The UV Index, shade, and sun-protective clothing

How to read the UV Index, use the shadow rule, pick UPF clothing, hats, and sunglasses, and account for windows, altitude, snow, and water.

Sunburn depends on the strength of ultraviolet (UV) radiation, and temperature tells you little about it. A cool, breezy day in May can deliver more UV than a sweltering afternoon in September. The UV Index was created to give people 1 number that captures UV strength for a given place and day.

Reading the UV Index

The National Weather Service and the Environmental Protection Agency (EPA) publish a daily forecast for every U.S. ZIP code, and most weather apps show it. The scale starts at 0 and has no fixed top. Values of 11 or more occur in summer at high altitude and near the equator. The World Health Organization (WHO) names the bands low (up to 2), moderate (3 to 5), high (6 to 7), very high (8 to 10), and extreme (11 and above).

The EPA groups its advice into 3 levels:

• 0 to 2, low: most people can be outside with minimal protection.
• 3 to 7, moderate to high: protection needed. Seek shade from late morning through mid-afternoon, wear protective clothing, a wide-brimmed hat, and sunglasses, and apply broad-spectrum sunscreen generously.
• 8 and above, very high to extreme: extra protection needed. Take all of the steps above and be especially careful around midday. If your shadow is shorter than you are, move into shade.

The EPA wording names SPF 15 as a minimum, while the American Academy of Dermatology and the American Cancer Society recommend SPF 30 or higher. WHO notes that below a UV Index of 3, the risk of damage is limited even for very fair skin.

The shadow rule

No phone is needed for this check. Look at your shadow. A shadow taller than you means the sun is low, as in early morning or late afternoon, and UV exposure is likely to be lower. A shadow shorter than you means the sun is high and UV is strong.

UV peaks in the 4 hours around solar noon. In most of the United States during daylight saving time, solar noon comes near 1 p.m., so the peak runs from about 11 a.m. to 3 p.m., and the EPA advises extra care from late morning through mid-afternoon.

Shade helps, with limits

A tree or umbrella blocks direct rays. UV also scatters through the open sky and bounces off the ground, and those rays still reach you. WHO cautions that a long stay in open shade, for example between buildings, can still burn a sensitive person on a high UV day.

Clouds are similar. Thick cloud cover reduces UV, while thin or broken clouds may have little effect.

What UPF on a clothing label means

UPF stands for ultraviolet protection factor. It measures how much UVA and UVB passes through a fabric. A shirt rated UPF 50 lets through about 1/50th of the UV that hits it, which means it blocks 98%. The Skin Cancer Foundation rates UPF 30 to 49 as very good and 50 or more as excellent, and requires UPF 50 for its Seal of Recommendation.

Regular clothes vary widely. A white cotton T-shirt has a UPF of about 7, and that falls to about 3 when the shirt is wet. A long-sleeved denim shirt, by the AAD's estimate, offers protection equal to about SPF 1,700. Several features raise a fabric's protection:

• Tight weave. Hold the cloth up to a light; the less you can see through it, the better.
• Dark or bright color, which absorbs more UV than white or pastel.
• Dry fabric. Wet cloth lets more UV through.
• A loose fit. Stretched fabric thins out and lets more light pass.

Hats and sunglasses

A hat brim of at least 3 inches all the way around shades the face, ears, scalp, and back of the neck. Baseball caps leave the ears and neck exposed, and straw hats with open weave let light through. The ears and scalp are common sites for skin cancer, particularly in men with thinning hair.

For sunglasses, the label matters more than the tint. Dark lenses without UV filtering can do harm, because the pupils widen behind them. Look for a label stating that the lenses block 99% to 100% of UVA and UVB. Large frames and wraparound styles protect the eyelids and the thin skin around the eyes, where skin cancers are hard to treat.

Cars and windows

Ordinary glass filters out most UVB, the rays that cause sunburn, so people rarely burn indoors. UVA passes through. In cars, the front windshield is laminated glass that is treated to block some UVA. Side windows, rear windows, and sunroofs are usually tempered glass, which blocks UVA poorly.

Over years, that adds up. The Skin Cancer Foundation notes that in the United States melanomas are more common on the left side of the body, the driver's side. Clear UV window film can block more than 99% of UVA and UVB. People who drive for a living or sit beside a sunny window all day may want to ask about it.

Altitude, snow, sand, and water

Thinner air filters less UV. WHO estimates that UV levels rise about 10% with every 1,000 meters (about 3,300 feet) of altitude. A ski resort at 3,000 meters receives roughly 30% more UV than a beach on the same day.

Reflection adds a second dose from below, reaching skin that a hat shades. Fresh snow can reflect up to 80% of UV, which nearly doubles exposure. Sea foam reflects about 25% and dry beach sand about 15%. Grass, soil, and still water reflect less than 10%. Skiers and climbers burn under the chin and nose for this reason, and they need sunscreen, lip protection, and goggles or sunglasses even in midwinter.