A good sunscreen guide starts with the label, because the label tells you almost everything you need. SPF, broad spectrum, UVA, UVB and water resistance each describe a different part of the protection. Once you can read them, choosing and using sunscreen becomes straightforward.
UVA and UVB: the two kinds of ultraviolet light
Sunlight that reaches the ground contains two types of ultraviolet (UV) rays.
| UVB | UVA | |
|---|---|---|
| Main effect | Sunburn | Skin aging, tanning |
| Reaches | The outer layer of the skin | Deeper into the skin |
| Through glass and cloud | Mostly blocked by glass | Passes through glass and cloud |
| Strength over the year | Strongest in summer and at midday | Present all day, all year |
Both types damage skin cells and both contribute to skin cancer.
What SPF means
SPF stands for sun protection factor. It measures protection against UVB, the rays that cause sunburn. The number comes from laboratory tests in which a generous, fixed amount of sunscreen is applied.
- SPF 15 filters about 93% of UVB rays.
- SPF 30 filters about 97%.
- SPF 50 filters about 98%.
No sunscreen blocks 100%. The step from SPF 30 to SPF 50 is small, and a higher number does not let you stay out longer without reapplying. Dermatology organizations commonly recommend SPF 30 or higher.
What "broad spectrum" means
SPF says nothing about UVA. A sunscreen labeled "broad spectrum" has passed a test showing that it also protects against UVA. Other UVA marks you may see:
- PA+ to PA++++ on products from Asia. More plus signs mean more UVA protection.
- A UVA logo in a circle on products sold in Europe.
- A star rating on some products in the United Kingdom.
Always choose a product with UVA protection as well as an SPF number.
What "water resistant" means
No sunscreen is waterproof or sweatproof. In the United States, a "water resistant" label must state whether the SPF holds for 40 or 80 minutes of swimming or sweating. After that time, or after toweling dry, you need to reapply.
How much to apply
Protection depends on the amount. Most people use far less than the tests assume.
- Face and neck: a generous layer. Two finger-lengths is a commonly used guide.
- Whole adult body: about 30 ml, roughly a shot glass.
- Often missed: ears, eyelids, lips, back of the neck, tops of the feet, hairline and scalp part.
Apply sunscreen to dry skin about 15 minutes before going outside.
When to reapply
- About every two hours outdoors.
- Straight after swimming, heavy sweating or toweling.
- More often if it rubs off on clothes or masks.
If you wear makeup, see How to Reapply Sunscreen Over Makeup.
Sunscreen types
Sunscreens use mineral filters, chemical filters or a mix. Both kinds protect well when used properly. Mineral vs Chemical Sunscreen: Key Differences compares them. The best product is the one you are willing to wear in a sufficient amount every day. Face Sunscreen Buying Guide: Protection and Finish helps you choose a face sunscreen by finish and skin type.
Expiry and storage
Check the expiry date. Where no date is shown, sunscreens are generally made to last about three years unopened. Heat shortens that time, so do not leave the bottle in a hot car. Throw away sunscreen that has changed color, smell or texture.
Sunscreen is one part of protection
Shade, clothing, a hat and sunglasses all reduce the UV that reaches your skin. Sun Protection Tips Beyond Sunscreen covers these. Sunscreen is the last step of your morning routine. See Morning Skincare Routine: A Simple Step-by-Step Guide.
Related on ArniSun
- Mineral vs Chemical Sunscreen: Key Differences
- Sun Protection Tips Beyond Sunscreen
- How to Reapply Sunscreen Over Makeup
- Face Sunscreen Buying Guide: Protection and Finish
- Morning Skincare Routine: A Simple Step-by-Step Guide
Sources and further reading
Frequently asked questions
Is SPF 100 twice as good as SPF 50?
No. SPF 50 filters about 98% of UVB and SPF 100 about 99%. Applying enough, and reapplying, matters more than a very high number.
Does a high SPF mean I can reapply less often?
No. All sunscreens wear off, break down in sunlight and rub away. Reapply about every two hours outdoors whatever the SPF.
Do people with dark skin need sunscreen?
Yes. Darker skin has more natural protection against burning, but it can still be damaged by UV, develop skin cancer and form dark marks. Sunscreen is recommended for all skin tones.