Vitamin D

So it turns out that UVB converts vitamin D to its active form in the skin. Completely blocking UVB with sunscreen, then, will reduce your activated vitamin D, resulting in reduced calcium absorption and weakening bones. Fortunately, most of us are not slathering on the stuff, so we probably don’t need to worry about our vitamin D levels. If you are worried, however, you can always have it tested or take a supplement. Aim for 600-800 IU of vitamin D per day.

Nanoparticles

Mineral sunscreens are often formulated into small particles to enhance the way it looks on your skin. This is understandable if you’ve ever used the zinc oxide from years ago. Some have wondered if shrinking the particle size might actually allow your body to absorb the particles. Studies, however, have confirmed the particles stay on the surface of your skin, sloughing off naturally with skin cell turnover.

Contact Dermatitis

Occasionally, chemical sunscreens (those with lengthy, difficult to pronounce active ingredients) can cause a contact dermatitis when exposed to sun on certain people. This is an itchy mess known as a photo-allergic contact dermatitis. It feels and looks like a reaction to poison ivy, but it’s limited to the sun-exposed areas. Some of the chemical sunscreens are more prone to inducing this reaction and are easily avoided with a quick perusal at the active ingredients or using mineral sunscreens.