Sunscreens cause cancer.
People are constantly being told to use sunscreens when in the sun...to prevent sunburn, wrinkling, and some forms of skin cancer. But sunscreens not only block out skin damaging UV rays, but also the body's ability to make vitamin D. Vitamin D deficiency might increase the risk of other kinds of cancer.
But tell people not to throw away their sunscreens. Explain that it doesn't take much sun for the skin to produce the necessary amounts of vitamin D.
Just 15 minutes of sun exposure to the face, arms, hands, and legs, 2 to 3 times a week, is probably enough for fair-skinned people. Darker-skinned people or those at more northern latitudes might need more. For longer exposure, recommend a broad spectrum sunscreen with an SPF 15 or greater to protect the skin.