Sun and UV outdoors: the UV Index, sunscreen that works, and why snow and altitude matter

CDC says to protect your skin whenever the UV Index is 3 or higher. The UV Index forecast is issued daily by the National Weather Service and EPA. Japan's weather agency publishes one too; it's an international scale, so you can use it anywhere you travel.

The UV Index (EPA)

EPA's quick check: if your shadow is shorter than you are, UV is high. Japan's environment ministry splits the same index into five steps, from "weak" (1-2) to "extremely strong" (11+).

Sunscreen that works

• Broad-spectrum (UVA and UVB), SPF 15 or higher, applied before you go outside (CDC). NIOSH recommends at least SPF 30 for outdoor workers.
• Use about one ounce to cover exposed skin, about 15 minutes before going out (EPA).
• Reapply every two hours, even on cloudy days, and after swimming, sweating or toweling off (EPA, CDC).
• Sunscreen without an expiration date has a shelf life of no more than 3 years (CDC).
• Sunscreen isn't recommended for babies 6 months or younger (CDC); use shade and clothing.

Shade, clothing and sunglasses

UV rays are strongest from 10 a.m. to 4 p.m. daylight saving time in the continental U.S. (CDC). Tightly woven fabric protects best, and a wet T-shirt protects less than a dry one. Choose sunglasses that block both UVA and UVB. Japan's environment ministry says UV-blocking sunglasses or glasses can cut eye exposure by up to 90%, and reminds people that you still get UV in the shade.

Snow, water and altitude

• Water, snow and sand reflect UV and raise your chance of sunburn (EPA). Japan's ministry puts fresh snow at about 80% reflection, sand at 10 to 20%.
• EPA says UV intensity rises about 6% per kilometer of elevation; Japan's ministry says 10 to 12% per 1,000 m. Either way, a mountain hike delivers more UV than the trailhead.
• Japan's ministry notes winter UV is weak, but reflection from snow can nearly double exposure, especially in high mountains.
• Clouds don't stop it all: EPA says broken clouds still let through 73% of UV.

Our suggestions

• Add sunscreen and sunglasses to your Ten Essentials; NPS lists sun protection as one of them.
• Plan strenuous stretches for early morning, which also helps with heat.
• Keep a small sunscreen and a hat in your emergency kit.