In a continuation on the theme of ultraviolet (UV) light, we answer the question of how UV light causes tanning or darkening of our skin. The answer is that there are multiple methods by which tanning occurs.
Depending on your baseline skin type, you may have noticed that after being out in the sun, your skin becomes tanned or darker (if you’re very fair-skinned, you may not tan at all, but rather, just burn). You may have noticed this change in your skin tone hours after your sun exposure, or perhaps days after.
Tanning/darkening occurs via two methods and thus can occur immediately as well as in a delayed form.
1) In immediate tanning, the darkening occurs hours after the exposure to ultraviolet light. The trigger is ultraviolet-A (UVA) light. The melanin (pigment giving our skin color) which is already sitting around in your skin cells becomes oxidized. Melanosomes (which are sacs of melanin in our cells) also get redistributed. These effects occur relatively quickly and the tan can be seen within hours. Note that in this form of tanning, there is no protective effect against additional UV exposure.
2) In delayed tanning, new melanin is made by your cells in a process called melanogenesis. Since it takes time for your cells to make melanin, the tan is not evident until 48-72 hours after exposure. This process is triggered by ultraviolet-B (UVB) light.
Need a review on the difference between UVA and UVB and wavelengths of light? See the previous DermBytes post on this topic.
What is ultraviolet (UV) light? Ultraviolet light is made of up light waves of a certain wavelength. What does that mean? Well, I think back to grade school where we learned about different wavelengths of light. Certain wavelengths would form certain colors of light. e.g. in a rainbow, each color reflects a different wavelength of light (recall the mnemonic of ROYGBIV for the colors of the rainbow? red, orange, yellow, green, blue, indigo, and violet). However, these colors of light were part of visible light. This is light that we can see with our eyes.
However, there’s light that we can’t see as well. We can’t see ultraviolet light.
So what does ultraviolet light have to do with the skin? (After all, DermBytes is a blog about all things skin.) Well, ultraviolet light can cause damage to the DNA of our cells and can also make our immune system less effective. UV light can lead to sunburns, cause darkening or tanning of the skin, and lead to increased risk of skin cancer. In some people, UV light can also lead to various rashes. In others, patients may find that the light helps their skin rash (e.g. psoriasis).
Because of the potential deleterious effects of UV on the skin and its potential to lead to skin cancer, dermatologists recommend using sunscreen. Remember from our previous post that we should choose a sunscreen that protects against both UVA and UVB. UVA and UVB simply refers to different sets of wavelengths. (Specifically, UVA includes wavelengths of 315-400 nm and UVB includes wavelengths of 290-315 nm).
One day after Tropical Storm/Hurricane Irene passed through town, the sun is back! Perfect time to learn about sunscreen.
A common question is: what is SPF? SPF stands for sunburn protection factor. (The FDA changed it from sun protection factor to sunburn protection factor as sunscreen does not completely protect you from the harmful UV of the sun.) The number is a measure of how many times more it takes for you to burn with the sunscreen on compared to not wearing the sunscreen. So for instance, if it takes a certain amount of UV rays for you to become red without sunscreen, it would take 15 times that amount of UV for you to become red when you wear SPF 15 sunscreen. The SPF label assumes that you correctly apply the sunscreen. You should use about 1 ounce to cover the surface of your body. Furthermore, you need to reapply the sunscreen as you sweat, or get wet. Additionally, the SPF value on bottles refers to harmful UVB (ultraviolet-B) rays, but not harmful UVA rays. When choosing a sunscreen, look for one that covers UVA and UVB (“broad spectrum” sunscreens).