Why don't all moisturizers include sunscreen?

Getting Beautiful Skin Image Gallery Sunscreen is sometimes included in moisturizing products for added skin protection, but why isn't it included in all products? See more pictures of ways to get beautiful skin.
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"Use a moisturizer so you don't get wrinkles!" "Put on sunscreen to prevent skin cancer and signs of aging!" Following all the recommended guidelines for skin protection can translate to you slathering product after product on your skin.

However, maybe you've noticed that many moisturizing products also contain sunscreen, which can help cut down on the number of products you should use. Yet why do some moisturizers contain sunscreen and others don't? Is the combined product less effective than if you use both separately?

Before digging into those questions, take a closer look at each of these products separately and see how they protect your skin.

Moisturizer helps hydrate and protect the skin. Without it, the skin can dry out and lose its natural protective barrier. But using a moisturizing product can protect and replenish this barrier, reducing the look of aging signs and keeping skin looking younger [source: Mayo Clinic: Moisturizers]. Moisturizers can hydrate your skin better than water alone because dry, damaged skin can't absorb water very well. It needs a product that traps water on the skin so the cells can better absorb and retain moisture. A number of ingredients found in moisturizers, including glycerin, collagen, elastin, proteins and amino acids, among others, can help the skin rebuild and replenish its outer surface [source: Begoun: Moisturizers].

Sunscreens, on the other hand, are made to protect your skin from the sun's ultraviolet (UV) rays. These invisible rays, whether UVA or UVB, can cause skin damage and increase the risk of developing skin cancer, so experts recommend wearing sunscreen whenever you're outside. Sunscreens come in two different varieties that provide either physical or chemical protection from the sun. Ingredients such as zinc oxide and titanium dioxide help physical sunscreens form an impenetrable layer over the skin that blocks UV rays. Chemical sunscreens include ingredients such as avobenzone or oxybenzone that sink into the skin and absorb UV rays before they can do any lasting damage to the skin. The efficacy of a sunscreen is determined by its SPF, or sun protection factor, which is the percentage of UVB rays the product can filter out. Over time, the sunscreen's ability to protect skin fades, which is why it's important to reapply the product throughout the day [source: Mayo Clinic: Sunscreen].

Now that you know how these products work, let's look at why they aren't always combined into a single product.