Ultraviolet Radiation From The Sun
The sun emits radiation, consisting of electromagnetic rays including ultraviolet A (UVA) and ultraviolet B (UVB). This ultraviolet radiation has many effects on you.
On one hand, sun exposure is responsible for some powerful health benefits, including catalyzing the production of Vitamin D in your body. Sunlight has also been shown to improve the symptoms of depression and Seasonal Affective Disorder and—correlated with optimal Vitamin D levels—may also positively affect many other adverse physical and mental conditions and even help prevent some cancers.
On the other hand, exposure to the sun’s radiation, especially UVA and UVB rays, damages human tissue such as your skin and eyes, causes sunburn and premature aging, induces inflammation, and suppresses the immune system. The longer skin tissue is exposed to sunlight, the greater the damage. In fact, nothing ages your skin more than sunlight.
Overexposure to the sun is known to:
- Cause sunburn
- Result in free-radical damage to cells
- Break down the collagen and elastin in your skin
- Lead to premature skin aging, wrinkles, and sunspots
- Induce inflammation
- Trigger immunosuppression (decreased effectiveness of the immune system)
- Mutate DNA and increase the chances of developing skin cancer
For further information on the following topics related to ultraviolet radiation, expand the respective sections below:
Skin Damage
Both ultraviolet A and ultraviolet B radiation damage your skin, but the primary method by which they do their damage is different.
UVB radiation (which is higher energy and has a shorter wavelength than UVA) does the majority of its damage directly to your DNA. When UVB radiation strikes a strand of DNA, it can result in the creation of pyrimidine dimers (either thymine dimers or cytosine dimers). Other names for these structures are cyclobutane pyrimidine dimers (CPDs) and cyclobutane photodimers (CPDs). These unnatural compounds are usually repaired by your skin cells’ natural repair mechanisms, but too much of this damage can result in cell death (as may occur in sunburn), erythema (skin redness), inflammation, and occasionally mutation. Mutations, over time, can cause cancer.
UVA radiation causes its damage via another mechanism. UVA radiation, which is a longer wavelength than UVB and as such can travel deeper into your skin, strikes other structures in skin cells, creating free radicals and reactive oxygen species (ROSs). These free radicals and ROSs can damage you DNA, but they also damage other structures in your skin such as collagen and elastin. This damage eventually results in many of the negative effects attributed to the sun. Furthermore, these free radicals and ROSs can travel through your blood stream and damage other areas of your body not directly reached by UV light.
UVB and UVA rays vary in intensity throughout the day. Because UVB radiation is partially blocked by the earth’s atmosphere, the strength of UVB rays that reach your skin varies considerably depending on the time of day. The further overhead the sun is, and thus the less atmosphere the sunlight travels through before it reaches you, the greater the intensity of the UVB rays. It takes you less time to burn in the middle of the day than the morning or evening.
Conversely, the longer wavelengths of UVA rays are relatively unfiltered by the earth’s atmosphere. The amount of UVA radiation that reaches your skin in the middle of the day does not vary much from the amount that reaches your skin in the morning or evening. Thus, while you may not notice the effects immediately and might not get burned, you are still damaging your skin while exposed to seemingly low-intensity sunlight.
The same holds true when it is overcast, or in the fall/winter/spring. While the intensity of UVB rays is much lower, the intensity of UVA rays remains little changed. While you are not getting sunburned, these periods of sun exposure can be extremely dangerous because the damage being sustained is often unnoticed. These periods of seemingly innocuous sun exposure can significantly contribute to the cumulative damage and aging of your skin.
Tanning
Color in your skin is primarily attributed to melanin. The more melanin present, the darker your skin. Your body produces melanin in specialized cells called melanocytes. When UVB radiation damages the DNA in melanocytes, creating pyrimidine dimers, melanocytes are triggered to increase their production of melanin. This melanin is released from melanocytes in little packets of melanin called melanosomes, which are eventually taken up by keratinocytes (a type of skin cell). This melanin colors keratinocytes, and as these cells migrate toward the surface of your skin the result is the appearance of your skin as tan. This process of melanogenesis takes 2 to 3 days.
Whereas UVB radiation stimulates your production of melanin, UVA radiation does not. UVA radiation does, however, cause the release of small amounts of preexisting melanin from melanocytes, resulting in an immediate but short-lived “tan.”
The increased production of melanin in your skin is caused by DNA damage from UVB radiation. So, in short, a tan is a marker of some level of DNA damage. However, melanin does protect the skin from additional sun damage by efficiently and harmlessly converting the energy in ultraviolet radiation into heat. The more melanin that is present in your skin, the less ultraviolet radiation is able to pass through to cause damage to other skin structures below it. This same melanin, however, also inhibits your production of Vitamin D.
Vitamin D Production
Ultraviolet B radiation, especially between the wavelengths of 295-297nm, is able to stimulate the production of Vitamin D in your skin. When UVB rays at these wavelengths hit a molecule called 7-dehydrocholesterol (a molecule that would otherwise eventually be converted into cholesterol in the body), it causes the conversion of this molecule into pre-vitamin D3. Over the course of approximately 12 days, this pre-vitamin D3 is converted to Vitamin D3 (also called cholecalciferol), and enters your blood stream. It is then transferred to your liver where it is converted into 25-hydroxycholecalciferol (also called calcidiol) and stored. Eventually it makes its way to the kidneys where it is converted into 1,25-dihydroxycholecalciferol (also called calcitriol): its final, active form.
Vitamin D is involved in a number of crucial pathways in your body, including the proper functioning of your immune system, normal bone growth and repair, the absorption of calcium, the concentrations of calcium and phosphate in your blood stream, neural and muscular function, and the regulation of a number of different genes. In short, it is an extremely important molecule, and production in sunlight is one of the main methods the body obtains its necessary levels of Vitamin D. It is possible, however, to obtain enough Vitamin D from food to meet your needs.
