If you’ve ever felt like the sun is especially intense on a particular day, you’re not imagining it. The daily amount of the sun’s ultraviolet (UV) radiation that reaches the ground can vary, causing one day to feel pleasant and the next to feel like your skin is burning the minute you step outside.

The National Weather Service calculates the UV index (sometimes referred to as UVI) daily for each ZIP code, and the Environmental Protection Agency shares the data to help people plan outdoor activities.

Factors that go into calculating the UV index include ozone concentration in the stratosphere (the second layer of the earth’s atmosphere up from ground level), cloud amounts and the elevation at each ZIP code.

The resulting index rates your risk of overexposure to UV radiation on a scale of 1 to 11. Each level also includes recommendations to help you stay sun safe:

• Level 1 – 2 (green): Minimal risk from unprotected sun exposure for the average person. The Aim at Melanoma organization says that most people can stay outside for an hour between 10 a.m. and 4 p.m. without burning. But if you burn easily, cover up and use sunscreen with a sun protection factor (SPF) of at least 30 and wear sunglasses with UV protection.
• Level 3 – 5 (yellow): A low risk of harm from unprotected sun exposure; those with fair skin may burn within 20 minutes. Wear protective clothing, sunglasses and a hat when outdoors. Stay in the shade around midday. Use sunscreen with at least SPF 30 and reapply every two hours, even on cloudy days. Wear sunglasses with UV protection.
• Level 6 – 7 (orange): A moderate risk of harm from unprotected sun exposure. Wear protective clothing, sunglasses and a hat when outdoors. Use sunscreen with at least SPF 30 and reapply every two hours, even on cloudy days. Reduce your time in the sun between 10 a.m. and 4 p.m. Wear sunglasses with UV protection.
• Level 8 – 10 (red): A high risk of harm from unprotected sun exposure. Wear protective clothing, sunglasses and a hat when outdoors. Use sunscreen with at least SPF 30 and reapply every two hours, even on cloudy days. Seek shade outdoors. Try to avoid the sun between 10 a.m. and 4 p.m. Wear sunglasses with UV protection.
• Level 11+ (purple): A very high risk of harm from unprotected sun exposure. Wear protective clothing, a hat and sunglasses with UV protection when outdoors. Apply sunscreen with at least SPF 30 every two hours, even on cloudy days. Seek shade outdoors. Try to avoid the sun between 10 a.m. and 4 p.m.

Be Sun Smart

Here are additional considerations to help you stay sun safe:

• Just because you’re in the shade doesn’t mean you are completely protected – snow, water, sand, pavement and glass can reflect UV rays onto you. Wear sunscreen and cover up accordingly, even in the shade.
• When choosing protective clothing for sun exposure, look for long-sleeved shirts and trousers made from tightly woven fabrics. Loosely knit materials can let the sun’s rays through. Look for clothing rated with an ultraviolet protection factor (UPF) of 50 for the best protection from UV rays.
• Use the “shadow rule” to help avoid times of day when UV exposure can be the highest: If your shadow is taller than you are (early morning and late afternoon/evening), your UV exposure is lower.
• Wearing a wide-brimmed hat can prevent about half of UV rays from reaching your eyes. It will also keep the sun off your scalp, which is just as susceptible to skin cancer as the rest of your body.
• Don’t forget to apply sunscreen to often-overlooked areas such as your neck, the tips of your ears and the top of your feet in low-vamp shoes. Use SPF balm on your lips.
• Reapply sunscreen every two hours or after exposure to water (such as swimming). No matter how high the SPF is or what the label claims, no sunscreen lasts all day.

Sources:
Aim at Melanoma
Environmental Protection Agency
Melanoma Research Alliance