UV exposure increases roughly 4-5% for every 1,000 feet (about 300 m) of elevation gain, so a high-altitude hike delivers far more UV than the same hours at sea level.
Dermatologists note that on long hikes sweat and rubbing steadily wear sunscreen away, so reapplying every couple of hours matters more than chasing a higher SPF number.
Questions we get a lot
How often should I reapply sunscreen on a long hike?
Reapply broad-spectrum sunscreen about every two hours, and sooner if you're sweating heavily or wiping your face. Sweat and friction rub product off, so a water-resistant (40- or 80-minute) formula plus a stick for the face, lips and ears keeps protection going all day on the trail.
Is sun exposure stronger at higher altitude?
Yes. UV intensity rises roughly 4-5% for every 1,000 feet of elevation because there's less atmosphere to filter it, so summit and ridge hikes carry a higher burn risk than valley walks. Cover up with SPF 30+, a hat and UV-blocking sunglasses, and reapply regularly.
How much sunscreen should I actually apply before a hike?
Most people apply far too little. Sunscreen is tested at 2 mg/cm², but typical use is a quarter to half that — which can drop an SPF 50 to real-world SPF 15 or lower. Aim for about a quarter to half teaspoon for the face and roughly a shot glass (1 oz) for the whole body before you head out.
r/Ultralight: 'What's the best sunscreen setup for an all-day exposed ridge hike in summer?'