Understanding the UV Index
By now, you’ve likely heard of Ultraviolet (UV) light. From weather forecasts to sunscreen and apparel labels, and even during regular skin checks with your dermatologist, the term “UV” is seemingly everywhere. But just what is it, and why should we pay attention to it? Here, we explore what’s behind this common acronym—how it’s measured, the ways it can impact your health, and how you can stay alert and informed about the UV conditions in your area each day.
What is Ultraviolet (UV) Light?
Often, when we talk about UV light, we mean the energy (or rays) emitted from the sun—although this energy can also come from tanning beds, as well as other artificial sources, such as Mercury vapor lighting, some halogen, fluorescent and incandescent lighting, and certain types of lasers.
Generally speaking, UV is a form of non-ionizing radiation that exists all around us. Backing up from there, radiation itself is defined by the American Cancer Society as “the emission (sending out) of energy from any source.” And UV radiation in particular comes in two primary forms—ionizing and non-ionizing—though there is a wide spectrum between the two.
UV sits in roughly the middle of this spectrum of energy, in between visible light—what we can see all around us—and the higher-frequency radiation of things like x-rays and gamma rays that we can’t see or directly feel.
While the scientific definition of UV can feel a bit cloudy or hard to grasp, think of it as the energy from the sun (or tanning beds, or certain kinds of lighting) that is capable of reaching and even penetrating the outer layers of our skin, where other kinds of radiation cannot. This is precisely why UV can be harmful.
Types of UV Light
Generally speaking, there are three known forms of UV, named alphabetically as UVA, UVB and UVC. The third is considered “far-range” energy, and is ultimately absorbed by the Earth’s ozone layer before it can reach us. So in terms of sun protection and risk, we typically focus on UVA and UVB.
- UVA: Accounting for about 95% of all UV radiation that we experience on Earth, these rays have a longer wavelength and are capable of penetrating the dermis (the middle layer of our skin). While they have the lowest energy of the UV forms, most signs of premature aging, such as wrinkles, are attributed to UVA—and this kind of UV can also contribute to the development of some skin cancers.
- UVB: While this short-range energy only accounts for 5% of natural UV radiation, UVB is believed to cause the most significant harm. UVB rays have shorter wavelengths that generally impact the epidermis (outer layer of the skin), causing sunburns and most forms of skin cancer over time.
The UV IndexNow that you know what UV is, how can you be sure about your exposure? The UV Index was designed to help you manage your risk. By understanding your risk on any given day and in any given place, you can take appropriate precautions—beyond wearing daily sunscreen (a given!)—to avoid sunburn and reduce your risk of exposure to the radiation factors that can contribute to skin cancer.
The UV Index is determined based on four primary factors:
- The thickness of the ozone layer over your location
- The cloud cover over your location, since clouds block can block some UV radiation
- The time of year, since UV radiation varies by season based on the sun’s distance (tip: UV radiation is higher in the summer months)
- The elevation of your location, since higher elevations are exposed to more UV radiation
The UV Index is mapped on a numerical scale of 1 (Low) to 11+ (Extreme), with 11+ being the highest UV exposure risk.