How often to reapply sunscreen, and why

Putting sunscreen on in the morning is the easy part. Keeping it working through the day is where most people slip. The American Academy of Dermatology (AAD) says that people who get sunburned "usually didn't reapply, used too little sunscreen, or used an expired sunscreen." This guide covers the reapply rule, the reasons behind it, and how it changes for swimming, sweating, children and indoor days. It is part of Skinli's guide on how to choose a sunscreen for your face.

The rule: at least every two hours outdoors

Every major source gives the same number. The FDA says to "reapply sunscreen at least every two hours, and more often if you're sweating or swimming." The AAD says to "reapply sunscreen every two hours, and immediately after swimming or sweating." The NHS says it is "recommended to reapply sunscreen every 2 hours."

The rule is also written into U.S. law. Federal labeling rules for sunscreen (21 CFR 201.327) set the exact directions a label must print:

Type of sunscreen	Required reapply directions on the label
Not water resistant	"reapply at least every 2 hours" and "use a water resistant sunscreen if swimming or sweating"
Water resistant (40 minutes)	Reapply after 40 minutes of swimming or sweating, immediately after towel drying, and at least every 2 hours
Water resistant (80 minutes)	Reapply after 80 minutes of swimming or sweating, immediately after towel drying, and at least every 2 hours

The FDA also bans any sunscreen from claiming "protection for more than two hours without reapplying." So a label may never promise all-day protection from one coat.

Why sunscreen stops working

Three things wear sunscreen down during the day.

Light breaks it down. The AAD says, "The sun's rays break down some sunscreens. Others clump and lose their effectiveness."

The sun dries it off. The NHS says to reapply every two hours "as the sun can dry it off your skin."

Water, sweat and towels remove it. The FDA says "all sunscreens eventually wash off." Rubbing your face, wiping sweat, or drying off with a towel takes sunscreen with it.

This is why a sunscreen can be strong at 9 a.m. and much weaker by lunchtime, even though the number on the tube has not changed.

A higher SPF does not mean longer

It is tempting to think SPF 50 lasts longer than SPF 30. It does not. The AAD says "high-number SPFs last the same amount of time as low-number SPFs," and that a high SPF "does not allow you to spend additional time outdoors without reapplication."

The FDA explains why the SPF number cannot act as a timer. SPF is "not directly related to time of solar exposure but to amount of solar exposure." The sun is stronger at midday and closer to the equator, so the same hour can bring different doses of light. Our guide on how sunscreen works and what SPF means goes through the numbers.

Swimming, sweating and towels

Water changes the clock. Look for the words "Water Resistant" on the front, followed by 40 or 80 minutes. The AAD explains what each means: water resistant sunscreen "stays effective for 40 minutes in the water," and the 80-minute kind "stays effective for 80 minutes in the water, so you need to reapply after 80 minutes."

Three points catch people out:

• No sunscreen is waterproof. The FDA does not allow the words "waterproof" or "sweatproof" on a label.
• Towel drying removes sunscreen. Water-resistant labels must say to reapply "immediately after towel drying." The NHS gives the same advice: reapply straight after you have been in water, even if the product is water resistant, and after towel drying or sweating.
• Dry skin still needs the two-hour rule. The AAD says that even if your skin stays dry while using water-resistant sunscreen, "you'll need to reapply the sunscreen every 2 hours."

The NHS also warns that the cool feel of water "can make you think you're not getting burned," and that water reflects ultraviolet rays, which raises your exposure.

How much to reapply

Reapplying a thin layer does little. The SPF on the label comes from a test that uses a thick coat: 2 milligrams of sunscreen per square centimeter of skin, set by federal rules. The AAD says many people apply only 20 to 50 percent of the amount needed to get the SPF on the label.

For the face, the AAD says to use "at least 1 teaspoon," which it describes as the amount needed to cover the length of your index and middle fingers. Skinli's plan uses the same measure: two lines of sunscreen, one on each of two fingers, for the face and neck. Use that full amount again each time you reapply.

For the body, the FDA says an average adult needs at least one ounce, which it compares to the amount that fills a shot glass. The NHS puts it at 6 to 8 teaspoons for the whole body.

Timing the first coat

The first coat matters for the rest of the day. The FDA says to "apply 15 minutes before sun exposure" because this "allows the sunscreen to have enough time to provide the maximum benefit." Federal rules require every sunscreen label to give that direction. The AAD warns that if you wait until you are in the sun, "your skin is unprotected and can burn."

In practice, this means putting sunscreen on at home as the last step of your morning routine, then counting your two hours from when you step outside. Our guide on what order to apply your acne products shows where sunscreen goes after cleanser and moisturizer.

Days spent mostly indoors

The two-hour rule is written for time outdoors. No source Skinli read gives a reapply schedule for a day spent inside.

Two facts help you decide. First, the AAD says UVA, the ultraviolet light that ages skin, "can pass through window glass," while glass blocks UVB, the light that burns. Second, the AAD says your skin is exposed to ultraviolet rays "every time you go outside, even on cloudy days and in the winter." If your day includes a walk at lunch or hours next to a sunny window, a second coat before that time is a reasonable step. The morning coat is the one never to skip.

Children

For children 6 months and older, the AAD says to reapply sunscreen every two hours outdoors, or as often as the label says. The NHS gives the same two-hour rule for babies over 6 months and adds reapplying immediately after swimming or sweating.

Babies younger than 6 months are different. The FDA and the AAD advise keeping them out of direct sun instead of relying on sunscreen. Our guide on when an infant can wear sunscreen covers what the sources say.

Makeup and moisturizer with SPF

Some moisturizers and makeup contain sunscreen. The AAD says these products are convenient, but that sunscreen still needs reapplying every two hours when you are outdoors. Most people use far less makeup than the teaspoon of sunscreen the AAD recommends for the face, so adding more makeup gives less protection than a full coat of sunscreen. For a day outside, a separate sunscreen gives you more control over the amount.

A simple reapply plan

1. Put on a full coat of broad-spectrum SPF 30 or higher sunscreen at home, 15 minutes before going out.
2. Outdoors, reapply the full amount every two hours.
3. After swimming or heavy sweating, reapply when the label's 40 or 80 minutes are up, and right after you towel dry.
4. Keep a small tube in your bag so reapplying is easy, and store it out of direct sun. The FDA says heat and direct sun can harm sunscreen, and every label must say to protect the product from excessive heat.
5. Wash sunscreen off at night with a gentle cleanser, as the first step of your evening routine.

If you are using an acne treatment that makes skin more sensitive to the sun, the reapply rule matters even more. Our starter acne routine shows how sunscreen fits into a simple morning plan, and our guide to sunscreen for acne-prone skin covers the label words to look for.

See a doctor if a sunburn blisters over a large area, or comes with fever, chills or a headache. The FDA says severe sunburn should be treated as a medical emergency.