"It's because of something called melanin," says, Dr. E. Vile, biochemist at AAHGG. "It is a special kind of pigment produced during the oxidation of the amino acid tyrosine."
How does this relate to our ghostly friends? When exposed to UV radiation, melanin darkens (i.e. causes you to "tan"). Because it also absorbs sunlight, it is believed to protect underlying skin cells.
The implications for the phantom species? Because they don't have melanin, they lack this protective barrier.
Then again, they also lack skin that would necessitate protection...
Stay tuned for forthcoming AAHGG updates. In the mean time, make sure to SCREEN up--unless, of course, you count yourself among the Undead.