Have you ever checked your weather app and noticed the UV index (UVI) embedded in all the other details for your daily report? But what is the UVI? Think of the UV index as a way to predict your risk of getting a sunburn that day.
Before we talk about the UVI, it’s important to mention some basic background information. Here comes the mini physics lesson. The sun produces ultraviolet light (UV) that reaches the earth’s surface. There are three main types of UV light based on their wavelengths: ultraviolet A (UVA), ultraviolet B (UVB), and ultraviolet C (UVC). UVA is the longest wavelength and reaches the earth’s surface. UVB is medium in length, and some of it reaches the ground. UVA is more constant throughout the year, penetrates deeper into the skin, is responsible for tanning, early wrinkling, and plays a role in skin cancer development. The amount of UVB we are exposed to varies throughout the year and is the main wavelength that causes skin burning and skin cancer. UVC rays are blocked by the ozone layer and do not reach the earth’s surface. So when we are talking skin—UVB and UVA rays are the main players. Think of the B in UVB as B for Burn and the A in UVA as A for Aging. Funny how that works perfectly!
How much UVA and UVB are we getting each day? That’s where the UV Index can help. The UVI was initially created in Canada in 1992 and later adopted by the United States National Weather Service (NWS) and Environmental Protection Agency (EPA) as well as the World Meteorological Organization (WMO) and World Health Organization (WHO) in 1994. The UV index measures UV intensity on a scale from 1-11+, with 1 being minimal risk and 11+ being a very high risk. In the United States, the NWS calculates the UV Index each day using a computer model. This calculation considers factors like clouds, the amount of ozone, and the amount and angle of the sunlight that may affect how much UV light reaches the earth’s surface. The UV index varies depending on the time of day, weather, and the season. However, it doesn’t take into account surfaces like snow or water and how they might affect the intensity of UV light.
The UVI is based on a Type 2 Fitzpatrick skin type (fair skin, burns easily, tans minimally) and predicts the risk around noontime since this is when the sun is highest in the sky. To give you an idea of risk, the scale looks like this:
- Low exposure (green): 1-2
- Moderate exposure (yellow): 3-5
- High exposure (orange): 6-7
- Very high exposure (red): 8-10
- Extreme exposure (violet): 11+
When the UVI reaches 5 or above, paying particular attention to safe sun practices and protecting yourself from overexposure is recommended. So does that mean that you don’t need sunscreen when the UVI is low? Absolutely not. Regardless of the UVI, it’s important to still wear sunscreen. Skin cancer development and premature aging of the skin are also linked to chronic sun exposure and not just burning. Knowing the UVI is not a sunscreen advisor but rather a measurement of sunburn risk for the day so you are prepared.
Trotter’s Take: So should you care? Yes, but not to the point that you’re only motivated to use sun protection if you see an index of 5 or higher. The UV Index is useful to better understand our risk of overexposure to the sun. However, I tell my patients to practice sun protection regardless of the UV index for the day to keep up good habits and for an added layer of protection.
We know UV exposure is linked to the development of skin cancer, including melanoma. If you want to know more about a patient’s journey with skin cancer, then check out my podcast with Leah Adams to hear an inspirational story of triumph over melanoma.
