Sunscreen or sunblock

When I went to the supermarket, I noticed that some of the product say sunscreen and some of them say sunblock.


My question split into two part.


1) What are the different chemical substances use in both item? (In general)


2)What is more effective to prevert sunburn? (In general) 




Giving advice on preventing sunburns may fall under health advice, which is not permitted on this site. For advice, you should ask a medical professional.


The following information is from [The Melanoma Foundation](http://www.melanomafoundation.org/prevention/facts.htm).


What is the difference between sunscreen and sunblock?
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> Sunscreens can be classified into two major types: chemical and physical. **Chemical sunscreens** contain special ingredients that **act as filters and reduce** ultraviolet radiation penetration to the skin. These sunscreens often are colorless and maintain a thin visible film on the skin. These **sunscreens usually contain UVB absorbing chemicals and more recently contain UVA absorbers as well**.
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> **Physical Sunscreens, most often referred to as sunblocks**, are products containing ingredients such a titanium dioxide ($\ce{TiO2}$) and zinc oxide ($\ce{ZnO}$) **which physically block** ultraviolet radiation (UVR). Sunblocks provide broad protection against both UVB and UVA light. They can be cosmetically unacceptable to many people, because they are often messy, visible and do not easily wash off. However, some new zinc oxide products are available in brightly colored preparations which are popular with young people. The amount of sun protection these sunblocks provide, while potentially high, cannot be quantified in the same manner as sunscreen SPFs. Physical sunscreen is recommended for individuals who have unusual sensitivity to UVR. Most recently on the sun protection scene is sun-protective clothing designed to block UVA and UVB radiation. The effective SPF is greater that 30.
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When should you use a sunscreen?
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> 
> **Sunscreens should be used daily if you are going to be in the sun for more than 20 minutes.** Most people will receive this amount of sun exposure while performing routine activities. They can be applied under makeup. There are many cosmetic products available today that contain sunscreens for daily use because sun protection is the principal means of preventing premature aging and skin cancer. Sunscreen used on a regular basis actually allows some repair of damaged skin. Because the sun's reflective powers are great - 17 percent on sand and 80 percent on snow - don't reserve the use of these products for only sunny summer days. Even on a cloudy day 80 percent of the sun's ultraviolet rays pass through the clouds. Skiers beware, ultraviolet radiation increases 4 percent for every 1,000-foot increase in altitude.
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To add to this last part, sunblock manufacturers usually include instructions with their products on the proper way to use them.


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One thing to add, the U.S. Food and Drug Administration (FDA) recently changed the labeling requirements for sunscreens. As part of it the word "sunblock" will no longer be allowed. You'll still find some sunscreen's using the term "sunblock" and this is usually because they were manufactured before the labeling changes went into effect.


Going forward everything will be labeled "sunscreen" whether it is a mineral or chemical sunscreen. So to tell in the future you'll need to look at the "Active Ingredients" in a sunscreen (the FDA requires sunscreens to list all their ingredients). If you see only zinc and/or titanium there then it's a mineral sunscreen. Any other ingredient listed under "Active Ingredients" is a chemical UV filter as currently 17 ingredients (2 minerals and 15 chemicals) are approved by the FDA as sunscreen filters.


Something you definitely want to look for on a sunscreen's label is the term "Broad Spectrum". According to the new FDA regulations only sunscreen's that protect against both UVA and UVB rays can be labeled as such.


If you're interested, here's a bit more about the new sunscreen labeling laws from the Skin Cancer Foundation: "[New Sunscreen Labeling Rules Issued by FDA](http://www.skincancer.org/prevention/sun-protection/sunscreen/labels)".


One other interesting thing is that some UV filtering ingredients protect against the full UVA and UVB spectrum while others protect against only parts. This is why some sunscreens use multiple active ingredients. If you're interested, we have bit more about that here "[UVA, UVB and UVC Rays: What They Are and How Sunscreen Protects You](http://www.blockislandorganics.com/Blog/post/2013/11/06/UVA-UVB-and-UVC-Rays-What-They-Are-and-How-Sunscreen-Protects-You.aspx)".


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