I’ve noticed within the past few years, whenever I mention how high the UV index is on a particular day, I’m always met with confusion. “What index? Are you reading a book?” I don’t know if awareness has faded with time or if it’s just my personal experience, but I want to share some basic information about the UV index. It’s an important thing to consider, especially since summer is coming in the Northern Hemisphere.

What is UV? Here is an explanation from the World Health Organization (WHO):

Ultraviolet (UV) radiation is part of the electromagnetic spectrum emitted by the sun. Whereas UVC rays (wavelengths of 100-280 nm) are absorbed by the atmospheric ozone, most radiation in the UVA range (315-400 nm) and about 10 % of the UVB rays (280-315 nm) reach the Earth’s surface. Both UVA and UVB are of major importance to human health.

Small amounts of UV are essential for the production of vitamin D in people, yet overexposure may result in acute and chronic health effects on the skin, eye and immune system.

What do the numbers mean? Here is a guide from the Environmental Protection Agency (EPA):

UV Index Scale

The UV Index scale used in the United States conforms with international guidelines for UVI reporting established by the World Health Organization. Learn how to read the UV index Scale to help you avoid harmful exposure to UV radiation.

0 to 2: Low

A UV Index reading of 0 to 2 means low danger from the sun’s UV rays for the average person.

• Wear sunglasses on bright days.
• If you burn easily, cover up and use broad spectrum SPF 30+ sunscreen.
• Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.

3 to 5: Moderate

A UV Index reading of 3 to 5 means moderate risk of harm from unprotected sun exposure.

• Stay in shade near midday when the sun is strongest.
• If outdoors, wear protective clothing, a wide-brimmed hat, and UV-blocking sunglasses.
• Generously apply broad spectrum SPF 30+ sunscreen every 2 hours, even on cloudy days, and after swimming or sweating.
• Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.

6 to 7: High

A UV Index reading of 6 to 7 means high risk of harm from unprotected sun exposure. Protection against skin and eye damage is needed.

• Reduce time in the sun between 10 a.m. and 4 p.m.
• If outdoors, seek shade and wear protective clothing, a wide-brimmed hat, and UV-blocking sunglasses.
• Generously apply broad spectrum SPF 30+ sunscreen every 2 hours, even on cloudy days, and after swimming or sweating.
• Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.

8 to 10: Very High

A UV Index reading of 8 to 10 means very high risk of harm from unprotected sun exposure. Take extra precautions because unprotected skin and eyes will be damaged and can burn quickly.

• Minimize sun exposure between 10 a.m. and 4 p.m.
• If outdoors, seek shade and wear protective clothing, a wide-brimmed hat, and UV-blocking sunglasses.
• Generously apply broad spectrum SPF 30+ sunscreen every 2 hours, even on cloudy days, and after swimming or sweating.
• Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.

11 or more: Extreme

A UV Index reading of 11 or more means extreme risk of harm from unprotected sun exposure. Take all precautions because unprotected skin and eyes can burn in minutes.

• Try to avoid sun exposure between 10 a.m. and 4 p.m.
• If outdoors, seek shade and wear protective clothing, a wide-brimmed hat, and UV-blocking sunglasses.
• Generously apply broad spectrum SPF 30+ sunscreen every 2 hours, even on cloudy days, and after swimming or sweating.
• Watch out for bright surfaces, like sand, water and snow, which reflect UV and increase exposure.

The Shadow Rule

An easy way to tell how much UV exposure you are getting is to look for your shadow:

• If your shadow is taller than you are (in the early morning and late afternoon), your UV exposure is likely to be lower.
• If your shadow is shorter than you are (around midday), you are being exposed to higher levels of UV radiation. Seek shade and protect your skin and eyes.

It’s important to take into account the UV index not just for our own protection, but when understanding others. I’ve seen situations in which friends and family living in different cities or countries could not understand why a loved one got a sunburn when it was “only __ degrees” outside and they weren’t out all day. “It’s hotter than that here and I was fine!” Sometimes temperatures that seem low in numbers can still be dangerous conditions in which to work, exercise, or play. Similar to how we take humidity levels into account (especially in certain regions), everyone should pay attention to the UV index when looking up the weather forecast. We may not be able to immediately feel it the way we do with temperature and humidity, but it should be considered when planning which activities to do, what time of day to do them, how long they can be done, what to wear, and how much sunscreen we will need.

Here’s a visual aid showing the mean UV index in the US for the summer months. (If you have relevant links or images for other countries, please share in the comments.)

For more information on UV index, here is a pdf from WHO, World Meteorological Organization,
United Nations Environment Programme, and International Commission on Non-Ionizing Radiation Protection.

Stay safe and have a great summer. To the readers in the Southern Hemisphere, I hope your winter is great and please remember that snow reflecting the sun can increase your UV exposure. So everyone please stay safe and have fun!