It is a well-known fact that too much exposure to the sun causes damage to our skin. However, many people are not aware that there are two kinds of ultraviolet rays in sunlight that can cause damage: UVA and UVB. Here’s how they compare to each other.
The sun emits radiation in the UVA, UVB and UVC bands. The ozone layer of the Earth blocks about 97-99% of this UV radiation from ever penetrating the atmosphere. Of the radiation that does reach the Earth’s surface, 98.7% is UVA. These rays are known as long-wave, or black light waves, at about 320-400 nanometers long.
UVA rays can pass through windows, light clothing, and windshields. Furthermore, these rays have the same strength year-round regardless of how close the earth is to the sun.
UVA was once thought to be safer than UVB but these rays may actually be more damaging to the body than UVB. While less likely than UVB to cause sunburn, UVA penetrates the skin more deeply, causing leathering, wrinkling, cracking and shrinking of collagen and elastin, as well as the destruction of vitamin A and D.
Recent studies also indicate that UVA rays contribute to and may even initiate the development of skin cancer. In particular, the rays damage skin cells in the basal layer of the epidermis, where most skin cancers occur.
UVB rays are known as short-wave rays and are at about 290-320 nanometers long. These rays are strongest in the summer months especially when the sun is highest in the sky around noon. They are also strongest at the equator. In short, less atmosphere means more UVB.
UVB rays do not penetrate as far into the skin as UVA rays but they are still damaging. They are the primary cause of skin reddening, sunburn, and skin cancer.
UVB rays do not pass through glass. This is why you can drive for a long time on a sunny day without getting a sunburn on your arms when the windows are up. Unfortunately, UVA radiation is not blocked by the glass so your skin is subjected to its damaging effects unless they are protected by clothing or a UVA sunblock.
There are some benefits to UVB exposure, such as the production of vitamin D in the skin. It also has regulatory roles in calcium metabolism, immunity, cell proliferation, insulin secretion, and blood pressure. The appropriate amount of UVB exposure varies according to skin color (less for light-skinned people, more for dark-skinned).
Tanning booths mostly emit UVA radiation, often at levels much higher than those naturally produced by the sun. Those who regularly use tanning salons are much more likely to develop squamous cell carcinoma.
For many years, sunscreens and sun blocks only blocked UVB rays because the damaging effects of UVA rays were not fully understood. The good news is that there are now products that block both UVA and UVB rays.