UVA and UVB activate melanocytes (a special type of cell in the skin) which produce melanin along tubules in the skin.
UVA is a longer waveform and penetrates the skin deeper, activates melanocytes and damages DNA.
UVB is a shorter waveform and penetrates the skin shallow and also damages DNA (at a shallower level).
UVA is the type of UV radiation that doesn't burn you, but still activates the melanocytes to produce color.
UVB is the type of UV that does burn you (along with activating melanocytes), UVB also aids in oxidizing melanin to become brown while UVA doesn't.
All types of UV do DNA damage. This is why the melanoytes release melanin. The melanin (pigment) is released into the skin as a pinkish color and oxidizes to brown (hence the tan). Melanin is responsible for absorbing UV light (and other forms of light), that's why we appear darker after we get a tan. The melanin is used as a natural sunscreen to absorb the UV light before it can do more DNA damage.
But, melanin isn't 100% blocking, it just permits you to be out in the sun longer than without it. Hence, people without a tan burn rapidly. The more tan you are, the longer you can remain in the sun.
To regain the original color, avoid the sun. In time, the skin cells filled with the color will wash off the surface and then underlying 'whiter' cells will become visible. If it needs to be done faster, it can be done with certain types of bleaching solutions (while faster than waiting for it to naturally go away, still takes a fair amount of time to bleach out).
Keep in mind that what really happens is that the DNA damage causes MSH (melanocyte stimulating hormone) to be released. This hormone is really what is responsible for activating the melanocytes to release melanin. People who are 'white' without color don't produce MSH constantly. Other people who have darker skin produce some level of MSH consistently that constantly has the melanocytes producing melanin.
Also if you are taking medications that affect the melanocytes or the melanin production that will cause tanning or change of color at minimal sun exposure
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Annual Checkup
It is recommended that all individuals see a Family Doctor for a full medical examination annually. Adults should get their blood pressure checked (US Preventive Task Force), a flu vaccine (CDC) and more each year.
The American Dental Association recommends dental cleanings for adults at least twice annually. Some stains can only be removed by a professional cleaning.
It’s recommended by the American Academy of Ophthalmology that every adult between the ages of 18 and 80 does an eye examination at least once every two years.
It's recommended by the American Cancer Association that every adult between the age of 20 and 80 does an annual skin screening by a licensed dermatologist.
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