I have been getting many questions about this topic, so I thought I would do a little research…
First thing: sunshine exposure is necessary for vitamin D synthesis. With sunblock, vitamin D synthesis does not happen. No vitamin D synthesis = immune system problems — which can lead to immunologic disease such as allergies, auto-immune conditions, cancer, etc. Also, you need vitamin D for calcium metabolism and the bones.
However, it is now recognized that not all people can synthesize vitamin D from the sun because of some gene or enzymatic pathway deficiency!!  Therefore, supplementing with vitamin D may be important even in the summer. My best advice would be to get your serum hydroxy vitamin D checked. This test will let you know if your stores are low since vitamin D is a fat-soluble vitamin – meaning that your body stores it in fat cells and does not excrete it.
So… applying sunscreen may be ok so long as vitamin D is being taken orally. However, certain sunscreens pose bigger risks than others so check your label.
For example, “sunscreens” (i.e. absorb UV light – thus prevent damage for UV radiation) containing any of the following have the potential for allergic reactions and, counter-productive, potential for free radicals because they are chemicals!:
PABA (especially if you have an allergy to sulfonamide medication), padimate A and O, benzophenones (oxybenzone, eusolex 4360, methanone, unival M40, diphenylketone), cinnamtes, salicylates, dibenzoylmethanes, octocrylene.
“Sunblocks” (reflect UV light) – titanium dioxide and zinc oxide – are highly effective. The downside to these is that they are thick and do not get absorbed in the skin. However, with nanotechnology, titanium dioxide and zinc oxide are formulated in nano size and are therefore much better absorbed in the skin and do not leave residues. Of course, there is concern with this because nanoparticles have the potential for toxicity, escaping the immune system, and free radical damage with exposure to sunlight. The research is inconclusive and further studies are necessary to determine the safety of these products . Some studies show that these particles do not penetrate to the deeper levels of the skin and so there is no safety concern. These studies, however, where done on animals or on non-intact (non-living) skin.
And then… to make things more complicated… how about the health of the skin? Eczema increases absorption of materials applied to the skin while psoriasis inhibits them. Sunburned skin is inflamed and less is absorbed.
The US FDA does not have a regulation that sunscreen manufacturers have to say whether they used nanoparticles or not in their sunscreens. And generally, concentrations of these agents are not posted on the bottle.
I also found that higher sunscreen is ok.
The answer? Hard to tell you! It is best to get informed and use your judgment. If you have a skin condition, you might want to ask your dermatologist about sunscreens, or do your research about finding the one that is best for you.
The most natural would be a sunscreen with mineral concentrations of titanium and/or zinc oxide to sun exposed areas. The other natural method is to use clothing for cover and limit your sun. Supplement with vitamin D. 10-15 min of sun exposure per day to hands and face does not pose a high risk for skin cancer.
 Binkley, N et al. (2007). Low vitamin D status despite abundant sun exposure. J Clin Endocrin and Met 92(6): 2130-2135.
 Newman, M et al. (2009). The safety of nanosized particles in titanium dioxide and zinc oxide based sunscreens. J Am Ac Dermatol.