How to Choose and Use Sunscreen: SPF, Broad Spectrum and Labels Decoded

If you only use one beauty product every day, dermatologists agree it should be sunscreen. Ultraviolet radiation causes sunburn, contributes to most skin cancers and is responsible for much of what people think of as skin aging: wrinkles, dark spots, rough texture and uneven tone. Yet many people apply too little, choose formulas they dislike and stop wearing, or misread the label. Here is how to choose a sunscreen and use it properly.

How sunscreen is regulated in the US

In the US, sunscreen is regulated by the Food and Drug Administration as an over-the-counter drug, not a cosmetic. That is why every bottle carries a Drug Facts panel listing active ingredients, uses, warnings and directions. The FDA sets rules for what labels may claim and how products must be tested. Its sunscreen pages are the place to check the current rules.

UVA, UVB and what SPF really means

Sunlight contains two kinds of ultraviolet radiation that matter for skin:

• UVB is the main cause of sunburn and plays a key role in skin cancer.
• UVA penetrates deeper, contributes to skin aging and also to skin cancer. It passes through clouds and window glass.

SPF (Sun Protection Factor) measures protection mainly against UVB. It is determined in lab testing by comparing how long it takes protected and unprotected skin to redden. Higher SPF means more protection, but the relationship is not linear, and no sunscreen blocks all UV rays.

Reading the label

Under FDA labeling rules, sunscreens cannot be labeled "waterproof" or "sweatproof," and manufacturers cannot call their products "sunblocks." If you see those words, be skeptical. According to the FDA, only products that are broad spectrum with an SPF of 15 or higher can claim to reduce the risk of skin cancer and early skin aging when used as directed with other sun-protection measures.

Choosing your SPF

The American Academy of Dermatology recommends that most people use a sunscreen that is:

• Broad spectrum,
• SPF 30 or higher, and
• Water resistant if you will be swimming or sweating.

Very high SPFs can offer a little extra protection, which may be valuable for people with very fair skin, a history of skin cancer or conditions that make them sensitive to light. But a higher number is not a license to apply less or reapply less often.

Mineral versus chemical filters

Sunscreens use two broad types of active ingredients.

• Mineral (sometimes called physical) filters: zinc oxide and titanium dioxide. They are often recommended for sensitive skin and for children, and tend to cause less stinging around the eyes. Older formulas could leave a white cast; many modern versions are tinted or more sheer.
• Chemical (organic) filters: ingredients such as avobenzone, octocrylene and homosalate, among others approved in the US. They tend to feel lighter and disappear more easily on deeper skin tones.

Both can be effective. The FDA has continued to review the safety data for sunscreen ingredients, and its pages explain the current status. For most people, the best choice is the formula they find pleasant enough to wear generously every day.

How much to apply

Most people apply far less sunscreen than was used in testing, which means they get less protection than the label promises.

• For the body, the AAD advises about one ounce, roughly enough to fill a shot glass, to cover exposed skin for an adult.
• For the face and neck, a common guide is a generous coating that covers the whole area evenly; many dermatologists suggest measuring by lines of product along your fingers until you get a feel for the amount.
• Do not forget ears, the back of the neck, the tops of feet, the hairline and the backs of hands.

Apply about 15 minutes before going outside so it can settle. Use a lip balm with SPF as well.

When to reapply

• Every two hours when you are outdoors.
• Immediately after swimming, heavy sweating or towel drying, even if the product is water resistant.
• On ordinary office days, the morning application may be enough if you are mostly indoors, but reapply before going outside at lunchtime or after work in strong sun.

Reapplying over makeup is the hardest part. SPF sprays, sticks and powders can help with touch-ups, though they work best as a top-up rather than the main layer.

Finding a formula you will actually wear

The texture is what decides whether sunscreen becomes a habit.

• Oily or acne-prone skin: gel, fluid or matte formulas labeled non-comedogenic.
• Dry skin: creamy or lotion-based sunscreens that double as moisturizer.
• Deeper skin tones: tinted mineral formulas or sheer chemical sunscreens that do not leave a gray cast.
• Sensitive skin: fragrance-free mineral formulas.
• Under makeup: lightweight fluids that dry down quickly.

Sprays are convenient for the body, but spray enough to leave an even sheen and rub it in. Do not spray directly onto your face; spray into your hands and apply, and avoid inhaling the mist.

Common sunscreen myths

• "Darker skin does not need sunscreen." Melanin offers some natural protection, but people of all skin tones can burn, develop sun damage and get skin cancer. Dermatologists recommend sunscreen for everyone.
• "I am indoors, so I am safe." UVA passes through window glass, so long days by a sunny window or in the car still add exposure.
• "A base tan protects me." A tan is a sign of skin damage, not a shield.
• "One application lasts all day." Sunscreen wears off with time, sweat, water and rubbing, which is why reapplication matters.
• "Last summer's bottle is fine." Check the expiration date, and replace sunscreen that has been stored in a hot car or has changed color or texture.

Sunscreen is one part of sun protection

The FDA and dermatologists recommend combining sunscreen with other measures:

• Seek shade, especially in the middle of the day when UV is strongest.
• Wear protective clothing, a wide-brimmed hat and UV-blocking sunglasses.
• Check the UV Index for your area, available from the US Environmental Protection Agency and many weather apps.
• Avoid tanning beds.

For babies under six months, the FDA advises keeping them out of the sun and asking a doctor before using sunscreen.

Make it automatic

Keep a sunscreen next to your toothbrush, another in your bag and one in the car. Apply it as the last step of your morning routine, every day of the year. It is the most effective beauty habit there is, and it costs only a minute.