There is plenty of evidence that the use of sunblock is what has caused the increase in melanoma over the last 25 years and not sun exposure.
Most people in this country are deficient in vitamin D and the vitamin D story is much, much more involved than simply taking a pill or casual sun exposure.
"Sensible sun exposure should be encouraged, not maligned. If one avoids sunburn, the sun’s health-giving benefits far outweigh its detrimental effects. A large body of evidence indicates that sunlight does not cause the most lethal form of skin cancer, malignant melanoma. A U.S. Navy study found that melanoma occurred more frequently in sailors who worked indoors all the time. Those who worked outdoors had the lowest incidence of melanoma. Also, most melanomas appear on parts of the body that are seldom exposed to sunlight (Arch Environ Health 1990;45:261–267). Sun exposure is associated with increased survival from melanoma (J Natl Cancer Inst 2005;97:195–199). Another study showed that people who had longer lifetime exposure to the sun without burning were less likely to get melanomas than those with less exposure (J Invest Dermatol 2003;120:1087–1093.)"
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http://www.lewrockwell.com/miller/miller25.html