The word "moisturizer" can be misleading. In fact, moisturizers mostly help your skin to retain its naturally occurring moisture. Your skin is far more moisture-rich than the surrounding air, so that moisture needs to be trapped in your skin. Oils help to make sure that your skin has an adequate barrier of protection to retain this moisture.
People also turn to moisturizers to soothe dry skin and protect it from the sun and the elements. Some moisturizers may also help to cover up wrinkles or even to unclog pores and tackle acne. But be careful of excessively bold claims, such as that a moisturizer will boost the skin's healing ability or will remove blemishes. Because moisturizers are classified as cosmetics rather than drugs, manufacturers' claims often aren't closely scrutinized or tested.
The main active ingredients in moisturizers generally fall under the categories of emollients, humectants, preservatives and fragrances. Emollients help to create smoother skin by fitting into spaces between skin cells. Emollients, which may be mineral oil, petroleum jelly or other compounds, can be oil- or non-oil-based. Oil-based emollients are less common than water-based ones, although oil-based emollients go on thicker and stay in place longer. You can often tell an oil-based moisturizer from a water-based one because it's thicker to the touch and seems to leave a palpable residue on the skin.
Humectants help to draw moisture out of the air. They're able to do so because they're hydrophilic -- literally, they love water. But a humectant-loaded moisturizer will also need to have some oil in order to trap that water against the skin. Or it'll need to have one of the many possible ingredients that simulate the effects of oil. Because, in fact, supposedly "oil-free" moisturizers are only oil-free in name. These moisturizers contain oil derivatives or compounds -- with hard-to-pronounce names like hydrogenated polyisobutene -- that mimic the effects of oil by sealing in moisture. It seems that the oil-based vs. oil-free distinction is largely a semantic one.
Moisturizers also contain preservatives in order to block the development of bacteria. Preservatives can sometimes cause allergies, as can fragrances, so you might opt for a fragrance-free moisturizer if you're willing to put up with its simple, vaguely chemical smell.
Various other ingredients, such as sugars, vitamins, fruit and plant extracts (like aloe vera or avocado oil), or even lanolin, an oil derived from wool, may also show up in moisturizers. In fact, moisturizers with plant and fruit extracts are often recommended -- aloe vera is renowned for its healing properties -- and some evidence indicates that products with alpha hydroxy acid are good at replenishing moisture [source: Bouchez]. Try a cream that includes sunscreen; it'll help protect against sun damage, even on cool days.
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