How does ultraviolet (UV) light cause tanning of our skin?

Tanned armIn 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 light?

Bahamas_beachWhat 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).

What is Athlete’s Foot?

Image taken by Dr James Heilman
Image taken by Dr James Heilman

Athlete’s foot is a common skin complaint and is also commonly known as “foot fungus” and ringworm of the foot (although it’s not caused by a worm!) It is more formally known as tinea pedis.  The name “Athlete’s Foot,” however, gives us a sense of conditions that predispose to getting this condition. In athlete’s foot, the skin is infected by a fungus that likes the surface of the skin. This fungus also favors moist conditions and you can pick it up at the gym, poolside, communal showers, contaminated shoes, etc.  One can thus think of the athlete who walks around barefoot in the communal gym & shower area, picking up fungus on the feet.  Afterwards, the feet are then stuffed into shoes, where the fungus thrives in a dark damp environment.

So what does athlete’s foot look like?  You may notice some scaling or redness between the toes or on the bottom of your feet.

What does it feel like? Athlete’s foot may be asymptomatic, or there can be associated itch.

And what should you do if you have athlete’s foot?  Keep the feet dry.  While athlete’s foot can go away on its own by keeping the feet dry and changing shoes, there are plenty of good over the counter creams to help it along.  The generic name for one cream that is frequently used is terbinafine (e.g. Lamisil) and this can be used twice a day. Of course, if you’re not sure whether you have foot fungus or something else, have it checked out. This is an easy diagnosis for a dermatologist or podiatrist to make.