1
Two kinds of UV light
The sun sends ultraviolet (UV) radiation to Earth. Two types reach your skin in meaningful amounts: UVA and UVB.
UVB is the main cause of sunburn. It's strongest in the middle of the day (roughly 10 a.m. to 4 p.m.) and in summer, but it can still reach you year-round.
UVA is present at almost the same strength all day, all year, and it passes through glass. It reaches deeper into the skin and plays a big part in premature aging, dark spots, and skin cancer risk.
Both types contribute to skin cancer. That's why sun protection targets both, not just the rays that burn you.
2
What SPF actually means
SPF stands for Sun Protection Factor. It compares how much UVB it takes to redden skin with sunscreen applied properly versus with no sunscreen at all.
It is not a timer. SPF 30 does not mean "30 times longer in the sun." Applied generously, SPF 30 filters out roughly 97% of UVB; SPF 50 filters slightly more. The jump from 50 to 100 adds very little, which is why dermatologists point to SPF 30+ rather than chasing the biggest number.
"Broad-spectrum" is the label that tells you a sunscreen also protects against UVA. Without it, you may be covered for burning but not for deeper damage.
No sunscreen blocks 100% of UV, and most people apply far less than the amount used in testing, so real-world protection is usually lower than the label suggests.
3
Applying it well
Sunscreen works best when it's the one you'll actually use every day. Texture, finish, and price matter less than consistency.
- Use about a shot glass (roughly 30 ml) for your body, and a generous nickel-sized amount for your face and neck.
- Apply 15 minutes before you go outside so it has time to settle.
- Reapply every two hours, and straight after swimming, heavy sweating, or towel-drying, even if the label says water resistant.
- Don't skip the easy-to-miss spots: ears, neck, hairline and parting, tops of feet, lips (use an SPF lip balm), and the backs of your hands.
- Check the expiry date and don't leave bottles baking in a hot car.
4
Sunscreen is one layer, not the whole plan
Shade, clothing, a wide-brimmed hat, and UV-blocking sunglasses protect in ways sunscreen can't: they don't wear off, and they don't depend on how evenly you applied.
UV passes through clouds, reflects off water, sand, snow, and concrete, and comes through car and window glass (UVA especially). Cloudy and cool days still deliver UV.
No sunscreen makes hours of strong midday sun safe. If the UV Index is high, timing and shade do more for you than a higher SPF.