun Protection Clothing Basics.

Skin Conditions and Skin Types

What if you have sensitive skin, skin conditions or skin allergies? Your better sunscreen choice is likely to be a mineral sunscreen (containing zinc oxide or titanium dioxide). You may also want to steer clear of formulas that include alcohol or fragrances.

Sun exposure affects every type of skin. People with fair skin (e.g., Scandinavians) must employ extra vigilance to shield their skin.

The Fitzgerald Skin Type Scale categorizes human skin types into 6 groups:

Example Sun History
I Pale white; red-headed, freckled, Irish/Scots/Welsh Always burns, never tans; extremely sun-sensitive
II White to beige; fair-skinned, fair-haired, blue or green-eyed, Caucasian Burns easily, tans minimally; very sun-sensitive
III Beige; average skin Burns moderately, tans gradually to light brown; minimally sun-sensitive
IV Light brown; Mediterranean-type Caucasians Burns minimally, tans well to moderate brown; minimally sun-sensitive
V Moderate brown; Middle Eastern, some Hispanics, some African-Americans Rarely burns, tans profusely to dark
VI Dark brown or black; African-American Never burns, tans profusely  

Other Sunscreen Factors

Other factors that influence the level of protection you need from UV rays:

Altitude: The higher you ascend, the thinner the atmosphere (and the UV-inhibiting gases contained in it). It is estimated that UV radiation increases 4% per 1,000 feet of elevation gain. Thus at higher elevations, apply and reapply sunscreen with greater diligence.

Latitude: Sun intensity becomes greater near the equator and both poles. During summer months at mid-range latitudes, too.

Sweat: Heavy perspiration due to temperature, humidity or exertion is likely to weaken your sunscreen.

Abrasion: Bushwhacking or scrambling typically means your skin will be brushing against other surfaces. Toweling off after a swim also compromises protection. Reapply regularly.

Ultraviolet Index: The UVI is a rating scale—0-2 (low; green on the UVI color scale) to 11+ (extreme; blue-purple)—which indicates the daily amount of UV rays reaching the earth’s surface. The U.S. Environmental Protection Agency offers UVI forecasts by zip code.

Sunscreen + insect repellent: No hard science on this question is known to exist. Using both will likely diminish the effectiveness of both products.

A suggestion: First apply sunscreen 15 minutes prior to the start of your activity. After activity begins, apply insect repellent. It works by creating a vapor barrier atop your skin.

Sunscreen Shelf Life and Storage

Manufacturers sometimes stamp a “best by” date on the product. If not, write the year you purchased the sunscreen on the cap or label. It is estimated that sunscreen is good for 3-4 years after purchase, perhaps longer if it is a lotion that retains a creamy texture.

Store sunscreen in a cool, dry place. Heat degrades the active ingredients in sunscreen. Avoid leaving it stashed in a glove box or car trunk during warm months.

How to Treat a Sunburn

These tips come from the American Academy of Dermatology and the Skin Cancer Foundation:

  • Get out of the sun. If you sense you’ve overdone sun exposure, cover up or find shade. Burned skin typically becomes evident in 4-6 hours.
  • Cool down. Take a cool bath. Apply a cold, damp towel to affected skin a few times a day.
  • Moisturize. Generously apply lotions containing aloe vera, vitamin C or vitamin E. Use hydrocortisone cream for 1-2 days to relieve discomfort. Avoid lotions or creams that include petroleum (a heat trap), benzocaine or lidocaine (potential irritants). Consider taking ibuprofen if discomfort is substantial.
  • Hydrate. Sunburn draws fluid from the body toward the skin. So drink ample water.
  • Hands off. Do not pop blisters. It might cause infection.

Is All Sun Exposure Bad?

The human body needs vitamin D. Our diet is one source. The sun is another. Sunshine is also thought to stimulate our brains’ ability to produce serotonin, a mood-boosting chemical.

Mike Carruthers, host of the syndicated radio program Something You Should Know, asked author Joe Graedon (The People’s Pharmacy Guide to Home and Herbal Remedies) about the value of vitamin D and if we should obtain some of our intake from the sun.

Graedon: “Vitamin D can reduce the risk of many cancers. It may be valuable in reducing the risk of osteoarthritis. There are many other conditions where vitamin D is absolutely crucial….Our recommendation is 10-15 minutes of unprotected sun exposure 3 or 4 times a week. That's not enough to do any damage, but it is enough to get adequate vitamin D levels in your body.”

Shop REI’s selection of sunscreen.

Other articles in REI’s sunscreen series:

Article contributors: Brian Adams, MD, MPH and Interim Chairman of the Department of Dermatology at the University of Cincinnati School of Medicine; Robert Friedman, Clinical Professor of Dermatology at the New York University School of Medicine; Susan M. Swetter, MD, Professor of Dermatology and Director of the Pigmented Lesion and Melanoma Program at Stanford University Medical Center; John E. Wolf, Jr., MD, MA and Professor and Chairman of the Department of Dermatology at the Baylor College of Medicine in Houston.