DEAR DOCTOR K:
You’ve said many times that the sun can harm our skin and increase the risk for skin cancer. How does it do that?
As you age, the single biggest cause of damage to skin is sun exposure. This damage is called “photoaging.” Over the years, sun exposure causes fine and coarse wrinkles; baggy skin with a yellow, leathery appearance; and dry, scaly skin. It also reduces collagen, a natural chemical that gives strength to tissues and that supports a network of blood vessels in the skin. As a result, the skin bruises more easily.
Finally and most significantly, sun exposure increases the risk for skin cancer. That includes melanoma — the very dangerous kind of skin cancer that can spread — and the less serious kinds of skin cancer that rarely spread.
Skin damage from sun exposure is caused by ultraviolet (UV) radiation. Three different wavelengths of UV light come from the sun. Not all the sun’s rays have the same effects, though all can cause damage to the skin. Some wavelengths of UV radiation penetrate the skin more deeply than others. (I’ve put an illustration of this, below.)
Three wavelengths of skin damage
Three different wavelengths of ultraviolet (UV) light come from the sun. UVA is the longest wavelength and penetrates most deeply. UVB is the next longest and can cause burns to the skin’s upper layer. UVC, the shortest wavelength, normally is blocked by the ozone layer in the atmosphere. All can cause damage to the skin.
The three main wavelengths of the sun’s UV radiation:
- UVC, the shortest of the three, has the highest energy level and is the most damaging. Fortunately, most UVC rays don’t reach the Earth’s surface.
- UVB is the next shortest. UVB rays are lower in energy than UVC rays, so they’re less damaging to skin. UVB rays make up around 5 percent of the ultraviolet radiation that reaches the Earth’s surface.
- UVA is the longest wavelength and penetrates most deeply. UVA rays play a role in photoaging.
The people most vulnerable to getting melanoma are those who are fair-skinned (particularly pale skin, lots of freckles and red hair), and who have major sun exposure early in life. Most dangerous are repeated bad sunburns (five or more).
As a fair-skinned kid who spent a lot of time as a child on the beaches of Southern California, I had many sunburns. It was a time before we recognized the dangers of too much sun exposure. You might say we worshipped the sun. It was false idolatry: A dermatologist now checks me out every six months.
All types of cancer start in one cell. The genes in that cell cause the cell to start dividing uncontrollably: One cell becomes two, two become four, four become eight, and so on. Pretty soon, there’s a cancer. This uncontrolled growth is driven by genes — genes that aren’t built properly, or that are turned on and off at the wrong time.
This applies to all types of skin cancer, including melanoma. In some cases of melanoma (roughly 10 percent) a person inherits a gene that makes them vulnerable. More often, DNA in skin cells is damaged by UVB and UVA radiation.
I wish I had known when I was a kid what we know now about the damage sun exposure can do. Fortunately, the knowledge that research has given us will protect our children and grandchildren.